262 



Scientific Geology. 



10. Flinty Slate. 11. Chert. 12. Jasper. I regard these rocks 

 as varieties of other rocks, produced by the proximity of granite, 

 porphyry, or trap : and in Massachusetts they are merely altered va- 

 rieties of the graywacke formation that has been described. Hence 

 I shall treat of them in this place. The sagacious observations of 

 Dr. Macculloch concerning the origin of these rocks,* receive strong 

 confirmation from their situation in New England. And since this 

 is a subject, concerning which geologists are as yet by no means 

 agreed in opinion, I shall exhibit the relative position of these rocks 

 as intelligibly as possible, from the examinations of them which I 

 have been able to make. 



Flinty Slate or Siliceous Schist. 



This rock I have found only in two places in the district which I 

 am describing ; viz. in Newport, R. Island, and on the promontory of 

 Nahant. It is interesting, however, that in the former place it oc- 

 curs contiguous to granite, and in the latter, to trap. 



It is not this slate alone which in Newport exhibits the influence 

 of the proximity of granite : and it will save space to give an account 

 here of the whole of this interesting spot, to which I was conducted 



* System of Geology, vol. 1. Chapter XL. 



