209 



Quincy Neck we can only conjecture ; in a general way, how- 

 ever, it is almost certainly synclinal. 



On Hough's Neck, in Quincy, along the north side of Rock 

 Island Cove, there are prominent ledges of conglomerate flank- 

 ing a large mass of amygdaloid, and the latter rock crops through 

 the former in isolated bands due to extravasation or faulting. 

 The conglomerate strikes about east-west, and shows nearly 

 vertical dips to the north and south, dipping away from the 

 amygdaloid. It holds unmistakable pebbles of Shawmut brec- 

 cia. This is clearly a faulted anticlinal fold. Toward the north, 

 over the area marked as slate, the rocks are all concealed by 

 drift ; but on the south the conglomerate shows very plain 

 indications of a passage to slate. 



Raccoon Island consists of a homogeneous, gray, sandy slate. 

 This is on the north side of the anticlinal ; strike, east-west ; 

 dip, vertical. Slate with irregular dips is exposed on the south 

 side of Grape Island ; and Slate Island is entirely composed of 

 a beautiful black variety, having a well-developed cleavage par- 

 allel with the bedding. Strike, N. 80° E. ; dip, vertical. The 

 disturbed condition of the rocks in the adjacent part of Hingham 

 makes it unsafe to speculate concerning the relations of Slate 

 Island to the anticlinal of Rock Island Cove. One and a half 

 miles east of the Old Colony Railroad, on the road to Hough's 

 Neck, there is slate with a high northerly, nearly vertical, dip. 

 Like the Raccoon Island slate, this is evidently on the north 

 side of the anticline. Along the railroad, just north of Black's 

 Creek, the conglomerate crops a second time. The pebbles 

 are small, and often distinguishable with difficulty from the 

 slaty' paste ; and on the north the conglomerate passes through 

 greenish and slaty sandstones to true slate. The latter rock is 

 bluish-black, and distinctly stratified. There is some exotic 

 diorite exposed here. The inclination of the beds is variable, 

 but a nearly vertical dip to the north prevails. 



West of the railroad the section is very interesting. On a 

 line parallel with the track, and nearly one-fourth mile distant, 

 the succession from the granite northward is as follows (PI. 4, 



OCCAS. PAPERS B. S. N. H. — III. 14 



