ORIGIN AND DEVELOPMENT OF OBE-DEPOSITS. 63 



through the killas in post-carboniferous times.* But it is also 

 certain that there had been much earlier and very protracted 

 periods, during which the intrusions were in part real ejections, 

 and chiefly basic, viz., the " dunstones " and "greenstones." 

 During these earlier troubled periods, some post-Devonian and 

 others far earlier, it is reasonable to suppose that the surround- 

 ing seas became more or less charged with metallic salts, so that 

 the stratified rocks then being formed, could not fail to be 

 impregnated with them, as also those lying immediately beneath, 

 if they were at all of a porous character. f This hypothesis may, 

 perhaps, account in some degree for the rarity or entire absence 

 of fossils in the distinctively mining regions of the West, though 

 there are of course well-known cases where newer-formed lodes 

 traverse highly fossiliferous rocks, as at Crinnis. Afterwards, 

 the long series of granitic intrusions took place which not only 

 brought up new supplies of ore, and especially of tin, but opened 

 up cavities,- into which subsequent segregations took place. 



This local charging of isolated seas and lakes with metall- 

 iferous substance, is a common circumstance even now, and it 

 must have been much more common in the earlier geologic 

 periods. M. Dieulfait has dealt with this subject very fully on 

 many occasions ; thus, in a lecture delivered before the Associa- 

 tion Scientifique de France, in 1883, he urges that the terrestrial 

 waters are the source from whence the mineral substances found in 

 lodes (i.e. lodes traversing stratified rocks) have been gathered 

 and concentrated. Mr. J. A. Phillips also devoted some attention 

 to this subject, and he stated as an illustration of the extent of 

 such operations in recent times that no less than 80,000 tons of 

 copper have been carried into the sea from the Eio Tinto Mines 

 alone since the Roman times. 



In a really good mining region, therefore, it is hardly too 

 much to say that all the rock-masses, whether eruptive or strati- 

 fied, contain or have contained, diffused throughout their sub- 



* This is certainly so as to the Dartmoor granite ; it is not so certain that all 

 the granite masses were intruded at the same time, but if not, those of Cornwall 

 were probably earlier rather than later. 



f '■ The waters were so strongly impregnated with chemical solutions from 

 mineral springs, that nothing could live." See Jour. Roy. Inst. Corn., 1884, p. 166 



