98 



texture of both species is so wide that we frequently have granite 

 more compact than some petrosilex, and vice versa. Such 

 lithologists as Von Cotta, who hold to the igneous origin of both 

 granite and petrosilex, regard these rocks as products of essen- 

 tially the same* mineral dough, differing only by reason of the 

 slower or more rapid coohng of different portions of the origi- 

 nal fluid mass. This view is diametrically opposed to that pro- 

 posed here, and yet they are parallel ; for I regard the granite 

 and petrosilex as unequally metamorphosed portions of an 

 originally stratified, compact, petrosilicious rock. They are 

 not strictly synchronous, but, as I conceive, the petrosilex 

 represents, in a general way, the higher and the granite the 

 lower part of the same vast formation. Hence the gradual 

 transition from the one rock to the other should occur, normally, 

 in a vertical direction ; and we can expect to find the intermediate 

 textures that prove the passage only where the denudation or 

 upturnings have exposed the original zone of contact. Now, 

 as previously shown, the granite, in its present condition, is 

 generally exotic ; although as a rule it has not probably been 

 extravasated very far ; and it were manifestly absurd to suppose 

 that any contact between these two rocks which we may chance 

 to traverse will or should afford evidence of a gradual transition, 

 regardless of the mode of formation of the contact. Contacts 

 resulting from extravasation will usually be found abrupt, and 

 most of the contacts are of this kind. 



It is worthy of note that the granite is very commonly of 

 about the same color as the adjoining petrosilex, especially 

 where there is any evidence of a transition ; and not infre- 

 quently they have other characters in common. Thus, in New- 

 bury the petrosilex, judging from its deep-red color, is de- 

 cidedly ferruginous ; moreover, it appears to occupy a basin 

 having a granitic border, and where the petrosilex adjoins the 

 granite, especially to the north of Kent's Island, there is not 

 only evidence of a transition in texture, but the granite 

 shares the ferruginous character of the petrosilex, portions of 

 it being so charged with iron, chiefly in the form of carbonate. 



