58 OEIGIN AND DEVELOPMENT OF ORE-DEPOSlTS. 



If we assume an average produce equal to 400 ounces of 

 silver per ton, which, is probably near the truth, the total silver 

 thus accounted for would be a little under 240,000 ounces. 

 This is trivial as compared with the silver contained in the 

 copper and lead ores raised duriag the same period of 20 years, 

 most of which was extracted by the smelters. Of copper-ores 

 about If million tons were sold, averaging probably 2^ ozs. of 

 silver to the ton, and of lead-ores about 114,000 tons, averaging 

 about 25 ozs., or, together, more than 7| million ounces. 



Notwithstanding, these occasional occurrences of gold, of true 

 silver-ores, and of argentiferous lead and copper ores, it is 

 evident that the district as a whole can neither be regarded as 

 auriferous nor argentiferous in the ordinary sense of the terms. 

 The statistics of gold and silver productions are very imperfect, 

 but it is certain that the whole yield, past and present, has not 

 equalled the yields respectively of a single gold or silver mine 

 of the first rank. 



Group 8. Carbon and Hydrocarhons. 



Carhon, in a free state, exists in small grains or scales in a 

 great many of the slaty rocks, as may be seen when their thin 

 sections are examined under the microscope ; and even when it is 

 not distinctly visible, its presence may be shewn by the deflag- 

 ration which ensues when the powdered rock is heated with nitre 

 or chlorate of potash. In some instances, as at Wheal Jane in 

 Kenwyn, and Treamble in Perranzabuloe, I have known it to 

 occur in large flakes, under conditions which suggest an organic 

 origin, while some of the slaty rocks on the south coast near 

 Looe are so highly charged with carbonized fish- remains, that 

 they have been described as " a catacomb of ancient fish." The 

 anthracite of the culm series in North Devon and East Cornwall 

 is the only case in which carbon can be said to be common. 



Graphite has been found in small particles and nodules in 

 several of the elvans, but it is always very rare. 



Hydrocarlons. Bitumen or asphalt has occurred in several 

 of the mines of Gwennap, Eedruth, Illogan, and Camborne, 

 while some of the shales of North Devon and of the Tamar 

 Valley have afforded traces of mineral oil. 



