572 GRANITE ROCKS. [Cn. XXXIII. 



also, notwithstanding their appearance of interstratification, have been 

 forcibly injected. Some of the porphyritic rocks above mentioned are 

 highly quartzose, others very felspathic. In proportion as the masses 

 are more voluminous, they become more granitic in their texture, less 

 conformable, and even begin to send forth veins into contiguous strata. 

 In a word, we have here a beautiful illustration of the intermediate gra- 

 dations between volcanic and plutonic rocks, not only in their mineral- 

 ogical composition and structure, but also in their relations of position 

 to associated formations. If the term overlying can in this instance be 

 applied to a plutonic rock, it is only in proportion as that rock begins to 

 acquire a trappean aspect. 



It has been already hinted that the heat, which in every active volca. 

 no extends downwards to indefinite depths, must produce simultaneously 

 very different effects near the surface, and far below it ; and we cannot 

 suppose that rocks resulting from the crystallizing of fused matter under 

 a pressure of several thousand feet, much less miles, of the earth's crust 

 can resemble those formed at or near the surface. Hence the production 

 at great depths of a class of rocks analogous to the volcanic, and yet 

 differing in many particulars, might also have been predicted, even had 

 we no plutonic formations to account for. How well these agree, both 

 in their positive and negative characters, with the theory of their deep 

 subterranean origin, the student will be able to judge by considering the 

 descriptions already given. 



It has, however, been objected, that if the granitic and volcanic rocks 

 were simply different parts of one great series, we ought to find in moun- 

 tain chains volcanic dikes passing upAvards into lava, and downwards into 

 granite. But we may answer, that our vertical sections are usually of 

 small extent; and if we find in certain places a transition from trap to 

 porous lava, and in others a passage from granite to trap, it is as much 

 as could be expected of this evidence. 



The prodigious extent of denudation Avhich has been already demon- 

 strated to have occurred at former periods, will reconcile the student to 

 the behef that crystalline rocks of high antiquity, although deep in the 

 earth's crust when originally formed, may have become uncovered and 

 exposed at the surface. Their actual elevation above the sea may be re- 

 ferred to the same causes to which we have attributed the upheaval of 

 marine sti'ata, even to the summits of some mountain chains. But to 

 these and other topics, I shall revert when speaking, in the next chapter, 

 of the relative ages of different masses of granite. 



