The metalliferous mining region of south-west England

By H G Dines, ARSM, AMICE with petrographical notes by J Phemister, MA, DSc and addenda and corrigenda by K E Beer, BSc

Bibliographical reference: Dines, H G. 1956 [1988 reprint]. The metalliferous mining region of south-west England. Economic Memoir of the Geological Survey of Great Britain.

British Geological Survey. London: Her Majesty's Stationery Office 1956 1988 reprint

First published 1956 by HMSO. Second impression with amendments 1969.  Third impression with further amendments 1988 issued by BGS. Printed in the United Kingdom for BGS by Derry and Sons Ltd, Nottingham C5 8/88 ISBN 0 85272 104 8. Crown copyright 1956. Addenda and corrigenda © NERC 1988

The British Geological Survey carries out the geological survey of Great Britain and Northern Ireland (the latter as an agency service for the government of Northern Ireland), and of the surrounding continental shelf, as well as its basic research projects. It also undertakes programmes of British technical aid in geology in developing countries as arranged by the Overseas Development Administration.

The British Geological Survey is a component body of the Natural Environment Research Council.

Preface

This account of the mining geology of south-west England summarizes the geological information gained during the long history of mining in the counties of Cornwall, Devon and west Somerset.

The region covered is classical ground, for it coincides with the territory in which De la Beche commenced geological surveying in 1820. When, later, he was commissioned to colour geologically the Ordnance Survey one-inch maps, his work in the south-west of England was the first to be published, and his 'Report on the Geology of Cornwall, Devon and West Somerset', 1839, was the first memoir published by the Geological Survey. Previous accounts of the mining district, notably by Borlase in 1758 and Pryce in 1778, had appeared, but De la Beche's Report was the first treatment of the geology and of the ore deposits based on scientific concepts supported by geological mapping.

Not for more than 60 years was the official geological survey resumed in this district. At the turn of the century it had become clear that complex and fundamental problems in the stratigraphy, metamorphism and petrology, as well as the mining activity of the region, required further scientific investigation. Accordingly, detailed mapping on the six-inch scale was put in hand under Clement Reid, and the following members of the staff took part in the resurvey: G. Barrow, H. Dewey, E. E. L. Dixon, J. S. Flett, J. B. Hill, D. A. MacAlister, W. Pollard, J. B. Scrivenor, R. L. Sherlock, W. A. E. Ussher and B. S. N. Wilkinson. The petrographic study of the igneous and metamorphic rocks and of the ore deposits was undertaken by Flett, whilst the mining side of the work was for the most part carried out by MacAlister.

The resurvey covered most of Cornwall and the southern part of Devon. It thus dealt with the greater part of the metalliferous region. Results were published in the one-inch sheet memoirs, each with a mining appendix, but no regional treatment of the mining geology was attempted. Several publications on the south-western region, some from the more technical mining standpoint, had appeared between De la Beche's work and completion of the resurvey, the most important being those by Henwood in 1843 and Collins in 1912.

Wartime investigations of mineral deposits by the Geological Survey during 1914–18 were published in the series of 'Special Reports on the Mineral Resources of Great Britain'. From this work it became clear that further investigation was essential and, to this end, periodical visits were made to mining districts by Survey officers. The south-western region was visited from time to time by Mr. H. G. Dines during the years 1921–35, and in 1938 he was instructed to commence a thorough study of the mining geology under the superintendence of Mr. T. Eastwood, then Assistant Director. For brief periods Mr. Dines was assisted by Dr. S. Buchan (1942) and Mr. J. A. Robbie (1942–3) in the field, and in the office he had the assistance of several of his colleagues in the work of plotting mine workings from the plans in the Mining Records Office. Reductions from the plans were chiefly to the six-inch scale, but in the more intensively mined districts of western Cornwall 25-inch scale maps were used; mine sections have been reduced consistently to a scale of 200 ft. to the inch.

In 1933 Mr. Dines had propounded his theory of 'emanative centres', a concept that has proved a valuable tool in the unravelling of many hitherto puzzling problems relating to the distribution of tin, copper and lead-zinc ores. The general conclusion reached by Mr. Dines as a result of his official investigations is that though it is improbable that many rich individual deposits are still to be found near surface, planned systematic prospecting in some areas is likely to lead to the discovery of hidden ore bodies. Under present economic conditions, a revival of mining on a scale such as existed 50 or 100 years ago cannot be anticipated but it is possible that by improvement in beneficiation of low-grade ores a greatly increased activity might be expected. There is scope for planned underground exploration and grounds for hope that this will result in discoveries which will prolong production on a scale sufficient to preserve the west of England mining tradition.

For ease of treatment the region has been divided into 14 districts, and under each of these are grouped and described the individual mining works or properties. The account of each mine contains all the information it has been possible to obtain from past records, from examination of the ground and, where possible, from the workings. The accounts of the various mines do not provide a mining report upon the property, but they set forth the known facts from which inferences may be drawn, thus giving a geological foundation upon which just mining appraisal may be based. Throughout the memoir great care has been taken in quoting the many figures of distance, depth, length of drives, angles of bearing and underlie and outputs. It has, however, not been practicable, because of the time and expense involved, to check all these figures to their ultimate sources. Petrographical descriptions and notes by Dr. James Phemister and a few by Professor K. C. Dunham, formerly Chief Petrographer to the Geological Survey and Museum, are incorporated in the text.

Throughout the periods of field work much assistance was given both by professional men and by persons who take an interest in the industry. Many who have mine plans in their possession, as listed in the Catalogue of Plans of Abandoned Mines (Mines Department) vol. ii, 1929, have freely given facilities for examination of them. In acknowledging the assistance so extensively afforded, we wish to offer thanks especially to the following gentlemen : the late E. H. Davison, of the Geological Department of the Cambome School of Mines, and his successor Mr. J. Robson; the late Lt.-Col. J. V. Ramsden, C.M.G., D.S.O., of Bridford Barytes Mine, who supplied information on the mines of Devonshire which he had collected from many sources and by his own investigations; Mr. H. St. L. Cookes, of Bideford Black Mine, for supplying information and specimens collected during his investigations of mines in north Devon and west Somerset; and Mr. J. H. Trounson, of South Crofty Mine and Chairman of the Cornish Mining Development Association, for information, useful discussion and advice. In carrying out underground inspection of mines, every facility was afforded by the late C. V. Paull (South Crofty Mine), Mr. D. D. Belcham (formerly of East Pool Mine, now of South Crofty), Mr. W. E. Sevier (formerly of Geevor Mine), Mr. J. H. Bennetts (formerly of Wheal Reeth, now of Geevor) and Mr. J. B. Fern (formerly of Polberro Mine). Thanks are due also to Col. F. F. Oats, formerly of Levant Mine; Mr. E. M. Cunnack, Helston, for the loan of the late R. J. Cunnack's manuscript notebook; Mr. D. W. Saunders, formerly of South Crofty, for the loan of many letters, books and reports by the late W. Thomas; Mrs. Giles Oats, for permission to see notes, reports and mining maps by the late Col. G. Oats; Mr. Edgar Trestrail, Redruth, for the loan of plans, reports and prospectuses of many mines; Mr. L. F. Caunter, Liskeard, for permission to see reports and plans; Mr. M. Gregory, Cam Brea, for advice and comment; Mr. A. K. Hamilton Jenkin, for useful discussion and for information contained in The Cornish Miner; Mr. C. C. James, Penzance, Mr. A. Trevena, Redruth, and Lt.-Cmdr. P. H. G. Richardson, R.N., all of whom lent their own tabulated information relating to mines in various districts; and to the Estate Agent for the Duchy of Bedford Estate, Tavistock, the Bailiff of the Duchy of Cornwall Estates, Liskeard, and the Agent for Tehidy Minerals Limited, for permission to inspect plans and reports of mines on these estates and holdings; also to the mining engineers employed by the Non-Ferrous Mineral Development Control, particularly Mr. R. H. Skelton, who supplied information acquired by them during the wartime investigations of the Control. Appreciation of the help given by the following gentlemen, now deceased, is also to be recorded: Josiah Paull (South Crofty) for a view of all the reports on mines in the west of England written by him, and C. F. Barclay, Yelverton, for the loan of reports and plans made by him during a survey, with others, of the south-western part of Devon and elsewhere.

W. J. Pugh, Director. Geological Survey Office, Exhibition Road, South Kensington, London, S.W.7 28th February, 1953

Preface to 1969 reprint

The first publication of this Memoir in 1956 anticipated the current intensified search for economically workable deposits of tin and other ores in south-west England, and the Memoir has been in steady demand.

At the present time the immense task of revision cannot be undertaken and the Memoir is therefore being reprinted without change, except for the insertion of a list of additions and corrections. This list has been compiled by Mr. E. A. Edmonds from the personal communications, records and published works of Mr. K. E. Beer, Mr. P. Best, Mr. J. Brooke, Mr. D. G. Broughton, Mr. H. St. L. Cookes, Dr. W. R. Dearman, the late H. G. Dines, Mr. B. W. Hester, Mr. A. K. Hamilton Jenkin, Mr. J. H. Trounson and Mr. B. T. Walsham, whose generous assistance is cordially acknowledged.

K. C. Dunham Director Institute of Geological Sciences Exhibition Road South Kensington London, S.W.7 15th May, 1969

Preface to 1988 reprint

In the thirty years since its first publication this Memoir has been in constant demand from an ever-widening range of mining, mineral, exploration, industrial archaeological and caving interests. So much so that it has now been out of print too long.

During this usage several errors and omissions have been reported to the Geological Survey. Collected and compiled by Mr K. E. Beer, they appear at the end of Volume II of the Memoir. (They are also available separately.) The generous assistance of all contributors to this list is gratefully acknowledged.

It is certain that other mistakes remain undetected or unrecorded. Further, it would clearly be desirable to update descriptions of mines worked and incorporate the results of exploration undertaken since 1956. However, the total and fundamental revision needed cannot be contemplated at the present time. In consequence the Memoir is reprinted from the 1956 version without change, and enlarged Addenda and Corrigenda at the end of Volume II supersede those from the 1969 reprint.

F. Geoffrey Larminie, OBE Director British Geological Survey Keyworth 1 September 1988

Note 2018 for this web version

Text from the 1988 Addenda/corrigenda has been incorporated into the main body of text for this web version. Grid references for the mines have been added. These have been derived from a number of sources but special thanks are given to Mike Gill of the Northern Mines Research Society who provided a list of mines and their Grid References.

Chapter 1 Introduction

The region selected for this memoir, comprising Cornwall, the southwestern and northern parts of Devon and the western part of Somerset, embraces the metallogenic province associated with the granitic intrusions of south-west England. The region coincides more or less with the country covered by De la Beche's report (1839). That part which lies north of an E.-W. line passing about a mile north of Launceston is represented by the Old Series, One-inch Geological Maps (hand-coloured) Nos. 20, 21, 22 (part), 25 (part), 26, 27 and 29, all of which were published or revised in 1839. In this northern area the New Series, One-inch Geological Maps No. 295 (Quantock Hills) and No. 325 (Exeter) were published as colour-printed editions, respectively, in 1907 and 1912. South of the E.-W. line the country was almost completely resurveyed on the 6-inch scale between 1899 and 1913 and is covered by the following New Series, One-inch Geological Maps, which are accompanied by memoirs : 335 (Trevose Head), 336 (Camelford), 337 (Tavistock), 338 (Dartmoor), 339 (Newton Abbot), 346 (Newquay), 347 (Bodmin), 348 (Plymouth), 349 (Ivybridge), 350 (Torquay), 351 with 358 (Penzance), 352 (Falmouth), 353 (Mevagissey), 355 (Kingsbridge), 356 (Start Point) and 359 (Lizard).

For convenience in the present work the region has been divided into 14 districts (see Key Map). Districts 1 to 7 are practically contiguous, their boundaries being determined by geological considerations, while the boundaries of the remainder are determined by the distribution of the mines. The memoir deals only with the hydrothermal mineral deposits; alluvial deposits, apart from the details of a few that have been worked comparatively recently or in conjunction with mines, are not discussed in a general way.

In this classical ground, mining had commenced many centuries before the science of geology was born, and observations made in the region have played an important part in the building up of that branch of the science known as mining geology and of theories on ore-deposition. Since the beginning of the scientific study, accounts of the mines and the mineral deposits have been published from time to time. In addition to publications relating to individual deposits or to some aspect or other of ore-deposition (many of which are referred to in the sequel), there are outstanding works dealing with the region as a whole or with large parts of it. Borlase (1758) and Pryce (1778) recorded many facts and points of interest, most of which afford valuable records, though some are merely curious. Thomas (1819) has left many useful notes relating to mines, lodes, etc. in the important area between Camborne and Chacewater. De la Beche's report (1839), the first memoir of the Geological Survey, deals mainly with the geology of the region but includes a considerable amount of information on the mines and mineral deposits. Henwood's chief work (1843) represents an enormous amount of painstaking care in the collecting and tabulating of details relating to lodes calculated to be of value to the mining geologist. Symons (1884) gives a brief but very clear account of the physical aspects of mineralization that could hardly be improved upon today. Collins 'Observations' (1912), perhaps the best known work, contains not only a comprehensive treatise on the geology of the mineral deposits, but also an account of over 1,800 mines with a large number of which he was personally familiar. Between De la Beche's time and the beginning of the 20th century, little attention was paid by the Geological Survey to mining work in the west of England, but the sheet memoirs, based on the 6-inch survey made between 1899 and 1913, contain accounts of the mines and minerals within the area covered by each sheet. These accounts are largely by D. A. MacAlister, who traced the lodes in the field, reduced lode positions from mine plans on to the 6-inch geological maps, figured a large number of mine sections in the memoirs and compiled statistics of outputs. His Part II of the memoir on the Geology of Falmouth and Truro (1906), dealing with the most important mining district, namely that of Camborne and Redruth, though essentially written for that particular district, might also apply in a general way to the whole mineral field. During the 1914–18 war attention was given by the Survey to certain minerals then in demand, and a series of memoirs under the general title 'Special Reports on the Mineral Resources of Great Britain' were published. Certain of these deal with the following west of England ores: copper, arsenic, antimony, lead, zinc, tungsten, manganese, iron, barytes and fluorspar.

The geology of the region is fully dealt with in the various sheet memoirs as well as by Collins (1912) and other authors, and the general theories of ore-deposition and the relation of ores to the igneous rocks, etc. have often been expounded in the literature of mining geology during the last few decades. It is, therefore, considered sufficient for the needs of the present memoir to give only a brief outline of these subjects and to confine the account mainly to the explanation of the mineral deposits, and to the extent of the individual workings in so far as these are known. For the history of mineral production the reader is referred to the works of historians (see Rickard 1932, vol. I, pp. 291–354; Jenkin, 1927), and, for past yields of tin and copper, to papers by Collins (1892, 1895), and of tin, copper and other minerals to MacAlister (1906, pp. 306–14; 1907; 1921). These authors have summarized and tabulated records which give the annual outputs as far back as possible. Collins, by estimation, goes back to the 12th century, while MacAlister starts in the 18th century and continues his records up to 1905 for Cornwall and to 1913 for Devon. Yields for the 20th century from individual mines are given, where known, in the present memoir.

MacAlister grouped statistics by parishes, and the following list is given to show how the parishes are distributed amongst the districts of this memoir. Districts 1 to 10 are in Cornwall, 11 is shared by Devon and Cornwall, 12 and 13 are in south-west Devon while 14 is in north Devon and west Somerset.

1. St. Just District. Madron, Morvah, St. Just, Sancreed, Zennor.
2. St. Ives District. St. Ives, Towednack, Uny Lelant, Zennor.
3. Gwinear District. Crowan, Gwinear, Gwithian, Phillack, St. Erth.
4. Mount's Bay District. Breage, Germoe, Ludgvan, Marazion, Mawgan, Paul, Penzance, Perranuthnoe, St. Erth, St. Hilary, Sithney.
5. Wendron and Falmouth District. Constantine, Crowan, Falmouth, Wendron.
6. Camborne, Redruth and St. Day District. Camborne, Gwennap, Mogan, Kenwyn, Kea, Redruth.
7. St. Agnes District. Cubert, Kenwyn, Newlyn, Perranzabuloe, St. Agnes.
8. St. Austell District. Bodmin, Ladock, Lanivet, Lanlivery, Luxulian, Roche, St. Austell, St. Blazey, St. Columb Major, St. Dennis, St. Enoder, St. Ewe, St. Mewan, St. Stephen in Brannel, St. Winnow, Tywardreath, Withiel.
9. Wadebridge District. Camelford, Egloshayle, Endellion, Minster, Padstow, St. Breward, St. Issey, St. Kew, St. Minver, St. Teath, Tintagel, Wadebridge.
10. Liskeard District. Altarnun, Cardinham, Lanreath, Linkinhorne, Liskeard, Menheniot, North Hill, St. Cleer, St. Neot, Warleggan.
11. Callington and Tavistock District. In Cornwall: Callington, Calstock, Launceston, Lezant, St. Mellion, South Hill, Stoke Climsland. In Devon: Bere Ferrers, Brentor, Buckland Monachorum, Coryton, Lamerton, Lewtrenchard, Lydford, Marystow, Marytavy, Meavy, Milton Abbot, Petertavy, Plympton St. Mary, St. Stephen, Shaugh Prior, Sydenham Damerel, Tavistock, Walkhampton, Whitchurch.
12. Dartmoor and Teign Valley District. Ashburton, Bovey Tracey, Bridford, Buckfastleigh, Chagford, Christow, Dartington, Harford, Hennock, Holne, Ilsington, Lustleigh, Lydford, Manaton, Newton Abbot, North Bovey, Wolborough.
13. Okehampton District. Belstone, Okehampton, South Tawton.
14. North Devon and West Somerset District.

The large number of parishes embraced by this district are not listed as there are no memoirs of New Series sheets except No. 295 (Quantock Hills) and in this the mines mentioned are not located under parishes.

References

BORLASE, W. 1758. The Natural History of Cornwall. Oxford.

CANTRILL, T. C., R. L. SHERLOCK and H. DEWEY, 1919. Iron Ores (Contd). Sundry unbedded ores of Durham, East Cumberland, North Wales, Derbyshire, Isle of Man, Bristol District and Somerset, Devon and Cornwall. Mem. Geol. Surv., Min. Resources, vol. ix.

CARRUTHERS, R. G. and R. W. POCOCK. 1922. Fluorspar. Mem. Geol. Surv.. MM. Resources, vol. iv, 3rd edit.

COLLINS, J. H. 1892. Seven Centuries of Tin Production in the West of England. Trans. Mining Assoc. and Inst. Corn., vol. iii, pp. 173–93.

COLLINS, J. H. 1895. Four centuries of copper production in the west of England . Miners Assoc., and Inst. Corn., vol. 6, pp. 209–42.

COLLINS, J. H. 1912. Observations on the West of England Mining Region. Trans. Roy. Geol. Soc. Corn., vol. xiv.

DAVISON, E. H. 1914. The Geological History of Cornish Tin Lodes. Trans. Corn. Inst. Eng., vol. ii, pp. 161–92.

DE LA BECHE, H. T. 1839. Report on the Geology of Cornwall, Devon and West Somerset. Mem. Geol. Surv.

DEWEY, H. 1920. Arsenic and Antimony Ores. Mem. Geol. Surv., Min. Resources, vol. xv.

DEWEY, H. 1921. Lead, Silver-Lead and Zinc Ores of Cornwall, Devon and Somerset. Mem. Geol. Surv., Min. Resources, vol. xxi.

DEWEY, H. 1923. Copper Ores of Cornwall and Devon. Mem. Geol. Surv., Min. Resources, vol. xxvii.

DEWEY, H. and H. G. DINES. 1923. Tungsten and Manganese Ores. Mem. Geol. Sum, Min. Resources, vol. i, 3rd edit.

HENWOOD, W. J. 1843. On the Metalliferous Deposits of Cornwall and Devon. Trans. Roy. Geol. Soc. Corn., vol. v.

JENKIN, A. K. H. 1927. The Cornish Miner. 2nd edit., 1948. London.

JONES, W. R. 1925. Tinfields of the World. London.

MACALISTER, D. A. 1906. In The Geology of Falmouth and Truro and of the Mining District of Cambome and Redruth (Sheet 352), Mem. Geol. Surv.

MACALISTER, D. A. 1907. Total Quantity of Tin, Copper and other Minerals produced in Cornwall, particularly with regard to the Quantities raised from each Parish. Sum. Prog. Geol. Surv. for 1906, pp. 132–9.

MACALISTER, D. A. 1921. Total Quantity of Tin, Copper and other Minerals produced in Devonshire, particularly with regard to the Amounts raised from each Parish. Sum. Prog. Geol. Surv. for 1920, pp. 96–102.

MOISSENET, L. 1877. Observations on the Rich Parts of the Lodes of Cornwall, their form and their relation with the directions of the stratigraphic systems. (Trans. J. H. Collins.) London.

PRYCE, W. 1778. Mineralogia Cornubiensis, a Treatise on Minerals, Mines and Mining. London.

RICKARD, T. 1932. Man and Metals, a history of mining in relation to the development of civilization. New York and London.

SPARGO, T. 1865. The Mines of Cornwall and Devon: Statistics and Observations. London. SYMONS, B. 1884. Sketch of the Geology of Cornwall with special reference to its Mineral Wealth. In A Gazetteer of Cornwall by R. Symons, Penzance.

THOMAS, C. 1867. The Mining Fields of the West; being a practical exposition of the Principal Mines and Mining Districts in Cornwall and Devon. London.

THOMAS, R. 1819. Report on a Survey of the Mining District of Cornwall from Chacewater to Camborne. (Accompanied by a map and section.) London.

WILSON, G. V., T. EASTWOOD, R. W. POCOCK, D. A. WRAY and T. ROBERTSON, with H. G. DINES. 1922. Barytes and Witherite. Mem. Geol. Surv., Min. Resources, vol. ii, 3rd edit.

Chapter 2 Geology

By far the greater part of the region is composed of slaty shales and mudstones with subordinate bands of grit and conglomerate which are all embraced by the local name 'killas'. In the main these strata are of Devonian age, but in the northern part, between the latitudes of Launceston and Barnstaple they include also Culm Measures of Carboniferous age; both systems contain contemporaneous beds of lava and tuffs and pene-contemporaneous sills, dykes and bosses of basic greenstone. In the east, about Torquay, Exeter and the Quantock Hills, the Palaeozoic killas is overlain by sandy and conglomeratic Permian and Triassic strata.

The Palaeozoic rocks had been subjected to earth movements that produced folding, cleavage and jointing with a roughly E.N.E. trend, when, in late Carboniferous or early Permian times, granite was injected forming large bosses or cupolas ranging over a length of 125 miles along this trend. The chief bosses are those of Scilly Isles, Land's End, Carnmenellis, St. Austell Moor, Bodmin Moor and Dartmoor. Each has an aureole of thermally metamorphosed killas.

On the consolidation of the crust of the granite, fissures were opened up along the same general E.N.E. lines of weakness and some of these became filled with quartz porphyry, forming the dykes locally known as 'elvans'. Later, emanations from the magma resulted in the deposition of lode minerals, chiefly of tin and copper ores in fissures with an E.N.E. trend, though some occur in fissures coursing E.S.E. (called 'caunter' lodes). For simplicity the lodes of these two trends are usually referred to as E.-W. lodes. The final stage of mineralization is represented by ores of lead, zinc and iron, which occur in places as peripheral deposits in E.-W. lodes, but chiefly in fissures ranging roughly N.-S. (crosscourses), which post-date the E.-W. lodes. Subsequent erosion has laid bare the granite bosses and the fissured mineral-bearing ground around them.

In depth, both the metallic and the gangue minerals are distributed in the lodes with a zonal arrangement, presumably controlled by temperatures and pressures existing when the emanations were released from the magma. Thus, some minerals which crystallize out at higher temperatures and pressures than others, occupy the deeper zones of the fissures. The order of deposition of the principal metallic minerals from below upwards is—tin, copper, zinc, lead, iron. The vertical range of the zones varies from place to place. Dewey (1925) gives a depth of 2,500 ft. to both the tin and the copper zones respectively and 1,500 ft. for the lead-zinc zone. There is, of course, some overlapping of the zones, those of lead and zinc especially being telescoped so as nearly to coincide. Also, the zones of accessory minerals such as ores of tungsten and arsenic are superimposed upon the others. Tungsten occurs generally at the top of the tin zone and base of the copper zone, while arsenic though often associated with tungsten, extends upwards throughout the copper zone. The gangue minerals are similarly zoned, for whereas quartz is present at all depths and was deposited during several stages, tourmaline is more or less confined to the deeper part of the tin zone, while chlorite occurs throughout the remainder of that zone and continues up into the overlying copper zone. Fine-grained masses and felted aggregates of these last two minerals in lodes are known to the Cornish miner as 'peach', blue and green respectively. Fluorspar is often present as an original constituent fairly deep in the tin zone and extends upwards through the copper zone and into the lead-zinc zone. It is present also in the deep zones as a filling in later openings that dislocate the original lode material. Calcite and other carbonates with barite belong chiefly to the lead-zinc and the iron zones.

The above very brief statement of the sequence of events and phases of deposition of the ores during the period of igneous activity of the region conforms to the generally accepted theories of ore-deposition as proved, not only in south-west England, but also in most other metallogenetic provinces throughout the world. The zonal arrangement is well established, but the relationship of the mineral deposits to the granite masses or cupolas is not so close as is generally supposed, for, though the tin and copper lodes are in some places situated on the margins of granite bosses and the zinc, lead and iron lodes at some distance from them, this is by no means the general rule. Tin lodes occur as far from granite outcrops as some of the lead-zinc lodes (e.g. at Wheal Jane, east of St. Day which is situated well beyond the aureole of thermal metamorphism), while iron lodes occur within the granite (e.g. Ruby Mine and Treffry Consols in the St. Austell district). It is true that tin and copper lodes occur at the granite margin at three once-important mining centres, namely, Camborne, St. Ives and St. Just, but it would appear that the economic importance of these centres has added weight to the observations made on the conditions prevailing there, with the result that generalizations have been based on these special cases. From evidence afforded by the whole region it is clear that the theory postulating that the mineral deposits are closely related to the granite bosses and that the zones are approximately parallel to the granite surface is not strictly true.

After the intrusion of the granite into the sedimentary rocks and the consequent heating of the country, the temperature was highest in the granite and fell gradually towards the then surface of the ground. Planes of uniform temperature in the rocks were, therefore, roughly parallel to the granite-killas contact in depth, but, upwards, became flatter and more horizontal or nearly parallel with the land surface. The isothermal planes, therefore, sloped at gentler angles than the granite-killas contacts and were closer together above the summits of the cupolas than above the troughs between them. Disregarding the unknown effects of pressure, the upper and lower limits of any particular mineral zone can be regarded as representations of isotherms within the country rocks at the time of crystallization of that particular mineral. That the zones, and therefore the isotherms, have a gentler slope than the adjacent granite surface is shown in a number of cases where we have sections of lodes crossing the granite margin, as at Levant and Geevor mines (Figure 2), Botallack Mine (Figure 6) and Dolcoath Mine (Figure 17), in this latter case the lode is oblique to the granite margin).

As the mineralizing solutions or vapours escaped upwards and outwards from the still molten granite core, through suitable channels in the consolidated granite crust and in the overlying sedimentary rocks, the high temperature minerals (cassiterite, wolfram, tourmaline, etc.) crystallized out in a zone where pressure and temperature permitted, while the minerals that crystallized at lower temperatures remained in solution until they had reached cooler zones above. The latter minerals thus had a greater distance to travel upwards, and, it would appear, more opportunity to migrate laterally, for their upward velocities would be reduced in each higher zone where fissuring was probably freer. It is probable that galena and blende remained in solution considerably longer than chalcopyrite, awaiting the fall in temperature that would 'fix' them in their appropriate zones. This supposition is supported by the fact that whereas galena and blende are found frequently in the higher parts of copper lodes coursing about E.-W., they occur also in the later formed N.-S. fissures or crosscourses. The question as to whether lead and zinc came up in solutions at the same time as tin and copper or at a later time, does not affect this view, which is based on the assumption that the mineralizing solutions of all the various minerals were derived from the same source. Each depth zone, therefore, has a wider lateral distribution than the one below, a fact clearly brought out by the distribution of the ores of south-west England (see Dines 1934).

So little is known of the behaviour of complex solutions under great pressures and high temperatures that it is impossible to visualize the chemical nd physical processes that governed crystallization during the upward passage through the fissures. It is clear, however, that the mineral-bearing solutions emanated from the still liquid core of the granite only in certain restricted places and that the ores, rather than being distributed around actual granite bosses, are located around certain restricted 'emanative centres' which are themselves not necessarily directly related to the bosses. Cassiterite (as well as other high temperature minerals), being the first to crystallize out from the solutions, is obviously found located near the emanative centres; it therefore acts as an indicator of the positions of these centres.

The conditions necessary for the formation of an emanative centre are clear. Firstly, the country rock must have been suitably fissured to receive the solutions. Secondly, the country rock must have been at the requisite temperature gradient for the deposition of the ores in their respective zones. Thirdly, the fissures must have been connected with the source of the mineral-bearing solutions. The fissures in which deposition eventually occurred in killas country may have been formed before the intrusion of the granite, and, in any case, they and the fissures in the granite were formed before the arrival of the mineral-bearing solutions. The granite supplied the heat that gave the necessary temperature gradient to the country. The deep-seated channels that permitted the release of the solutions or vapours must have been formed after the consolidation of the crust or hood of the granite and at a time when the metallic elements had segregated from the magma and were accumulated under pressure in the still liquid core, and may even have been due to the pressure developed by them (see Emmons 1934).

With a few minor exceptions the ores do not favour any particular country rock. Their distribution was determined by the three conditions enumerated above and, provided each of these was present, emanative centres may occur equally well in the centre of a granite boss, on the margin of a boss or in killas country at a distance from granite, as shown diagrammatically in (Figure 1). In some cases emanative centres have the appearance of being connected with elvan dykes as at Clifford Amalgamated Mines (Figure 27) and Wheal Jane, both east of St. Day. This is due to the fact that both the dykes and the emanative centre are situated in an area of crustal weakness when deep-seated fissuring took place , first to release the elvan magma and later the ore-bearing solutions.

Since the ores of each depth zone in upward succession have a wider lateral distribution than the one below, it will be seen that the sequence of the depth zones can only be complete at an emanative centre. Away from the centre the lower zones are missing though the higher ones may be present. This accounts for the fact that workable bodies of tin ore are not found everywhere below the copper zone even where the latter may be well developed, as in the case of the Roskear and Seton mines, north of Camborne, and Devon Great Consols near Callington. Similarly lead and zinc deposits may exist without the underlying copper zone even where ores of these occur in E.-W. lodes as at the Chiverton group of mines, east of St. Agnes.

The recognition of the restriction of tin deposits to the emanative centres is important in the future exploration of the region. The haphazard reopening of old tin mines is unlikely to lead to satisfactory results unless the mine is favourably situated in regard to a centre. It should, however, be emphasized that the only evidence for an emanative centre is that afforded by past workings for tin. Centres, as yet undetected, may possibly occur, but there are no means of locating these other than prospecting in unproved ground, and no known clues by which an emanative centre can be detected by other means. Again, it should be stressed that the actual shape, extent and size of an emanative centre carrying tin deposits can only be known in so far as existing mine workings have proved it. The maps (Figure 3a) and  (Figure 3b) show the distribution of known emanative centres throughout the region, as indicated by the presence of cassiterite deposits, and also the surrounding, more widespread copper and lead-zinc deposits. The size and shape of the patches representing the centres is no indication of their importance. By far the most important is that of Camborne, north of the Cam Brea granite mass. The centres that cover larger areas as, say, those on Bodmin Moor and Dartmoor are of far less importance from the production standpoint for they represent only the lower parts of tin lodes occurring within the granite masses, the upper parts of which have been eroded. A discussion on suitable places for prospecting would be out of place in this memoir, but, as an example, attention may be drawn to the emanative centre on the coast of St. Agnes. In the emanative centre at St. Just, the lodes are often at right angles to the coast, and workings for tin have followed them for up to about a mile under the sea. At St. Agnes, however, the lodes are parallel with the coast, and though very rich tin deposits have been worked up to the coastline, no crosscut has been driven beyond. It seems most unlikely that the boundary of the St. Agnes emanative centre coincides with the coast, and probable that satisfactory results might be obtained from investigations seawards in this locality.

Lodes

Earlier writers have recorded the trends of lodes in compass direction vectorially, but, with the irregularities of strike that occur in individual lodes, it is doubtful whether such representation serves a useful purpose from the economic standpoint. In (Figure 4a) and (Figure 4b) are shown the general lode trend throughout the greater part of the region as proved in underground workings, and the various district sketch maps illustrate the lode trend in each of the 14 districts sufficiently clearly for general study. As previously stated, the general trend of alignment of the granite masses, of elvan dykes and of the lodes is E.N.E. and this direction was determined chiefly by earth movements initiated before and continuing after the emplacement of the granite. Many mining geologists hold that lode fissures are brought into being by the actual upthrusting of the sedimentary rocks by the granite cupolas and assume that in the case of an elongated mass the lodes will be parallel to its major axis while in the case of a circular mass they will be radial. This hardly seems applicable in south-west England. The elongated mass of Cam Brea, with lodes running parallel to its longer axis has been cited as a case in point. Yet these lodes maintain their trend beyond the immediate confines of the Carn Brea mass for miles in either direction and even enter the adjacent Land's End mass at right angles to its margin, and are tangential to the Carnmenellis mass, of which Cam Brea is only a small satellite. It seems clear, therefore, that both the form of Cam Brea and the lode trend were determined by pre-existing conditions. In addition to the general E.N.E. trend, lodes crossing this in a direction E.S.E., known as caunter lodes, are fairly prevalent. The name caunter, however, is locally applied loosely to any lode crossing the general trend at a more or less acute angle, and in some rare cases has even been used to refer to a lode that underlies contrary to the usual underlie of a locality. All the districts are traversed by N.-S. trending crosscourses which post-date the more E.-W. lodes and are occasionally mineralized with low temperature minerals.

The normal E.N.E. lode trend is very persistent in the area from St. Just to St. Agnes. The only important variations are groups of caunter lodes coursing E.S.E. or S.E. on the edge of and near the centre of the Land's End granite mass, and also in the killas of the coastal belt west of the Godolphin granite in Mount's Bay district. Throughout the St. Ives, Gwinear, Camborne and St. Agnes districts, apart from a few caunter lodes, as around Leedstown, the E.N.E. trend is almost unbroken, but ceases north-east of St. Agnes, where the Ferran Iron Lode courses E.S.E. and there are a number of N.-S. crosscourses carrying lead and zinc ores. From St. Austell district eastwards, the average trends are more easterly than in the country to the west. Near the centres of the granites of St. Austell Moor, Bodmin Moor and Dartmoor, all lodes maintain the E.N.E. direction, but in the surrounding killas they seem to follow, more or less, the directions of the granite margins and here may have been due to the upthrust of the granite; this phenomenon is, perhaps, most marked along the southern edge of St. Austell Moor. The lodes to the north of St. Austell Moor and in the Wadebridge district, however, belong mainly to the crosscourse series and have yielded lead, zinc, antimony and iron ores. South of Bodmin Moor the lodes range more nearly east and west and this trend obtains across the mineralized belt connecting Bodmin Moor and Dartmoor, through the ground occupied by the small granite cupolas of Kit Hill and Gunnislake. Traced westwards, these latter lodes pass into the faulted granite margin on the south-eastern corner of Bodmin Moor, showing that the movement that produced these fissures occurred after the consolidation of the granite or that earlier formed fissures were extended later into the granite. To the south, in the Bere Alston neighbourhood, there are some important N.-S. lead lodes. Eastward, the E.-W. lode-trend continues across the southern part of Dartmoor and beyond into the Ashburton neighbourhood. On either side of Dartmoor there are N.-S. lead lodes, while around its northern edge there are copper deposits, generally parallel with its margin, though these are, in some cases, impregnations of certain bands of country rock and not true fissure lodes.

The majority of the lodes dip at angles steeper than 20° from the vertical. There are, however, a few 'flat' lodes which dip at 45° or less, notably at Wheal Kitty, West Wheal Kitty and Polberro Mine, in the St. Agnes district and the Great Flat Lode, south of Carn Brea.

It is difficult, in most cases, to determine whether lode fissures are faults or merely fractures, since there are usually no distinctive beds or changes in texture of the rocks to serve as a clue to the amount and direction of movement of the walls. In a few cases, lodes passing from granite to killas country dislocate the contact. The best examples are in the Phoenix and Caradon mines of the Liskeard district where the throw is considerable and here, near the margin of the granite, whether the lodes underlie north or south, the hangingwall is usually killas and the footwall granite.

In the complex systems of lodes throughout the region as a whole, there is no sure criterion as to the respective ages of the lode fissures, and each group of intersections requires individual consideration. Lodes have been classified according to age, direction and dip by Carne (1822), and subsequent writers have found only small points on which to differ from his grouping (see Collins 1912, pp. 352, 378). The fissures appear to have been formed and in some cases cross-faulted before the advent of the mineralizing solutions. Some of those which cross, either in the line of strike or on the dip, and appear to heave others, may not necessarily be of later date. It is well known that a fault fissure may be deflected or refracted from its course on passing through a dyke of harder rock than the country, a phenomenon that has given rise to the supposition, in some cases erroneous, that some elvans have displaced lodes. In the same way it seems that, in a number of cases where lodes cross, an apparently heaved lode may be later than the lode that appears to do the heaving, owing to extreme refraction caused by the open fissure of the first formed. This is shown in some cases by the mineral contents, thus, an apparently unfaulted lode may contain a certain mineral suite, while another, apparently faulted by it, has a different suite which may be found to pass along the first lode between the points of junction with the dislocated parts of the second. This state of affairs occurs where Cudna Reeth and Crowns lodes cross the Hazard Lode of Botallack Mine, St. Just. The problem is, of course, open to other interpretations, thus, Collins (1912, p. 140) suggests that the sequence of events was: (1) the formation of the Cudna Reeth fissure; (2) the formation of the Hazard fissure, faulting the former and becoming mineralized; (3) reopening of and deposition of another set of minerals in the Cudna Reeth fissure and in that part of the Hazard Lode that lies between the two heaved parts of the Cudna Reeth Lode.

In the case of the north-dipping and south-dipping lodes of the Camborne neighbourhood (see (Figure 16)) a simple interpretation of the faulting leads to the conclusion that the north-dipping fissures were heaved by those dipping south, but, in this case, the area may have been the seat of large-scale and intensive shattering of the country rocks and, in point of time, the two sets of fissures may have been contemporaneous and pre-date the mineralization. It is not known whether the ridges in the underground surface of the granite in this locality are due to faulting or to an irregular form of the granite cupola.

Crosscourses

Crosscourses have a general N.-S. trend and usually consist of a belt of country, often many feet wide, in which there are a number of roughly parallel fissures. This, in places, results in the E.-W. lodes faulted by them being cut up into a number of dislocated blocks between the two parts of maximum heave. Crosscourses are more persistent in strike, more nearly vertical and traceable over longer distances than E.-W. lodes. At surface they affect the drainage of the land, often determining the course of streams, while underground they are frequently a source of water trouble in that they afford easy passage of water into working mines from abandoned ones, and even from the sea. The crosscourse fissures are generally clay filled due to comminution of country rock, but where mineralized the chief constituent is banded white quartz, in places up to 20 ft. wide, occasionally with pyrite. In a few localities they contain economic minerals. The most important part of the region for crosscourses embraces the Wadebridge district, the northern part of the St. Austell district and the northeastern part of the St. Agnes district, where the N.-S. lodes carry zinc, lead, silver, antimony, arsenic and iron ores. Other important occurrences are north of Tavistock and south of Callington where the N.-S. lodes carry lead ores with fluorspar and in the Teign valley, east of Dartmoor, where lead ores occur with barytes. Small amounts of cobalt, nickel and uranium ores have been recovered from crosscourses south of St. Austell Moor. In some areas where N.-S. lodes have been worked, E.-W. fissures have locally been termed 'crosscourses', but this is contrary to the general rule.

The crosscourses are, in nearly every case, definitely of late formation, for not only do they heave lodes of normal and taunter trend, but, where mineralized, they carry a suite of low temperature minerals. The N.-S. wolfram lode at Castle-an-Dinas Mine, north of St. Austell Moor, which is mineralized with a high temperature suite and, moreover, clearly pre-dates the local granite, is regarded as belonging to an earlier period at least than the crosscourse stage. Since crosscourses are only rarely mineralized, they have not been explored to anything like the same extent as the other lodes. Some observers assign a Tertiary age to them (see De la Beche 1839, p. 311; Collins 1897, p. 311; Hill 1901, p. 586; MacAlister 1906, p. 157), but there is no conclusive evidence for this. Since their mineral content has every appearance of belonging to the granitic episode they probably represent the final stage of that mineralization, when some of the earlier E.-W. lodes were reopened to receive their final influx of mineralizing solutions (see Finlayson 1910, p. 291). The silver ore in the crosscourse of Herland Mine of the Gwinear district, for instance, was confined to the vicinity of the intersection with an E.-W. lode, as if the solutions that deposited the ore in the crosscourse had found their way into it through the original E.-W. lode in depth. That all crosscourses may not be of precisely the same age is supported by the fact that when two intersect, one may heave the other. Clay-filled crosscourses are referred to as 'fluccans', and the clayey matter in them, as well as that found occasionally near the walls of E.-W. lodes, is termed 'fluccan'. Owing to their usually soft or shattered nature, crosscuts have often been driven in them and, for this reason they have been, in some cases, known as 'guides', a term not used to-day. In St. Ives district crosscourses are known as 'trawns', and in St. Agnes district, where they are often quartz-filled, as 'gossans' though this word is strictly applicable only to the weathered parts of lodes.

Slides

This name is used to refer to faults, formed subsequent to mineralization, that trend more or less E.-W. and have a flat underlie. They are usually under 3 ft. in width and are filled with clay, and heave both the lodes and the crosscourses.

Mineralization

The lodes are essentially fissure veins and are frequently accompanied by an alteration and mineralization of the wall-rock, locally called 'capel'. Wall-rock mineralization is usually confined to a foot or so on either wall, but, in places, the capel may be wider and, particularly in granite country, may form irregular masses of ore known as 'carbonas', as in the St. Ives district. Occasionally ores occur in nearly horizontal deposits known as 'floors' which are usually arranged in tiers, due to mineralization along horizontal tear-faults joining two vertical veins or to mineralization along horizontal joint planes in granite country. Other than the capel alongside lodes, killas country is rarely mineralized, but in the Okehampton district some altered beds of garnet rock are impregnated with sulphide ores; there are sulphide impregnations at Virtuous Lady Mine, south of Tavistock, and an unusual example of cassiterite in killas at Parka Mine, west of St. Austell. Stockworks, consisting of numerous interlaced or parallel veinlets occur both in killas, as at the Mulberry and Prosper openworks of the St. Austell district, and in granite, as at Cligga Head, St. Agnes, and Hemerdon Wolfram Mine, north-east of Plympton. In some cases lodes pass through elvan dykes as a number of small parallel cracks, the mineralization of which gives rise to a small stockworks within the elvan as at Parbola Mine in the Gwinear district.

The character of the country rock has little effect upon the deposition of tin and copper ores and their associated minerals, except that harder rocks afford cleaner and more open fissures than the softer ones. Apart from the cases mentioned below, these minerals appear to favour no particular country. It has been claimed that, since some ore shoots pitch in the direction of the dip of the enclosing sedimentary rocks, they must therefore follow certain beds in preference to others. This, however, appears, in most cases, to be due to the fact that the dip of the sediments is generally away from the granite bosses, while, as shown above, the mineral zones, determined by temperature gradient, themselves pitch in this direction. While it is true that many minerals show no preference for particular rock types, the country rock has an effect in the case of tungsten ores in that whereas wolfram is the principal primary mineral of tungsten in granite and killas country, scheelite occurs where lodes pass through greenstone country, as at Wheals Jewell and Friendship, in the Tavistock neighbourhood and Maudlin Mine in the St. Austell district. The lead lode worked at Chiverton Mine, St. Agnes district, was more productive in argillaceous than in arenaceous killas country (Figure 32), and another case of an ore being restricted to horizons bordered by beds of a certain type is that of the manganese ore which seems to be more or less confined to chert beds in the Culm Measures, as in the ground north of Callington and Tavistock. This latter ore, however, occurs as irregular ramifying strings hardly to be regarded as true lodes, and may not be attributable to the granitic mineralization, but, more probably, originated earlier from the greenstone intrusions in the Culm Measures, or else by alteration resulting in the segregation of indigenous manganese oxide deposited in the chert beds.

Lodes may be simple fissures filled with minerals, generally showing banding parallel to the walls and with vughs at the centre as in the case of North Lode, Geevor Mine, St. Just (Figure 9) or of a composite nature, either due to brecciation of the country rock, giving rise to a lode consisting of fragments of impregnated country rock interlaced by numerous small veins, or to a series of infillings due to successive reopening of the fissure as in the case of the lodes of South Crofty Mine, Camborne, shown in (Figure 18) (Figure19) (Figure 20), (Figure 21) (Figure 22). In addition to the lode structures figured in this memoir, many have been figured and described by earlier authors whose works are quoted in appropriate places in Chapters IV and V.

Since mine workings in depth are usually abandoned because of the falling off of values or because of insufficient values to offset the increasing costs of hoisting and pumping that arise with increasing depth, the nature of the changes in lode character in depth are not easily studied. Lodes occurring near the centre of granite masses represent the lowest parts of mineralized fissures because here the tin zone attained its highest elevation relative to present-day sea level on account of the disposition of the temperature zones within the granite (see (Figure 1)). The lodes in these places are seldom wider than a foot or so and usually consist of groups of small, more or less parallel, veins in greisenized granite as at Ding Dong Mine, St. Just, and elsewhere. This suggests that in depth the lodes split or branch.

Alteration of the country rock alongside the lodes may take various forms; for example, granite, by alteration of the feldspars and micas may be tourmalinized, or chloritized, or greisenized, and each type of alteration is usually accompanied by some silicification and some impregnation by cassiterite or chalcopyrite; the latter usually fills cavities or cracks but the former is deposited within the pseudomorphs of the feldspars and micas. In rare cases, complete pseudomorphs of cassiterite after feldspar occur, as at Wheal Coates, St. Agnes, but generally the impregnation cassiterite is variable in amount and of fine grain, being sometimes difficult to recover economically by gravity mill methods. Carbonas, though sometimes large in extent are seldom of high grade ores. Beyond the altered granite walls adjoining lodes, the country is often irregularly kaolinized. Modifications of killas adjacent to lodes are less frequent, but the alteration takes the form of silicification or tourmalinization—the latter following the more argillaceous laminae—along with sporadic impregnations of cassiterite, either disseminated or filling cracks and partings. In greenstone the hydrothermal alteration results in garnetization and the formation of other lime-bearing minerals such as axinite, fluorspar and epidote.

When lode material is examined either macroscopically or microscopically, the minerals are seen to have been deposited in a certain order. The sequence may vary from lode to lode, but it has been established that the order of deposition is similar to the order of arrangement of the depth zones (see Flett 1903; Cronshaw 1912). Wolfram, cassiterite and mispickel precede chalcopyrite, which is followed by blende and galena, while the latest metallic minerals are siderite, haematite and pyrite. The gangue minerals show a similar correspondence with the depth zones, being formed in the order: tourmaline, feldspar, chlorite, fluorspar, barite and carbonates; quartz persists through nearly all the stages of mineral deposition. Fluorspar, though occasionally present as a primary constituent along with cassiterite and chalcopyrite, occurs also as veins filling the last opening of the fissure, its usual position in the depth zones being in the upper part of the copper zone and in the lead-zinc zone (see Dewey 1925).

Secondary alteration of the lode minerals above the ground-water level has been complete throughout the region. The depths to which it takes place are variable owing to irregularities in the depth to water table. While secondary minerals resulting from the oxidation of unstable primary minerals are still to be found, the secondary zones, both of leaching and of enrichment, have long since been worked away. Perhaps the results of weathering can now best be studied only in the iron lodes, which have not been so extensively and thoroughly worked in depth as the others. Here, siderite, which is usually encountered in the bottoms of iron ore workings, is replaced in the weathered zone by limonite and manganese oxides.

The alluvial deposits of the region were not of high grade, the tin content of the 'pay-dirt' being usually much less than that of the poorest of the worked lodes. This, combined with the sporadic and irregular occurrence, characteristic of alluvials, renders any that may remain unprofitable for exploitation today, except, perhaps in a small way. Alluvial tinstone is, however, freer from objectionable gangue minerals than that of lode ores.

References

CARNE, J. 1822. On the Relative Age of the Veins of Cornwall. Trans. Roy. Geol. Soc. Corn., vol. ii, pp. 49–128.

COLLINS, J. H. 1871. Handbook to the Mineralogy of Cornwall and Devon. Truro.

COLLINS, J. H. 1897. On the origin and development of ore deposits in the West of England. Chapter V.

COLLINS, J. H. 1912. Observations on the West of England Mining Region. Trans. Roy. Geol. Soc. Corn., vol. xiv.

CRONSHAW, H. B. 1912. Structure and Genesis of some Tin-lodes occurring in the Camborne District of West Cornwall. Trans. Inst. Min. Met., vol. xxx, pp. 408–67.

DE LA BECHE, H. T. 1839. Report on the Geology of Cornwall, Devon and West Somerset. Mem. Geol. Surv.

DEWEY, H. 1925. The Mineral Zones of Cornwall. Proc. Geol. Assoc., vol. xxxvi, pp. 107–35.

DINES, H. G. 1934. The Lateral Extent of the Ore-shoots in the Primary Depth Zones of Cornwall. Trans. Roy. Geol. Soc. Corn., vol. xvi, pp. 279–96.

EMMONS, W. H. 1934. On the Origin of Certain Systems of Ore-bearing Fractures. Amer. Inst. Min. Met. Eng., No. 561.

FINLAYSON, A. M. 1910. The Metallogeny of the British Isles. Quart. Journ. Geol. Soc., vol. lxvi, pp. 281–98.

FLETT, J. S. 1902. Note on some Brecciated Stanniferous Veinstones from Cornwall. Sum. Prog. Geol. Surv. for 1901, pp. 154–9.

HILL, J. B. 1901. The Plutonic and Other Intrusive Rocks of West Cornwall in relation to the Mineral Ores. Trans. Roy. Geol. Soc. Corn., vol. xii, pp. 546–615.

MACALISTER, D. A. 1906. In The Geology of Falmouth and Truro and of the Mining District of Camborne and Redruth (Sheet 352). Mem. Geol. Surv.

MACALISTER, D. A. 1908. Geological Aspects of the Lodes of Cornwall. Econ. Geol., vol. iii, pp. 363–80.

SYMONS, B. 1884. Sketch of the Geology of Cornwall, with special reference to its Mineral Wealth. In A Gazetteer of Cornwall by R. Symons, Penzance.

Chapter 3 Economic minerals

THE principal economic minerals of south-west England are, of course, tin and copper ores and considerable amounts of ores of lead, zinc, silver, arsenic, antimony, sulphur, iron and manganese have also been raised. Uranium was a product of some importance during the latter half of the last century and wolfram has been during the present, but neither of these ores has been produced in large amounts. Minerals of bismuth, cobalt, nickel, etc. have only been obtained in insignificant quantities and also very small but unknown amounts of gold. The region, as a whole, has not been an important producer of spar minerals, but fluorspar and barite have been raised in economic amounts.

Tin

The chief ore-mineral is the oxide, cassiterite, commonly known as 'tinstone'. Varieties known as 'wood' tin and 'toad's-eye' tin are comparatively rare. The sulphide of tin, copper and iron, known as stannite, also occurs in some lodes but is not of economic importance. In hand specimens of ore, cassiterite may occasionally be seen as black crystals with adamantine lustre. Granular or massive cassiterite is also to be found, but more often the mineral is disseminated through the gangue in a fine state, not easily detected by the unaided eye. The gangue minerals associated with cassiterite are tourmaline, chlorite and quartz. Wolfram and mispickel are often present in the higher parts of the tin zone.

Separation of cassiterite from the ore is affected by fine grinding or crushing and gravity separation, wolfram and iron-oxides of similar specific gravity to cassiterite being removed magnetically and residues of sulphides by calcining or flotation. Some impurities cannot be separated, however, and the resulting concentrates, known as 'black tin', generally contain under 65 per cent of metallic tin. The smelted metal is referred to as 'white tin'.

The date of the discovery of tin in the west of England is not known, but it was being produced about 2,500 years ago. The early workings were probably in alluvial deposits but there seems little doubt that workings along the lode outcrops or gossans were in progress in very distant times. The exposures of lodes cropping out in cliff faces as at St. Just and St. Agnes, led to exploitation by adit levels, and deep mining inland above ground-water level seems to have commenced about the 15th century.

Collins (1892) estimates that the annual yield of metallic tin during the early part of the 13th century was about 120 tons, of which Devon produced about three-quarters, and in the early part of the 14th century nearly 600 tons. The increase was largely from Cornwall, for Devon then contributed only about one third. There seems to have been a fall in output towards the end of the 14th century and, as far as can be estimated, only some 450 tons were produced yearly, Devon again supplying about one third. From the latter part of the15th century to the beginning of the 17th, the annual yield seems to have fluctuated around 450 tons, the Devon output gradually declining until in the early 1600's it was only about one sixth of the total. Hunt (1884, p. 823), however, gives a much higher yield for this period. In the 17th century there was a rise of over 1,000 tons. This may have been partly due to a demand for tin for the manufacture of pewter ware and partly to the introduction of explosives for blasting. From this time, records are more certain and show an increase in output which reached 3,000 tons a year by the middle of the 18th century, Devon producing some 50 tons. Following a fall to about 2,500 tons in the early part of the 19th century, there was a steady increase due in some measure to the development of steam pumping engines which permitted mining at deeper levels. In 1850, the output totalled 6,000 tons, and by 1870, when the tin-plate industry reached a peak, 10,000 tons was attained which was about half the world yields. A decline then set in and by 1890 the figure had fallen to 8,000 tons which was followed by a sudden drop to 4,000 tons in 1900 (see MacAlister 1906, p. 307) and again to only a few hundred tons in 1920. After the post 1914–18 war slump there was a recovery to 2,000 tons, a figure that has been maintained, more or less, since (see (Figure 5)).

The total yield from Cornwall and Devon is estimated to be considerably over 2,000,000 tons of metallic tin. The relative importance of the mining areas around the five chief granite masses in recent years is shown by the proportion of the total output produced: Land's End 21, Carnmenellis 67, St. Austell Moor 7, Bodmin Moor 3 and Dartmoor 2 per cent (see also Davies 1919; and table on p. 32). On the Scilly Isles there are a few ancient surface workings, but the amount of tin obtained there is not known.

Tin is used chiefly in the manufacture of tin-plate, and in smaller amounts in alloys. With lead, in various proportions, it forms pewter or solder, and, with the addition of bismuth or cadmium, 'fusible' metals which melt below the boiling point of water. Its addition to copper results in alloys harder and tougher than that metal such as bronze, gun metal, bell metal, etc.; the further addition of phosphorus increases the resistance to wear and gives phosphor bronze, used for machine bearings. Tin is only used in the pure metallic state as tinfoil wrappings and for tubes used in certain chemical apparatus.

Copper

The chief ore is the primary copper-iron sulphide, chalcopyrite, once popularly known as 'yellow copper ore', though the rich secondary sulphide, chalcocite, referred to by earlier writers as 'grey copper ore' has been found in large quantities and has constituted the principal ore at some mines. This mineral, owing to its common occurrence in the locality, has also been given the name 'Redruthite'. The carbonates, azurite and malachite, and the oxides, melaconite or black copper ore and cuprite or red copper ore, have also been found in abundance in the zone of secondary alteration. Many other products of weathering occur and are referred to in the details of mines in Chapters IV and V. Perhaps the most prolific mines, in so far as the variety of minerals is concerned, are those of the St. Just district, especially Botallack, which attracted the attention of mineralogists in the early part of the 19th century. The gangue minerals associated with copper ores are mainly quartz, sometimes with chlorite and fluorspar, but the latter is usually a late arrival occupying cracks within the lodes. Pyrite is also present, often in large proportion, and its decomposition to limonite in the gossans results in the formation of 'earthy brown iron ore' so frequently referred to by early writers. Mispickel is a common associate of chalcopyrite. Since the copper deposits were exhausted during the 19th century no ore-bodies remain exposed for study to-day.

In dressing the ores, the sulphides were cobbed and hand-picked, large masses being separated from disseminated lode-stuff and waste. The first was ready for the smelter and the second, after crushing and sizing, was concentrated by various methods of gravity separation. The industry had declined before flotation had been brought to perfection. The Elmore flotation plant erected at Tywarnhayle in 1906 was apparently the first of its kind in the west of England; it was intended to separate complex sulphide ores, but the project was not a success. Flotation cells are now used to separate the small amount of sulphides associated with tin ore. Precipitation on scrap-iron is used in some cases for the recovery of copper from low-grade tailings and dump material.

The copper industry did not develop so early as that of tin. Bronze-age celts found in the district were probably made from local copper, but systematic mining for copper does not appear to have commenced before the 16th century and it continued only in a small way until the 18th. Copper mining then began in earnest and for the first half of the 19th century more than 40 per cent of the world's output was obtained from Cornwall and Devon, and Britain was the largest producing country. The peak was reached about 1860 when something like 15,500 tons of metal were produced annually (see (Figure 5)). A rapid decline then set in and by the end of the century the output was negligible. The mines of Devon seem to have lagged behind those of Cornwall and it was not until 1855 that the former county contributed one third of the total amount; by 1870 it was producing nearly half, and in 1886 slightly exceeded the Cornish yield. After that date the two counties produced roughly equal amounts yearly. To-day, the recovery from the whole region, as a by-product from the tin mines, is usually less than 50 tons of copper metal a year. The metal content of the copper concentrates has varied considerably at each mine, and has shown a general tendency to decrease, probably due to the working away of richer secondary ores above the primary zone. Collins (1895, p. 299) shows that, on the average, at the end of the 18th century it was 12 per cent and during the 19th century it fell from 9.6 to 6.5 per cent. Some of the mines have produced concentrates with considerably less than this latter figure, while, on the other hand, a few others have reached as much as 20 per cent.

Copper was one of the earliest metals known to man, and has always been an important commodity. Next to iron, it is the most valuable of the base metals. The greater part of the copper used to-day is taken up by the electrical industry, and much of the remainder is used in the alloys brass and bronze. In most of its uses copper has no substitute, though in some it is being replaced by aluminium.

Arsenic

Arsenic is a constituent of many sulphide minerals, but the only one of economic value in the west of England is mispickel or arsenopyrite, the sulpharsenide of iron. It is locally known as 'mundic', a term formerly used to include both pyrite and mispickel, the latter being distinguished as white mundic Mispickel contains up to 46 per cent of arsenic, but when the content falls below 20 per cent it is not of sufficient value to work. The mineral is a primary sulphide and occurs in the sulphide zone mainly associated with chalcopyrite and pyrite, but occasionally with cassiterite in the upper part of the tin zone. Though generally found in lodes of E.-W. trend it is known to occur in crosscourses as at Treore, near Wadebridge, and is present in the sulphide impregnation deposits of Virtuous Lady, Tavistock, and in the Okehampton district. It is fairly stable, and is not affected by weathering to the same extent as iron and copper sulphides though in some gossans the secondary mineral scorodite, hydrated ferrous arsenate, is to be found.

Mispickel is generally worked as a by-product to copper and tin, and though in a few mines it has been the predominant mineral, as at Calstock and Danescombe mines, none has been worked solely as an arsenic concern except during the 1914–18 war when some old copper mines were reopened for the purpose.

Though often sold in its natural state as 'arsenical pyrites', the ore is usually treated. After crushing and concentration it is calcined and the vapours passed through chambers or flues where arsenic oxide settles as a grey powder known as arsenic soot or 'crude arsenic'. The crude arsenic contains from 80 to over 90 per cent of As₂O₃ along with certain impurities such as carbon and sulphur compounds. It is often sold in this form but at South Crofty Mine is refined by further calcining and gives, in the collection chambers, white or translucent crystals that are subsequently ground to a fine white powder, known as 'refined arsenic' or 'white arsenic', containing 99.5 per cent As₂O₃. The refining process does not greatly increase the As₂O₃ content, and, therefore, in order to economize space, the outputs of these products from the various mines described in Chapters IV and V are usually referred to simply as 'arsenic'.

In the early part of the 19th century, mispickel was regarded as an undesirable constituent of the lode-stuff and often large masses were left standing in the stopes on copper lodes. With a demand for arsenic in the chemical industry attention was given to arsenic deposits and by 1870 a number of mines were raising mispickel as a by-product and several had installed their own flues for the collection of the crude oxide. It is stated that at this time about half a dozen mines in the Callington and Tavistock district, including Devon Great Consols, Okel Tor, Gawton and Friendship, were producing half the world's output (see Toll 1938). Between 1902 and 1907, a fall in price caused a slump in production, but with a subsequent demand, output again increased and continued during the 1914–18 war when the price was high. At this period several old mines in which mispickel was known to have been left were reopened, and dumps were worked over. Production continued until the post-war slump of 1920–21, but in 1923 a further demand for arsenic for insecticide in American cotton-fields again raised price and output. This boom, however, ceased in 1925, since when the arsenic industry has only been maintained as a subsidiary of tin works.

Cornwall and Devon are the only counties in Great Britain that have yielded arsenic in quantity. From 1870 to 1902, the annual output varied between 4,000 and 8,000 tons of arsenic oxide. In 1903–4, only a little over 900 tons was produced each year, but the output increased gradually to over 2,000 tons for the years between 1914 and 1926. Since then there has been a steady decline. From 1931 to 1936, the output stood between 100 and 200 tons and has now fallen to less than 100 tons.

Apart from its use in insecticides, arsenic is used in the glass industry for decolourizing, as a preservative to prevent decay in vegetable matter, in medicine, and for a number of other uses.

Sulphur

Pyrite, or iron pyrites, the sulphide of iron, was once locally known as 'mundic', a term which included also mispickel, but to-day is restricted, by some at least, to mispickel only. The mineral is widely distributed throughout the lodes of the region and is usually associated with the other sulphide ores. It also occurs as a late arrival, being found as crystals encrusting the insides of vughs in lodes, and, with white quartz, in the crosscourses. In the gossan or weathered parts of lodes it is readily decomposed and gives rise to earthy brown iron ore. In some mine workings its decomposition has resulted in the generation of heat, as at Duchy Peru Mine on the Perran Iron Lode. Pyrite has been recovered in economic amounts from mines in all the districts except those of St. Just, in the west, and Dartmoor and Okehampton, in the east. It is usually a by-product, though some small mines have produced larger tonnages of this than of other minerals. The chief producing districts have been those of Callington and Camborne. The largest outputs have come from Wheal Jane and West Wheal Jane, east of St. Day, which have, between them, raised over 60,000 tons. The next mines in the importance of their pyrite production are Okel Tor Mine, Devon Great Consols, Prince of Wales Mine and Wheal Friendship, all in the Callington and Tavistock district.

Pyrite, which may contain up to 53 per cent of sulphur, is used in the manufacture of sulphuric acid; the demand for it seems to have commenced shortly before 1850 and the tonnages produced are mainly confined to the succeeding 25 years, during which some 150,000 tons are recorded from about 70 different mines. The presence of arsenic, which is common in the pyrite of this region, renders it unsuitable for the manufacture of sulphuric acid which is to be used in sugar refining.

Tungsten

Wolfram, the chief tungsten mineral, is an isomorphous mixture of manganese and iron tungstates. In some cases the manganese content may be sufficiently low to bring the mineral into the range of iron tungstates known as ferberite. Scheelite, or calcium tungstate, also occurs, under certain conditions, in sufficient quantities to be recovered as an ore. Wolfram is often present in the lodes as large sheaf-like aggregates of black crystals up to 2 or 3 inches long, and is nearly always coarse-grained. It is a common associate of cassiterite, generally in the upper part of the tin zone, and is often found intimately inter-grown with mispickel. Unlike cassiterite, it is rarely found in the impregnated country adjoining the lodes, and, though normally a constituent of E.N.E. lodes, is present in a N.-S. lode as at Castle-an-Dinas Mine in the St. Austell district. Scattered throughout the region are occurrences of white quartz and wolfram usually occupying irregular fissures or in lens-like masses as for example at Hawk's Wood, Wheal Vincent and elsewhere on the east side of Bodmin Moor. Deposits of a similar nature fill horizontal fissures in granite country around Robinson's Shaft of South Crofty Mine, Camborne. These latter pre-date the nearly vertical tin lodes, and there is other evidence, though scanty, suggesting that these occurrences may perhaps represent an earlier stage of mineralization than the tin lodes. Complex Lode of South Crofty and the Castle-an-Dinas lode may belong to this group.

Wolfram was once an undesirable gangue mineral in tin lodes, since, owing to its similar specific gravity it was impossible to separate it from cassiterite and penalties were imposed by smelters on a content of more than 3 per cent of the mineral in the concentrates. About the middle of the 19th century the Oxland process, in which wolfram was converted to sodium tungstate, was introduced to effect separation, but in recent years the slight magnetic quality of wolfram has been utilized in magnetic separation. Owing to its friable nature much is carried away in slimes on milling and, where fine crushing has to be resorted to, little more than 50 per cent of the wolfram content of the ore is recoverable. The variable amount of tungsten in the ore necessitates concentrates being valued at so much per 'unit', a unit being 1 per cent of WO₃ present. Mill concentrates normally contain from 60 to 70 per cent of WO₃.

Production in the west of England has never been large in comparison with other tungsten producing countries. The first yields were recorded in 1858, but its recovery was not seriously commenced until around the beginning of the 20th century. Between 1910 and 1919, the average annual yield for the region was about 250 tons, but from 1920 to 1934 was only 41 tons, fluctuating between 1 ton and a little over 100 tons, according to demand. Since 1934 there has been a fairly steady yearly yield of about 200 tons of concentrates.

Apart from its use as a metal filament for electric lamps, the main use of the metal is in alloys; it hardens steel and bronze and, with cobalt and chromium, forms an alloy unaffected by organic acids that is used for surgical instruments; it replaces platinum to some extent in electrical apparatus. Tungsten is also used as a mordant in dyeing and printing, as a pigment and, in the form of sodium tungstate, for fireproofing inflammable materials.

Zinc

Blende, or zinc sulphide, sometimes known as sphalerite, and popularly as 'black jack', is the chief ore. The carbonate, calamine, and the silicate, hemimorphite, are also found but not in large quantities. Blende occurs mainly in N.-S. crosscourses, in association with galena, but workable quantities have also been met with in copper lodes, as in the Seton and Roskear mines, north of Camborne, and in some of the St. Day mines; its position here is near the top of copper lodes or at lower levels where it fills cracks formed in the original lode-stuff. Thick masses of blende coat the walls of the Perran Iron Lode where it appears to have been deposited before the iron ore; the mines on this lode have been amongst the largest producers of zinc. The concentrates produced in the region may contain 20 to 45 per cent of metal.

Until the early part of the 19th century, blende was frequently discarded or cast upon the dumps, which have subsequently been worked over for its recovery. The maximum amounts were produced between 1850 and 1885, and production continued in a small way, chiefly from dumps, until 1911. The most important district was that of St. Agnes, which raised 75 per cent of the total of 85,000 tons, produced in the region. The Gwinear, Camborne, St. Austell, Callington and Dartmoor districts have produced small amounts.

Metallic zinc contains certain metals as impurities, chiefly lead, cadmium and iron, and in the market is graded according to purity. Commercial standard zinc, or spelter, contains upwards of 98 per cent of zinc and not more than 1.5 per cent of lead. For special purposes it is refined by distillation. The principal use of the metal is for galvanizing iron, but a considerable amount is used as plain or perforated zinc sheets. It is alloyed with copper for brass. Zinc white, the oxide, is used as a pigment and the sulphate and chloride are used in the textile industry.

Lead

Galena, or lead sulphide, is the principal ore; its alteration products, the carbonate, cerussite, and the chlorophosphate, pyromorphite, occur in the weathered parts of lodes. The ore lies chiefly in N.-S. crosscourses and is often accompanied by blende. It also occurs in lodes of E.-W. trend, as at the Chiverton mines, St. Agnes. In places, therefore, it may be found in the higher parts of copper lodes, though its occurrence here is not so frequent as that of blende, and in one instance, Budnick Consols, St. Agnes, it is found in association with cassiterite, though probably of a later period of deposition than that mineral. Galena often carries some silver, as sulphide, and has constituted the most important source of silver from the region. The silver content varies from place to place within the lodes, and is said to have been generally higher in the upper levels. The average silver content of all the ore produced was about 40 oz. to the ton of lead, but as much as 170 oz. has been recorded, though in some cases the high figures may have been due to bad lead smelting. The weathering of galena in the gossans has resulted in the formation of secondary silver ores, including native silver. Lead lodes have often been found to deteriorate and eventually die out in depth, but in a few instances lead has given place to copper in depth. The lead workings in the St. Agnes district do not much exceed 140 fms. in depth, though Wheal Mary Ann, near Liskeard, has reached 300 fms. and copper ores were not encountered.

The gangue minerals of the lead zone vary from place to place but quartz is always present. In the St. Agnes district, fluorspar and barite occur but are rare. In the Liskeard district, fluorspar has not been found at Herodsfoot mine, though dolomite is recorded, but in the nearby mines of Wrey, Ludcott and Trelawny, purple, green and colourless fluorspar is present as well as calcite, crystals of barite and siderite, the last filling vughs in the lode. In Bere Old Mines of the Callington district fluorspar has been encountered in sufficient quantities to be worked, and some siderite occurs. In the Teign Valley Mines fluorspar is present towards the south and barite to the north.

The lead workings date back many centuries. The Bere Old Mines were active in the 13th, and the mines of the Mount's Bay district in the 16th century; some of these were worked chiefly for their silver content. The period of maximum output was in the earlier half of the 19th century. Between 1845 and 1850 the annual output averaged over 10,000 tons of ore. From then onwards there was a gradual decline, the yield falling below 1,000 tons in 1878 and ceasing in 1887. During this whole period (1845 to 1886) 251,813 tons of lead ore containing 5,721,153 oz. of silver were raised in Cornwall and 59,236 tons of lead with 1,341,597 oz. of silver from Devon. Since that time, apart from a few tons raised in 1892, 1896 and 1903, there has only been about 725 tons produced during the period 1905 to 1908, some of which has come from dumps. The metallic content of lead ore concentrates has ranged between 60 and 75 per cent. Lead has been produced in nearly all the districts of the region, but chiefly in those of St. Agnes, Liskeard and Callington.

Lead has provided pipes and sheets since Roman times and is still largely used for these purposes. Large amounts are used for sheathing electric cables and for accumulator plates. With tin and antimony it is alloyed for white metal for anti-friction bearings, for printers' type metal, for pewter and for soft solders, and with tin and bismuth for fusible metals. It is also used as a constituent of bronzes. White lead, the carbonate, and red lead, the oxide, are used as pigments and many other compounds are utilized in industry.

Silver

Galena may contain varying amounts of silver, ranging from nothing up to over 170 oz. per ton of lead, and has been the chief source of silver from the west of England. The variation is rapid and the silver content often changes considerably from place to place in a lode. Small amounts have been recovered from copper sulphides and also from true silver ores which include native silver, often present in the gossans of lead lodes, the sulphide, argentite, the silver-antimony sulphides, stephanite and pyrargyrite, and the chloride, cerargyrite. These ores are usually found in crosscourses, frequently associated with ores of bismuth, cobalt, nickel and antimony.

The chief silver districts have been St. Agnes, St. Austell, Liskeard and Callington, where the bulk of the production has come from lead ore. Silver has been reclaimed from copper ores at Levant, in St. Just, at Credis and Tregonna in the Wadebridge district, and at Prince of Wales Mine, Callington. The best known silver-ore occurrences are at Rosewarne and Herland, near Gwinear, and Wheal Newton, Callington.

Much silver is believed to have been obtained from the region in mediaeval times, when the lead lodes were exploited chiefly for their silver content. The first discovery of true silver ores appears to have been that at Wheal Mexico, St. Agnes, in 1788, though the deposits were not extensive. The amounts raised in recent times have been insignificant, but during the 19th century the chief producing mines were the Chivertons, Cargoll and East Wheal Rose of St. Agnes, Herodsfoot, Wrey, Ludcott, Mary Ann and Trewetha of Liskeard, and the Bere Old Mines of Callington. Perran Silver Mine, near Marazion, was active between 1903 and 1907. Collins (1904) estimates that 2,000 tons of true silver ore were raised in the 19th century.

Antimony

The minerals of this metal include the sulphide, antimonite, the sulphide of lead and antimony, jamesonite, and of copper and antimony, bournonite. They occur in N.-S. crosscourses, usually scattered on the fringe of the mineralized areas beyond the lead and zinc deposits, and are, in places, associated with galena, occasionally in workable quantities. Some of the ores carry gold. The occurrences are small, consisting chiefly of thin strings or veins of mineral running through crushed killas country, with, here and there, small bunches of ore. The sites of some of the workings on these small ore-bodies cannot now be traced.

The chief locality is the Wadebridge district, and there are two mines about 6 miles S.S.E. of Callington. The recorded output of the region does not much exceed 300 tons, but may have been considerably more since the ore was chiefly exploited in the 18th century, or earlier. From 1915 to 1918 about 10 tons of ore was produced from lead mines.

Antimony is used in alloys for anti-friction metal, for printers' type metal and for hardening lead. It is also used as a colouring agent for glass, pottery (lustre-ware), pigments, and dyes, in medicines, as a mordant and for eye-shade cosmetic.

Iron

The chief sources of iron in the region are the lodes of Exmoor, and those in and around the St. Austell granite mass and extending north into the Wade-bridge district and west into the St. Agnes district. With the exception of the lodes of Exmoor and the Perran Iron Lode, St. Agnes, which course E.S.E., these occur mainly in N.-S. crosscourses, and, though usually in killas country, are to be found also near the centre of the granite as at Ruby Mine, Wheal Edith and Treffry Consols. The workable ore-bodies are often over 10 ft. wide and widths between 20 and 30 ft. are not uncommon. Very few have been worked below the water-table and only the weathered zones of the lodes are, therefore, exposed. There is little doubt that the primary mineral of the Perran Lode was the carbonate known as siderite, chalybite, or spathic ore, for this has been proved in the bottoms of some of the workings and the hydrated oxide, limonite, which has constituted the chief ore, contains some unweathered cores of this mineral. Siderite has also been raised from a number of other iron lodes, and it was probably the original constituent of most of the lodes of the St. Austell and the North Devon and West Somerset districts, although a few, which are usually associated with greenstone, contain magnetite as at South Terms Mine and at Tower Consols, near Roche. The ore obtained from the iron lodes has often been referred to as haematite, but it is chiefly limonite or brown haematite; gothite is also occasionally present. In the Dartmoor granite there are a number of lodes carrying a variety of specular ore, known as micaceous haematite either as a main constituent, as at Great Rock Mine, or in association with cassiterite, as at Birch Tor. Deposits of magnetite in killas at Haytor and Smallacombe, east of Dartmoor, appear to be bedded ores, and these and other deposits referred to as lodes, as at South Terras Mine, are associated with a mineral assemblage that suggests hydrothermal action by basic igneous rocks or green-stones. In addition to the above-mentioned iron deposits, a number of mines have sold iron ore from the gossans of sulphide lodes, notably at Wheal Jane near St. Day.

The iron lodes of Cornwall, though referred to by writers as far back as the 17th century, were not worked until the early part of the 19th. Owing to the low prices commanded by the ore and the distance of the deposits from furnaces, combined with the difficulty of working vertical ore bodies, as compared with the large replacement deposits of the north of England and the bedded ores of the Midlands, none of the workings on the lodes has been carried far below adit level. The output of ore between 1855 and 1865 fluctuated between 24,000 and 87,000 tons per annum. In 1867 it had fallen to 37,000 tons, but by 1875 had risen to 87,000 tons, after which date the yield declined. Since 1884, about 6,500 tons in all has been raised. The metallic content of the ores is given under the description of the mines; it ranges between 48 and 65 per cent.

In addition to use in the iron and steel industry (the spathic ore, which often contains some manganese, being particularly suitable for the latter) some of the ore raised, especially that from the gossans, has been used for pigments. The micaceous haematite of Dartmoor is still used for making a rust-resisting paint of metallic-grey colour; its original use was as pounce.

Manganese

The manganese ores of the west of England occur chiefly in the cherty beds of the Culm Measures, where they consist of irregular, ramifying or roughly lens-shaped masses of the black oxide pyrolusite or wad, and occasionally of the silicate, rhodonite. The principal area for these ores is the northern part of the Callington and Tavistock district. In addition to these deposits there are iron lodes in the area north of St. Austell Moor in which manganese occurs as oxide and silicate, probably resulting from the alteration of siderite.

There are no large ore bodies in the region. The deposits in the Culm chert beds are notoriously patchy and irregular and have produced only a few hundred tons each. The chief period of working appears to have been about the middle of the 19th century, but Chillaton and Hogstor Mine continued active until 1907, producing altogether about 50,000 tons. The lodes north of St. Austell Moor are essentially iron lodes with only small amounts of manganese ore; they have not been extensively worked.

The chief use of manganese is as an alloy with steel and with bronze. The former requires an ore containing at least 40 per cent of metallic manganese, while for the latter only ores with a low iron content are suitable. The oxide is used as an oxidizing agent in certain chemical processes, including the decolouring of glass. Manganese compounds are used as brown, green and violet colouring agents in glass, pottery dyes and paints.

Rarer minerals

Uranium, in the form of the oxide pitchblende and its alteration products autunite (calcium uranite) and torbernite (copper uranite), known as the 'uranites', as well as zippeite, a hydrated uranium sulphate, are found in tracesin most parts of the region (see Dines 1930, p. 213). They occur in association with low-temperature minerals such as those of iron, cobalt, nickel, bismuth and occasionally with lead and zinc ores, and are, therefore, found mainly in crosscourses, but are also present in some lodes of normal trend. The secondary minerals are also found lining joints in granite. In the early part of the 19th century, pitchblende, which occurs usually in the massive form, was found to be affecting adversely the quality of copper concentrates at Wheal Trenwith, and much ore was subsequently discarded.

The earliest use of uranium was to impart a yellowish-green colour to glass, and most of the outputs from 1845 onwards were sold for this purpose. Since the discovery of the radioactive properties of radium, the uranium minerals have been much sought for in mines of the region and a number of occurrences have been found, though most turned out to be of no economic value. The chief producers were Wheal Trenwith, St. Ives, and South Terras Mine, St. Austell. Small outputs, measurable in hundredweights rather than tons, have been recorded at three other mines, Wheal Owles at St. Just, East Pool Mine near Camborne, and St. Austell Consols. The total output of uranium ore for the region is not known, but probably does not much exceed 2,000 tons.

Bismuth, chiefly as small flakes of the native metal, and also as the sulphide, bismuthine, is frequently found in the region. Native bismuth occurs in crosscourses, probably as a weathering product of the sulphide. It is also found in lodes of normal trend, but its mode of occurrence is not known since it is usually detected as small particles in sand table and jig concentrates. The sulphide occurs in lodes, often lining vughs as at Fowey Consols, St. Austell, but is more frequent in crosscourses, and is usually associated with cobalt and nickel ores and occasionally with silver and uranium ores. Only four mines are known to have recovered bismuth ore and only one of these, East Pool Mine, Camborne, has a recorded output, namely of one ton of mixed bismuth, cobalt and nickel ore.

Cobaltoccurs as the arsenides, smaltite and erythrine, and as the sulpharsenide, cobaltite, as well as in oxide form, and is found mainly in crosscourses associated with nickel and bismuth ores. Though the minerals have been recorded at about a dozen mines, only about six have raised the ore. The outputs are known only in a few cases. Dolcoath Mine is believed to have produced a few tons, and East Pool Mine has a recorded output of 4L tons. In the St. Austell district, Dowgas Mine has yielded 4 tons and St. Austell Consols about 250 tons of mixed cobalt and nickel ores.

Nickel, which generally occurs in close association with cobalt, is present in the form of the arsenides, kupfernickel and chloanthite, and the sulphide, millerite. The chief producers have been St. Austell Consols with 61 tons of nickel ore in addition to the mixed cobalt and nickel ores mentioned above, Fowey Consols with 8 tons and East Pool Mine with 4 tons.

Molybdenumoccurring as the sulphide, molybdenite, has been found in a few localities but is only known to have been recovered at Drakewalls (Drake Walls) near Callington, though the amount obtained is not recorded.

Goldin small grains has been recovered in practically every alluvial tin deposit in Cornwall and Devon, and one nugget from the Carnon Valley weighed 1 oz. 18 dwt. 8.6 grains (Collins 1904, pp. 103, 106). The source was probably the gossans of copper lodes, for many fragments found were attached to vein quartz, and assays of chalcopyrite and pyrite from copper lodes often show a gold content. Levant Mine, St. Just, produced from its copper ores 4 oz. of gold, as well as some silver. Gold is present in the antimony sulphides of the Wadebridge district, notably at Treore Mine, and has been found native in adjacent alluvials; it has also been recorded in crosscourses at Wheal Sparnon, Redruth, in Woolf's Crosscourse in the Mount's Bay district and at Bampfylde and Britannia mines in North Devon. The metal seems never to have been systematically recovered. Picking of gold from the alluvial tin during concentration was apparently, at one time, considered the privilege of the 'streamers'. It is doubtful whether the sulphide ores, other than those of antimony, were assayed for their gold content before sale to the smelters.

Spar minerals

Barite, or barium sulphate, is not widely distributed in the west of England. It occurs as an occasional gangue mineral in small groups of crystals in some lodes, especially those of lead and zinc, but is only known in workable quantities in the N.-S. lodes of the Teign Valley in the extreme east of the region where it occurs in massive banded form in lodes up to many feet wide.

The quality of the mineral varies, not so much according to its purity as to its colour, white being more in demand than pink or iron-stained varieties. Ground to a fine powder it has many uses including the manufacture of paints and distempers, and as a filling to give weight and body to papers and shoddy cloths, etc. It is also used in making artificial marble and as a refractory material. Barium compounds are generally made from the carbonate, witherite, of which there are no deposits in the region. The known yield of barite comes only from Bridford Mine in the Teign Valley, and the output is given under the description of that mine.

Fluorspar, or calcium fluoride, is a common spar mineral in some of the lead lodes, and may occur in workable quantity in some copper lodes where it occupies fissures formed during a late stage in the mineralization. When associated with copper ore it is usually massive and of green colour, though some rare specimens are indigo blue, but in the lead lodes it often shows crystal forms, chiefly cubes, sometimes large, and may be green, purple, brown or colourless.

Fluorspar has been exploited in copper lodes in the Camborne district, at South Crofty Mine and near St. Day at West Wheal Damsel, Poldice and other mines. The chief centre of production, however, was in the Callington district where it occurs in lead lodes of the Bere Old Mines. Trelawny, Wrey and Ludcott mines, in the Liskeard district, contain fluorspar but do not appear to have marketed any. The recorded outputs are incomplete, but the region has never been a large producer. The Camborne district is known to have yielded over 1,500 tons between 1815 and 1893, and the Bere Old Mines over 3,000 tons between 1845 and 1886. The chief uses of the mineral are as a flux in metallurgical operations, in the manufacture of enamel and glassware and of hydrofluoric acid and other chemicals.

Alteration products

Ochre, an earthy hydrated iron oxide, has been recovered at about a score of mines throughout the region; it is formed by the decomposition of pyrite and accumulates in the cavities of gossans. When washed out, either naturally or artificially, the coarse and gritty particles are removed, and a fine paste of yellow, brown or red colour suitable for pigments results. In some cases irony gossans and iron lodes have been worked especially for ochre, as at Indian Queens Mine, St. Austell, but in some cases the material collects in the bottom of adits and is periodically removed, as at the County Adit of the St. Day district which opens into the Carnon Valley at Bissoe Pool.

A number of ochre workings exploit decomposed basic or pyroclastic rocks, as at Larrick and Halwell in the Liskeard district.

The production of ochre as a by-product is usually intermittent, and tonnages are not large. The recorded output for the whole region is about 7,000 tons, though this is probably much below the true figure.

Umber is a term commercially applied to ochre of dark brown shades, but in a stricter mineralogical sense refers to a mixture of hydrated oxides of iron and manganese with some silica and alumina. It may result from the decomposition of limestone as at Devon and Cornwall Umber Works near Dartmoor. Umber is recorded as having been produced at nine of the mines

Fullers'Earth isa complex hydrated aluminium silicate, of non-plastic clay-like texture, which possesses certain properties such as the ability to absorb grease from wool or cloth and also colouring matter from liquids.

Adjacent to some lodes, the killas country beyond the capel is in places altered to a pale grey or buff, friable shale with a smooth talc-like feel. This is particularly well developed along parts of the Perran Iron Lode particularly at Treamble Mine; though not fullers' earth in a mineralogical sense, it is being sold, after special treatment, for certain fullering purposes.

China Clayor kaolin, an alteration product of the granite, is a result of pneumatolitic action upon feldspar. The china clay industry, however, being distinct from that of ore mining, does not come within the scope of the present memoir.

The following table gives the outputs of various minerals for each of the districts, expressed as a percentage of the total recorded production of the whole region. It indicates the outstanding importance of the Camborne, Redruth and St. Day district in so far as tin and copper are concerned, the importance of the Callington and Tavistock district in arsenic, pyrite and lead production and of the North Devon and West Somerset district in iron-ore output.

List of mines and minerals produced

In the following tables the ores are shown as far as possible in their order of importance at each mine, that indicated by figure 1 in the columns being the chief mineral produced, and so on. In cases where the output is large the figure is printed in heavier type. The tonnage figure above which an output qualifies for the heavier type is a purely arbitrary one and is based on the general level of outputs from west of England mines. The tonnages chosen are as follows: tin 1,000, copper 5,000, arsenic 2,000, sulphur (pyrite) 500, zinc 750, lead 500, wolfram 200, manganese 1,500, iron 2,000 and silver 100,000 oz. Minerals which are rare or which have been produced only in small amounts are given in the last column of ores by symbols thus: Gold—Au, Cobalt—Co, Nickel—Ni, Bismuth—Bi, Molybdenum—Mo, Antimony—Sb, Uranium—U, Fluorspar—F, Barytes—Ba, Ochre—Oc, Umber—Um, Fullers' Earth—Ful. Owing to the paucity of records for district 14, North Devon and West Somerset, it has been omitted from the following analysis.

The lists of mine names given in the tables do not agree exactly with the names of the mines described under the corresponding districts in Chapter IV (Vol. I) and Chapter V (Vol. II), because some of the names given in the tables are of mines about which we have no further information beyond output, and, on the other hand, some of the names in the chapters are of mines with no recorded output or had no production before 1912, when the records for individual mines ceased to be shown in the Mineral Statistics of the Home Office.

1. St Just district

2. St Ives district

3. Gwinear district

4. Mount's Bay district

5. Wendron and Falmouth district

6. Camborne, Redruth and St. Day district

7. St. Agnes district

8. St. Austell district