AND STRATIFICATION. 83 



masses of trap are also found incumbent on similar 

 beds. In such cases, the clay is often converted into 

 jasper, in others it is partially changed, while, in a 

 third case it has escaped entirely. Sky presents the 

 most perfect display of all these phenomena. Thus, 

 under minor degrees of heat, alternating beds might 

 have escaped its action, without contravening the 

 general principle, should any geologists be inclined to 

 maintain it. But there is still another well-known 

 fact which might be adduced in proof of the possibi- 

 lity of an alternation between consolidated and un- 

 consoli dated strata, even if heat were the indurating 

 cause. This is the decomposition which rocks 

 undergo, though deeply situated beneath the sur- 

 face ; a fact common in granite and gneiss, and 

 even in trap to a great extent. It is as easy to 

 imagine that rocks whose constitution admitted of 

 it, should have undergone this change, since their 

 consolidation, while the adjacent ones have escaped, 

 as that, under the circumstances already stated, 

 they should have escaped consolidation altoge- 

 ther. 



The fact, of the uncertain recurrence of different 

 specific gravities among the strata, is clearly no 

 obstacle to the theory of stratification here laid down. 

 That objection, commonly made, is founded on an 

 equal ignorance of the chemistry of rocks and of 

 mathematical principles ; even were it admitted that 

 all the strata had been deposited from a single mixture 

 of earth and water. The subsidence of solids through 

 water is regulated by other principles, as I have 

 already shown, than that of mere specific gravity. 

 But whatever the specific gravity of the materials 

 may have been, that of the ultimate solid is parllv 



G 2 



