POSITION OF THE SOUIKM': OF HEAT. 441 



phur Bank, where the associated volcanic plienoniena are of considerable 

 age, probably thousands of years, the depths at which these lieated rocks 

 lie must be great. It is true that a body of lava covered with dry rock of 

 a very moderate tliicknes.s would remain hot for a very long time; but, at 

 the localities mentioned above, constant and copious streams of cold water 

 from the surface are heated and returned to tlie surface. In both localities, 

 also, this very effectual cooling process has been in operation for ages, and 

 probably from the era of the latest volcanic outbursts. The rocks hot 

 enough to heat rapid water currents to such an extent that tliey reach the 

 surface with a temperature of nearly or ([uite 100° must therefore lie at 

 great depths. On the Conistock lode the heat increment is 1° F. for every 

 33 feet. If the same increment obtain for Steamboat Springs and if the 

 rock mass which heats its waters be at a very low red heat (about 500'' C), 

 the depth of the uiass below the surface is, in round numljers, five miles. 

 The Sierra Nevada has been a land area from the Carboniferous onwards, 

 and during a great portion of this immense interval it has been a mountain 

 range undergoing rapid erosion. Its granitic surface must for the most 

 part be extremely ancient, and at a depth of five miles from the surface it 

 is very questionable whether there can be any rock which has ever been 

 exposed to daylight.^ The waters rising from a depth of five nnles, and 

 very possibly more, pass through granite which bears no evidence of meta- 

 morpliic origin, and possibly through other rocks. 



Relations of the deposits to various rocks— Grranitc is thc decp-scated rock beneath 

 all the ore deposits mentioned in this volume. This has been alluded to in 

 former chapters, in which it was shown that granite underlies the entire Coast 

 Ranges and supplied the material of which the sedimentary rocks of that 

 region are composed. The ore deposits themselves are found in various 

 rocks : At Steamboat Springs, in granite and to a small extent in basalt ; 

 at Sulphur Bank, in basalt, in Neocomian sand.stone, and in recent lake 

 deposits ; at Mt. Konocti, in andesite ; at Knoxville and New Almaden, in 

 metamorphosed Neocomian strata ; at Oathill and to a slight extent in Knox- 

 ville, in unmetamorphosed Neocomian strata ; at New Idri a, in the meta- 



' Compare "Origin of the massive rocljs," Chapter IV, p. I(i4. 



