52 Geology, as illustrated by Chemistry and Physics. 



ness. It is possible that they reach the depth of 115,000 

 feet, and perhaps exceed it, for these strata have not been 

 penetrated at any part of the earth. But if it were possible 

 that clay-slate could at this depth be converted into a crystal- 

 line rock by the central heat of the earth, then a rocky mass 

 must be removed to nearly such a depth, in order that the 

 metamorphic rocks should be accessible. The frequent deep 

 erosion of river valleys shews that considerable masses of 

 dry land may be removed and carried into the sea by running 

 water. If masses have been deposited at the bottom of the 

 ocean to the thickness of several miles, then it is evident that 

 equal quantities of debris must have been conveyed to the 

 sea by this means. From this point of view, the hypothesis 

 of a metamorphosis of the central heat of the earth would 

 encounter less difficulties, if it were at all possible to regard 

 such an hypothesis as a correct one. 



With regard to the favourite hypothesis, according to 

 which aqueous vapours are supposed to be the agents which 

 cause the transformation of sedimentary into crystalline rocks, 

 abundant opportunities are presented in nature, as well as in 

 chemical laboratories and metallurgical furnaces, for acquir- 

 ing a knowledge of their action upon rocks. During a space 

 of 190 years, aqueous vapour has issued from fissures in clay- 

 slate above the burning coal strata near Duttwiler, in such 

 quantities, that in cold moist weather, the mountain valley is 

 enveloped in dense clouds. Atmospheric air mixed with aque- 

 ous vapour and sulphurous acid at a temperature of 80° to 

 158° R., issues from some fissures. But besides a red colour- 

 ing of the slate-clay throughout its mass, making it resemble 

 burnt brick, besides sublimation of crystallised sulphur and 

 chloride of ammonium, no other changes are recognisable. 

 At other places, where plastic clay lies very near the focus of 

 the combustion, this has been converted into porcelain jasper. 

 The suffioni of Tuscany cause no other alteration in the 

 adjoining jasper and hornstone than destroying their red 

 and dark grey colour, and making the rock friable. At Ter- 

 ceira, aqueous vapours decompose the trachyte to white clay, 

 silica is extracted and again deposited as hyalite.* Accord - 

 * Lehrbuch tier Chem. Physikal. Geologic, Bd. IT., pp. 353 and 354. 



