316 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. 



This supposed metamorphosis of sedimentary rocks is, 

 however, said to belong not only to strata older than the 

 carboniferous, but actually to occur in strata belonging to 

 some of the more recent secondary groups. 



The nature of the transition between these groups .and 

 primary slates having been already considered, it remains to 

 enquire whether caloric alone is capable of accounting for the 

 difference between these two kinds of rocks. It has been 

 shown, that the schistose rocks, at their junction with granite, 

 not only differ from fossiliferous slates in their texture, as in 

 the case of such slates next trap, but also in their com- 

 position ; a chemical difference, which heat alone cannot be 

 supposed to have effected. This is rather a serious difficulty 

 in limine, for it cannot be attributed to the union of the 

 granite and slate by fusion, since the lines of stratification, 

 though obscured in gneiss, are allowed to be very distinct in 

 limestone, hornblende, and argillaceous schists, and similar 

 rocks ; and it appears to us that this difficulty is not dimi- 

 nished when the scrutiny is extended to the whole mass of 

 primary slates. 



Presuming that the analogy of our furnaces, through the 

 sides of which the effects of the fire do not extend beyond a 

 certain point, though continued for several years ; and of the 

 glacier on Etna *, preserved from the action of a burning 

 flood of lava by the sole intervention of a layer of volcanic 

 sand, to be objectionable; and, admitting that the central 

 fire can exert its influence through such an immense mass of 

 non-conductors of caloric as the primary strata, we proceed 

 to examine the changes which have been attributed to this 

 igneous agent. 



In the first place, the contact of melted granite (how called 

 into existence is at present immaterial) has, according to the 

 Plutonists, converted argillaceous sandstone, or some other 

 sedimentary rock, into gneiss ; which, adjacent to the granite, 



* Lyell's Principles of Geology, vol. i. p. 371. 



