167 



half of Kent's Island, and the exposures are good. It is rarely 

 distinctly amygdaloidal, but is usually compact, of a dull green- 

 ish color, and slaty appearance. The bedding is frequently 

 quite distinct, and the ledges trend in a direction parallel with 

 the strike, N.E.-S.W. to E.-W. The dip is always northerly 

 and steep ; and there is little evidence of extravasation. Much 

 of the rock is decidedly chloritic, and might be properly called 

 a massive chlorite slate. The chlorite sometimes occurs in 

 minute, scale-like layers. Small strings and patches of epidote 

 are common. At several points on Kent's Island, and likewise 

 along the railroad, on the south side of the River Parker, the 

 rock is of a deep purplish-brown or chocolate color, and is 

 evidently little more than a slightly arenaceous slate. The 

 joint surfaces of this variety are covered with chlorite and small 

 scales of mica. Other portions of the so-called amygdaloid re- 

 semble felsite, and are perceptibly crystalline. These lead 

 naturally into the breccia ; which, however, is not certainly 

 known to occur on Kent's Island, but is found in several small 

 patches to the westward, near the Newburyport turnpike. The 

 pebbles are usually quite small, and appear to be derived 

 mainly from the red, banded petrosilex of the region. The 

 base is compact, hard, felsitic, and of a pale greenish color. 

 The rock sometimes affects an imperfectly schistose or banded 

 structure, and is always distinct from the Primordial conglom- 

 erate of that district. The amygdaloid follows the course of 

 the River Parker westward, nearly to By field Parish, present- 

 ing everywhere substantially the same characters as on Kent's 

 Island. Along the Newburyport turnpike I have observed the 

 amygdaloid at a point one-half mile south of Mill River ; where 

 it includes irregular patches of mottled green and brown, which 

 have the aspect of petrosilex, are quite compact, and hard 

 as jasper. East of Kent's Island the breccia and amygdaloid 

 have been traced to the edge of the salt marshes, beyond which 

 there are no ledges. 



Shawmut Group in the Marhlehead Region. — On Mar- 

 blehead Neck and the adjacent islands the Shawmut group is 



