18 
not only the border, but the floor of this part of the Boston Basin. 
They are alike the foundation upon which the newer sediments 
and lavas of Nantasket are piled, and the principal source from 
which the material for building the Nantasket strata was de- 
rived. 
Among the granitic rocks of Cohasset are included chiefly 
the diorite and the granite proper. The diorite is in every in- 
stance clearly the older, as well as the less abundant and less 
import: int, rock ; the relations of the two rocks bei ing essentially 
the same here as elsewhere about the Boston Basin. To a large 
extent they are quite intimately associated, the diorite occurring 
very generally in the form of irregular fragments or masses, of 
all sizes, enclosed in the granite; while in other cases the gran- 
ite forms irregular, branching dikes in the diorite. In fact, the 
granite, however massive it may be, is rarely entirely free from 
inclusions of diorite ; and the diorite, even when farthest from a 
main body of granite, almost invariably exhibits a net-work of 
granite intrusions. In other words, the diorite has been very 
generally fissured, and in large part completely shattered, and 
then injected by the granite. Hence, although the two rocks 
are always perfectly distinct in their Se relations, ob- 
serving an invariable sequence, it is, for considerable areas, а 
hopeless task to trace or define the кли of either sepa- 
rately from the other; and it is for- this reason alone that they 
are not distinguished on the m: ap. Any boundaries that. might. 
be drawn upon the map would have but ve ry little significance, 
since in no ease could they be either exclusive or inclusive. The 
granite very largely predominates; and tho diorite, except ав 
isolated inclusions in the granite, is quite restricted in its distri- 
bution. It has been observed chiefly in the ledges south of Co- 
hasset Harbor, north and west of Little Harbor and along the 
shore between Little Harbor and Nantasket. The large ledge 
south of the Cove and west of the Gulf — Kent Rocks — 
18 mainly granite, but encloses considerable diorite, while the 
pointed, rocky hill immediately east of the Gulf — Government 
