METALLIFEROUS DEPOSITS. 583 



near Dolgelly, on the north side of the river Mawddach, Merionethshire, where, at 

 the Clogau quartz-lode and Gwynfynydd mines it has been obtained from quartz- 

 veins in the Lingula flags and older Cambrian rocks ; ' it has also been found in 

 small quantities in the alluvium of this district. The total weight of gold obtained 

 in Merionethshire, from 1844 to 1866, amounted to 12,800 ounces, having an 

 average value of /^;^ 4s. peroz.- (See p. 62.) 



The occurrence of gold has been noticed in the tin stream-works of Ladock, 

 in Cornwall, while an auriferous quartz-rock has been observed at Davidstowe in 

 north Cornwall.'* In Devonshire gold has been detected at North Molton, in 

 Somersetshire at Clevedon, and in Lancashire at Seathwaite, near Broughton-in- 

 Furness. 



Silver Ores. 



Silver occurs native, but is generally found combined with lead-ore (galena). 

 There is no silver-mine proper in Great Britain ; but the presence of silver mills in 

 Cardiganshire was mentioned by Ray in 1691, and silver-lead ores were worked in 

 Britain before the Roman invasion. Silver-ores have been met with in Cornwall, 

 at Beer Alston in Devon, at Alston Moor in the Carboniferous rocks, in mines 

 at Caldbeck in Cumberland, and in Sark ; but the highest yield has been from 

 the Great Laxey lead-mine in the Isle of Man. Argentite (sulphide of silver) 

 occurs in Cornwall, fine examples having been obtained at Huel Newton and 

 Huel Ludcott. 



Manganese Ores. 



These include pyrolusite (binoxide), psilomelane (hydrous peroxide), manganite 

 (hydrous sesquioxide) and dialogite (carbonate) ; they occur chiefly in Cornwall, 

 Devon, Somersetshire, and Merionethshire. 



The dendritic or arborescent markings (resembling trees, etc.) found on the 

 faces of cracks and joints in many rocks are often due to manganese-ore. (See 

 p. 244.) 



Graphite. 



Graphite, plumbago, or black lead, although not a metalliferous mineral, 

 may yet be mentioned here. It occurs at Borrowdale and Bannerdale in 

 Cumberland, in Cornwall, and in the Isle of Man. It is a crystallized form 

 of carbon, containing about 91 per cent, of it, with silicate of alumina, sesquioxide 

 of iron, etc. It is used in the manufacture of black lead pencils, crucibles, etc. 

 In Cumberland it is known as "wad," and the wad-holes near Seathwaite in 

 Borrowdale, occur in the Borrowdale series, and were worked in the sixteenth 

 century : they now constitute the only black-lead-mine in England.* 



Graphite probably originated from vegetable matter, which has undergone 

 great alteration at a considerable depth beneath the earth's surface. 



Platinum has been detected in veins of Keuper Sandstone in Shropshire, and 

 ores of antimony, bismuth, and cobalt have been obtained in Devonshire and 

 Cornwall. Nickel occurs in association with meteoric iron." 



^ A. C. Ramsay, Q. J. x. 242 ; Murchison, Silurian System, p. 367 ; T. A. 

 Readwin, Mineralog. Mag. 1879 ; D. Forljes, Phil. Mag. 1867. 

 '^ J. A. Phillips, Treatise on Ore Deposits, p. 204. 

 3 S. R. Pattison, Q. J. x. 247. 



* J. C. Ward, Geol. Lake District (Geol. Surv.) ; J. Postlethwaite, Mines and 

 Mining of the Lake District, 1877. 



* For an account of the Minerals of England and Wales, see R. P. Greg and W. 

 G. Lettsom, Manual of the Mineralogy of Great Britain, etc., 1S58 ; T. M. Hall, 

 Mineralogists' Directory, 1868; H. W. Bristow, Glossary of Mineralogy, 1861 ; 

 F. Rutley, Mineralogy, 1874 ; H. Bauerman, Text- Book of Descriptive 

 Mineralogy, 1884. 



