GEOLOGY: SOUTH SHORE. 



Prof. W. 0. Crosby. 



THE ODTER ISLANDS OF BOSTON HARBOR. 



Lovell's, Gallop's, George's and Great Brewster islands are 

 drift-covered. The rest have little or no glacial detritus, and form 

 part of a great synclinal fold of slate, with intrusions of diabase, 

 in part sills. Calf island lies on the north side of the fold, and 

 the Brewsters on the south. The south side of Middle Brewster 

 island gives the best exposures of alternating diabase and slate. 

 The former possesses a very perfect flow-structure, and shows 

 concentric weathering on a large scale. 



NANTASKET AND COHASSET. 



Contemporaneous flows, dikes, faults, plutonic rocks, etc. 



Route. — By Nantasket steamers from Rowe's wharf, Boston, to Nantas- 

 ket direct, fare, §.25; or to Pemberton by same steamers and thence by 

 the Nantasket Beach railroad to Nantasket station, or by all rail from the 

 Old Colony station (Kneeland street) to Nantasket station, fare $.35. 



The district in the southern part of Hull known as Nantasket, and 

 the adjacent shore of Cohasset, embrace in a limited area many 

 interesting and instructive outcrops, and afford an abundance of 

 material for two excursions. The rocks, above the granitic series, 

 are chiefly conglomerate of probable Carboniferous age interstiati- 

 fied with many contemporaneous flows of basic and neutral lavas 

 (melaphyr and porphyrite) and beds of tuff ; and the whole is 

 intersected by several systems of dikes. 



1. Nantasket beach to Green lull. 



This coastal area of Nantasket can be studied to the best advan- 

 tage when the tide is out. 



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