130 



LYELL'S ELEMENTS OF GEOLOGY. 



Conformable Porphyries .' Granitic Rocks. 



servations respecting localities referred to by Von Buch, I have 

 lately had opportunities of verifying. There are, however, on 

 a smaller scale, certain beds of euritic porphyry > some a few 

 feet, others many yards in thickness, which pass into granite, 

 and deserve perhaps to be classed as plutonic rather than trap- 

 pean rocks, which may truly be described as interposed con- 

 formably between fossiliferous strata, as the porphyries (a c, 

 Fig. 121.), which divide the bituminous shales and argillaceous 

 limestones, ff. But some of these same porphyries are par- 

 Fig. 121. 



Euritic porphyry alternating with fossiliferous transition strata, near*Christiania. 



tially unconformable, as &, and may lead us to suspect that the 

 others also, notwithstanding their appearance of interstratifica- 

 tion, 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 gradations be- 

 tween 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 in- 

 stance be applied to a plutonic rock, it is cnly in proportion as 

 that rock begins to acquire a trappean aspect. 



It has been already hinted that the heat, which in every ac- 

 tive volcano extends downwards to indefinite depths, must pro- 

 duce 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 

 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 m many 

 particulars, might almost 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 vol- 



