125 
The three-foot dike (67) so clearly exposed on the north 
shore of Melaphyr Peninsula and traceable across West 
Porphyrite Hill certainly does not extend so far in this line as 
Granite Plateau, probably ending under the water against the 
N. E.-S. W. fault. Nearly 200 feet west of this line, on the 
north shore of the plateau, is 68, which сап be traced about 
150 feet back from the water; and about 500 feet farther west, 
following the shore, is 69, of which only about 50 feet in 
length are exposed. 
DIKES ON TIIE COHASSET AND SCITUATE SHORE. 
Dikes are abundant in. the granitic rocks south of Nantasket 
and the boundary faults ; in faet, almost every large ledge or 
considerable exposure of the granite and diorite shows upon 
careful examination one or more dikes of diabase. As a rule, 
however, except along the shore, where the almost continuous 
ledges are clean and bare, the outcrops of the dikes are obscure 
and сап be correlated only to a limited extent; 7. e., the same 
dike can rarely be traced with certainty from one outerop to 
another. And when we further consider that the dikes 
naturally tend, through their more rapid erosion, to follow the 
depressions and the drift-covered portions of the surface ; and 
also that the drift-deposits and marshes are continuous over 
large areas, especially toward the south and south-east, the 
futility of attempting to trace out or map the dikes of Cohasset 
and Scituate becomes apparent. In the more limited Nantasket 
area the conditions are comparatively favorable for such 
thorough work ; but it is probable that even here many dikes are 
wholly concealed, and the map claims to represent with only 
approximate accuracy and completeness the dikes which are 
actually exposed. Particular attention was given to the Nan- 
tasket dikes, because it was seen that the outcrops are sufficient 
to permit of their correlation with the faults of the district and 
in systems of different ages; and about all that it has seemed 
wise to attempt in the study of the dikes of Cohasset and 
