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at tlie Cohasset boundary, in the vicinity of the railroad, the 

 diorite is observed most abundantly in the ledges aloug Hull 

 Street, Weir River Lane, Kilby and East Streets ; and then, 

 after a partial gap of more than a mile where the ledges of all 

 kinds are very generally concealed by the Lower Plain, north- 

 west of Fort Hill, near West Hingham, for a short distance, 

 until all the ledges are again blotted out by the modified drift. 

 It should not be supposed, however, that these marks represent, 

 in every case, continuous ledges of diorite, for the luost of the 

 ledges are of a mixed character, granite penetrating or inclosing 

 diorite. This irregular belt, it will be observed, is in about the 

 same latitude as the }»rincipal occurrences of diorite in Cohasset, 

 and on the same east-west line are found the chief outcrops of 

 diorite in Weymouth ; but south of this belt we have observed 

 no im[)ortiint or notable masses of diorite. In lithological 

 character the diorite of Hingham is scarcely to be distinguished 

 from that of Cohasset, showing similar variations, excej)t that 

 it nowhere exhibits a distinct flow-structure. It is usually 

 finely crystalline and dark colored ; but occasionally it is 

 coarser, with the hornblende either very clearly and })rominently 

 developed or mainly wanting, giving a light-colored, feldspathic 

 variety which might be readily mistaken for syenite or even 

 granite. Epidote is, as usual, the principal secondary mineral, 

 occurrin<T chiefly as narrow and irregular scijreoations and 

 veinlets, especially along the joint cracks. 



Although the granite of Hingham almost certainly embraces 

 the three distinct types — distinct in age and character — observed 

 on the Cohasset shore, few ledges have been observed in which 

 the relations of the two older types — the basic hornblendic 

 granite of mediian texture and the coarse acid granite — are 

 clearly exposed. Perhaps the most favorable exposure of this 

 kind is that afforded by Button Island, in Hingham Harbor. 

 The bedrock of the island is wholly granite, with only a thin 

 covering of drift. The granite is chiefly an miusually coarse 

 and distinct example of the second or more acid type, enclosing 



