37 



are metamorphosed sediments, I conceive that the peculiar 

 planes of separation (joint structure) referred to by Prof. Shaler 

 demand a different interpretation from that proposed by him, 

 for evidence is not wanting of the extravasation of the granite 

 at many points along the Blue Hill or Quincy and Milton 

 range. Following are the clearest cases with which I am 

 acquainted. It is well known that the Quincy granite is met 

 along its northern border by conglomerate and slate. Inter- 

 posed between these uncrystalline sediments and the granite at 

 some points is a compact, greenish, slaty rock, which is occa- 

 sionally felsitic, but offcener heavy and semi-crystalline, ap- 

 proaching diorite. It exhibits obscure traces of bedding in a 

 few places. The actual contact of this rock with the granite 

 is seldom observed ; it is displayed, however, at a place about 

 one-half mile west of the Old Colony R.R., and immediately 

 north of a quarry situated a few rods south of the southern- 

 most part of Adams Street, on the west side of the private 

 road that runs from Adams Street to the quarries. The con- 

 tact line is extremely irregular ; and the relation of the granite 

 to the semi-crystalline rock is unquestionably that of an exotic. 

 Some three miles to the south-west, near the centre of this 

 large granite range, about three-fourths of a mile east of 

 Randolph Turnpike, and near the reentrant angle in the 

 western boundary of Quincy, is an island of this same dark- 

 colored, slaty rock, but more distinctly argillaceous, and the 

 traces of bedding a little less obscure. It is very distinctly 

 cut by dykes and irregular strings of the underlying and 

 surrounding granite. According to Prof. W. H. Niles, the 

 relations of the granite and slate on Weymouth Fore River, 

 near the trilobite quarry, afford equally conclusive evidence 

 that at least a portion of the granite has experienced some 

 extravasation since the deposition of the slate. The slates 

 on the South Shore R.R., immediately east of the station 

 at Weymouth Landing, are in contact with the granite, which 

 cuts through, and overlies them in a manner possible, appar- 

 ently, only with an exotic ; and at the contact of the granite 



