93 



Final Report, p. 151, we find it stated that the beach, a mile 

 or two in length, skirting the foot of this hill, is largely 

 composed of pebbles and boulders of petrosilex. The small 

 area of petrosilex in Hingham is twenty-five miles distant 

 in a north-westerly direction, while more nearly to the north 

 petrosilex does not appear above the water at a point nearer 

 than Marblehead, fully forty miles distant. In short, the 

 abundance of these loose masses of petrosilex at a point 

 so remote from the nearest known source suggests the exist- 

 ence of a less distant source now unknown ; and I should not 

 be surprised to learn that Manomet Hill itself has an axis of 

 petrosilex. 1 



The analyses of the petrosilicious rocks recorded in the fore- 

 going descriptions do not represent any one locality more than 

 another, and in selecting the specimens for analysis those were 

 chosen which seemed to be most characteristic of the areas 

 which they respectively represent, and without regard to 

 whether they would probably prove to be acidic or basic. 

 Hence, nothwithstanding their small number, these analyses 

 indicate approximately the relative importance of the petrosilex 

 and felsite ; the former rock being far more abundant than the 

 latter. The felsite is nearly always of a pale color, — drab, gray, 

 or greenish-white ; on Marblehead Rock and the small island 

 east of Lowell's Island, however, it is black, and the banded 

 felsite in Milton contains some purple. These light tints are 

 common to portions of the petrosilex also, but the colors emi- 

 nently characteristic of this rock and peculiar to it, or nearly 

 so, are the following : red, brown, brownish-purple, and black. 

 In some cases the felsite can be recognized by the triclinic form 

 of the feldspar crystals ; it may also be usually known by its 



J This is precisely the conclusion reached hy Prof. Hitchcock. He describes (p. 374) 

 a block of porphyry on Manomet Hill sixty feet in horizontal circumference, and, on 

 p. 379 he adds: "Indeed, so abundant and so large are the porphyry boulders here 

 that I can hardly conceive they could have been brought from the Blue Hills, which are 

 nearly thirty miles distant; and I am disposed to believe that this rock exists in place 

 not far north of Plymouth." In yet another place he describes the granite at one point 

 in Kingston as having the texture and general appearance of the Blue Hill porphyry. 



