184 
(157) is the only really doubtful one. This is the dike shown 
in Fig. 19. In tracing it across the ledge to the mainland, its 
sasterly trend, hade, and thickness are all greatly. increased; 
and it would, perhaps, be best to class it as a highly aberrant 
member of the first system. 
FIG. 19.— DIKEZ157 CUTTING A LEDGE OF GRANITE, NEAR WHITE 
HEAD, ом Hominy POINT, Conasser. 
SEQUENCE OF EVENTS RECORDED IN THE NANTASKET 
LEDGES. 
It is unnecessary to review or summarize farther the evidence 
supporting the conclusions that have been reached in the preced- 
ing pages concerning the succession of the beds of conglomerate 
and the flows of lava; but we may fitly close this section with 
a general statement of the geological history of Nantasket so 
far as it is recorded in the hard rocks. The granite, with the 
associated diorite and felsite, is the fundamental rock in this 
part of the Boston Basin; and it had, apparently, been 
exposed to erosion for a long time before the basal conglom- 
erate was deposited over it, The character of this conglomerate 
