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easterly ; and under the harbor they may be more numerous 

 than on the land. The axis of a synclinal appears to pass near 

 the mouth of Black's Creek and the southern end of Half Moon 

 Island, and then, bending to the north, to run in the direction 

 of the outer end of Hull, parallel with Peddock's Island and 

 the north shore of Hough's Neck. Slate occurs in situ, on 

 the south side of the village of Hull, precisely as stated by 

 President Hitchcock. It is exposed at the base of the hill, 

 perhaps thirty rods east of the steamboat wharf ; and is a gray- 

 ish and brownish-black variety, distinctly stratified, but with 

 the bedding very much disturbed. The contortions of this slate, 

 no less than its geographic position, indicate that it lies in a 

 synclinal. An erratic of slate observed south of Strawberry 

 Hill, but probably derived from this synclinal, is decidedly 

 calcareous. 



Hangman's Island is marked by Hitchcock as syenitic ; but 

 I find that this rocky islet consists largely of a grayish slate, — 

 much contorted and broken, — but still distinctly, stratified. 

 The strike and dip are very variable, but the former is mainly 

 east-west. The "syenite" is really a fine-grained, but dis- 

 tinctly crystalline, basic rock, probably diorite. It cuts the 

 slate in every direction, in dykes and large irregular masses ; 

 and forms the shore around the whole island, protecting the 

 slate from the wash of the sea. Some of the diorite is of a 

 fine-grained, slaty texture, but this is no less clearly eruptive 

 than that which is coarse. These exotics do not belong to the 

 Shawmut group, except, perhaps, the slaty variety. I am of 

 the opinion that Hangman's Island lies on a broken anticlinal. 

 This hypothetical arch may be continued to the north-east by 

 the Shag Rocks, which appear to be of similar formation ; and 

 it is almost certainly, represented on the main land by the 

 elongated area of petrosilex and rocks of the Shawmut group 

 crossing Blue Hill Avenue, in Milton, and extending into 

 Hyde Park. Exposures are few in this direction, but enough 

 exist to prove a patch or island of the older rocks named, 

 extended in an east-north-east direction, and surrounded by 



