100 



nomena are presented along the western border of the petrosilex 

 in West Dedham and Dover ; as already noticed, the petrosilex 

 along this line becomes very distinctly, and, apparently, very 

 gradually, more crystalline westward, and is finally indistin- 

 guishable from the granite bordering the petrosilex in that direc- 

 tion. Similarly, in traversing the zone of contact between the 

 granite of West Roxbury and the petrosilex on the east and south 

 there is an undoubted transition. In the case of the Blue Hill 

 petrosilex I consider the evidence irrefragable ; it unquestionably 

 passes into the granite on the north and east, and probably into 

 the Dedham granite. A broad band of eurite forms the southern 

 border of the Blue Hill granite, extending from the petrosilex 

 eastward to Weymouth Fore River. Patches of granitic 

 petrosilex (elvanite) have been observed in the granite on the 

 Cohasset shore. 



In a paper on the Geology of the Vicinity of Boston,^ by 

 Dr. T. Sterry Hunt, I find a partial expression of a view of 

 the relations of the granite and petrosilex similar to that held 

 by President Hitchcock, and insisted upon in this paper. After 

 speaking of the "felsite" and " felsite-porphyries," he says: 

 ' ' Associated with them is a granular quartzo-feldspathic rock 

 which is often itself porphyritic, with feldspar crystals, and 

 sometimes appears as a fine-grained syenitic or gneissoid rock, 

 often distinctly stratified. This has been described by Hitch- 

 cock as intermediate between porphyry and syenite ; his syenites 

 with 'a nearly or quite compact feldspar base,' and some of his 

 porphyritic syenites will probably be found to belong to these 

 granular eurites, which I connect with the porphyries." 



Again, in his Chemical and Geological Essays, pp. 186-187, 

 the same author says : " The rocks having the mineralogical 

 composition of granites present a gradual passage from the 

 coarse structure of ordinary micaceous, hornblendic, and binary 

 granites to finely granular and even impalpable mixtures of the 

 constituent minerals, constituting the rocks known as felsite, 

 eurite, and petrosilex. These rocks are often porphyritic from 

 ^ Proo. Bost. Soo. Nat. Hist., vol. xiv., 45. 



