716 



CONFORMABLE PORPHYRIES. 



[Ch. XXXIII 



classed as plutonic rather than trappean rocks, which may truly he 

 described as interposed conformably between fossiliferous strata, as 

 the porphyries (a, c, fig. 14:8), which divide the bituminous shales 



Euritic porphyry alternating with primary fossiliferous strata, near Christiania. 



and argillaceous limestones, //. But some of these same porphyries 

 are partially unconformable, as 6, and may lead us to suspect that the 

 others also, notwithstanding their appearance of interstratification, 

 have been forcibly injected. Some of the porphyritic rocks above 

 mentioned are highly quartzose, others very felspathic. In propor- 

 tion as the masses are more volumiuous, 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 illustra- 

 tion of the intermediate gradations between volcanic and plutonic 

 rocks, not only in their mineralogical 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 vol- 

 cano extends downwards to indefinite depths, must produce simulta- 

 neously 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 mountain chains volcanic dikes passing upwards into lava and 

 downwards into granite. But we may answer that our vertical sec- 

 tions 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 which has been already de- 

 monstrated to have occurred at former periods, will reconcile the 



