Crosby. J j 
500 
[May iS, 
lavas are limited almost wholly to the northwest corner of the 
town, extending but little south of the railroad and having only 
a slight areal development east of the harbor, while over the re- 
mainder of the town, embracing more than five sixths of the total 
area, the numerous ledges comprise only granitic rocks (gran- 
ite, diorite, and felsite) and intersecting dikes of diabase. 
The granitic area of Hingham is similar to and continuous with 
that of Cohasset and Nantasket on the east, and Weymouth and 
Braintree on the west, the entire South Shore district being a 
unit in this respect. But the sedimentary and volcanic rocks, 
bordering the granite on the north and forming the immediate 
shore of Boston Harbor, are far less uniform in character and 
structure, and by their diversity warrant the division of the South 
Shore into several distinct areas, which agree approximately with 
the political divisions, the geology of North Hingham contrasting 
with that of Weymouth on the west and still more with that of 
the Nantasket area on the east. The promontory of Rocky Neck, 
northeast of Planter’s Hill, at the mouth of Weir River Bay, is, 
however, essentially a part of the Nantasket area, the true or nat- 
ural boundary between the Hingham and Nantasket areas being, 
approximately, the eastern shore of Hingham Harbor. 
In comparing the geological structure of Hingham with that of 
Nantasket, it is found that plication to a large extent takes the 
place of faulting, the sedimentary and volcanic rocks being in- 
volved in deep and almost isoclinal folds ; and while Nantasket 
shows repeated alternations of beds of conglomerate with flows of 
both basic and acidic lavas (melaphyr and porphyrite) , the por- 
phyrite, so far as known, is wholly wanting in Hingham, and the 
melaphyr is limited to one flow or bed of great thickness ; and 
the principal problem of the Nantasket area — the identification 
of the successive flows of lava — is really not presented to the 
student of Hingham geology. On the other hand, while the 
sedimentary rocks of the Nantasket area are almost exclusively 
conglomerate, the Hingham series embraces many beds of sand- 
stone and brownish slate, and a great volume of gray slate ; and 
the special feature of the geology of Hingham, the feature in 
which it excels not only Nantasket but the entire Boston Basin, 
is the extended series of alternating beds of conglomerate, sand- 
stone, and slate which it presents in three different sections, and 
the seemingly clear exhibition of the relations of this conglomer- 
