SUBTERRANEAN HEAT. 73 



into coal, and the bituminous coal into a hard and nameless 

 anthracite. 



But whatever the amount of rock-change brought about 

 by chemical agency, it is far less noticeable than that in- 

 duced by the operations of subterranean heat, whether 

 acting in the dry way, as in volcanoes, or in the wet, as in 

 hot springs and vaporiform exhalations. Whatever be the 

 source of volcanic heat, its effect by contact on all the 

 stratified rocks is at once marked and decisive. Sand- 

 stones are frequently converted into crystalline quartzites, 

 shales baked into splintery porcellanite and porcelain jas- 

 per, and bituminous coals changed into coke-like anthracites. 

 We every day witness the effects of dry heat in our brick- 

 kilns, coke-ovens, glass-houses, and iron-furnaces ; and if 

 the effect of such temporary heat be to melt and bake and 

 calcine, how much more the results of those subterranean 

 fires, that may continue to operate in certain areas unabated 

 for ages ! In the earth's crust, then, we may rest assured 

 that volcanic heat has effected, and is still effecting, exten- 

 sive metamorphism in all rock-masses, and that those lying 

 at great depths and under vast pressure will be affected very 

 differently from those occurring near the surface. But be- 

 yond this contact with dry heat there is the permeation of 

 heated water and vapours, which greatly facilitate the dis- 

 solution and recombination of mineral matters. As the tem- 

 perature of the crust increases at the rate of one degree 

 Fahr. for every 60 or 65 feet of descent, the transforming 

 effect of this heat at the depth of a few miles must be 

 enormous ; and we can readily conceive (though we cannot 

 witness the operation) how ordinary strata may be con- 

 verted into crystalline schists and igneous rocks, as the 

 traps and lavas, undergo a process of re- crystallisation, and 

 assume the character of granites. Heat, in whatever form 

 it operates, is, indeed, the great transformer of mineral 



