380 Dr. Boase's Inquiry into the Nature 



stance, a similar structure in all the other granitic rocks, 

 whether on a larger or smaller scale, must be referred to the 

 same cause. But I subscribe not to the Professor's inference, 

 that the slate and the laminated granite and shorl-rock must 

 have been nearly in the same condition ; for no evidence can 

 be detected in favour of such a nice distinction to warrant the 

 conclusion that the circumstances under which these rocks 

 crystallized were not perfectly identical. If crystalline action 

 produced the lamination of the granite as well as of the schist, 

 in what respect do these rocks differ from each other, except 

 in the degree of fineness of their laminations? It may be said 

 that they differ in mineral composition and in appearance. 

 I have elsewhere, at great length, attempted to show that in 

 the former respect they do not differ ; and in their appearance 

 the difference is not so great as is generally supposed. In the 

 case of Carclaze, just alluded to, there is a great similarity; 

 indeed, there is not nearly so great a difference between the 

 granitic rock and the schist at their junction, as -between the 

 former in this position, and the perfectly crystalline rock at 

 a distance therefrom : and the striking resemblance which 

 often occurs between granite and gneiss, and mica-slate, at 

 their contact with each other, needs no remark. Why then, 

 in these instances, is a difference in structure made to establish 

 a distinction of character and of origin ? and that too in very 

 opposition to the facts, that igneous rocks do possess the same 

 structure as aqueous formations: even granite, at Arran, 

 having a foliated structure; hornblende-rock, greenstone, and 

 other trappean rocks being frequently schistose; and more- 

 over, the lava of an extinct volcano in Auvergne not only 

 being fissile, but absolutely affording roofing-slates. 



After alluding to St. Austel Moor, the Professor adds: 

 " Now all phenomena of this kind accord perfectly with the 

 igneous theory of granite, and its protrusion among stratified 

 slates : yet have they been urged as proofs that the slate rocks 

 of Cornwall (including in the list the fossiliferous slates of 

 Tintagel, &c. &c.) are all contemporaneous with the central 

 granite." This is not a correct representation of my opinion ; 

 for the Professor might have known that in the fourth volume 

 of the [Royal Cornwall Geological] Society's Transactions, 

 published in 1832, I virtually retracted the notion that the 

 calcareous series of slates were as old as the granite, by admit- 

 ting that they contained organic remains; and last year, in my 

 work on " Primary Geology," I excluded this series altogether 

 from the oldest non-fossiliferous slates. 



Having said thus much in my own justification, I proceed 

 to consider " the changes which mechanical stratified rocks " 



