— 
— a 
21 
ing directly upon it so clearly and instructively exposed. On 
the other hand, the eccentric position of Nantasket with reference 
to the basin as a whole renders it a natural if not a necessary 
inference that the beds which are here seen to repose upon the 
granite are not the lowest or oldest sediments of this series; 
but these must be sought farther north, along the axis 
of the basin, which, according to the view of a progressive sub- 
sidence during the deposition of the conglomerate and slate, 
must be the deepest and oldest part of the trough. 
Throughout the Nantasket area the true conglomerate, as 
distinguished from the tuff, is largely composed of granitic 
detritus ; but this material is especially prominent where the 
lower beds of conglomerate lie immediately upon the granite. 
The volcanic rocks intercalated in the sedimentary series include 
three principal kinds: melaphyr, melaphyr-tuff and porphy- 
rite; and, as will appear in the detailed descriptions, the evidence 
for the contemporaneous origin of these igneous sheets is well 
nigh perfeet at every point. — It has been found impracticable, 
in the main, to determine satisfactorily the positions of the vol- 
canic vents. It appears probable, however, that the eruptions 
were chiefly submarine; while the tuffs would seem to indicate 
that the lavas issued to some extent from craters rather than 
fissures ; and we may well suppose that these ancient igneous 
channels, of whatever form, are still buried beneath their own 
ejectamenta. It will be shown, however, that the source of the 
more acid lava, the porphyrite, is probably indicated in part at 
least by the dikes of porphyrite along the Cohasset shore; and 
of the oldest melaphyr by the great dike of melaphyr (2) in the 
western area. s 
Briefly stated, then, the history of the Nantasket strata is sub- 
stantially as follows : The district now known as the Boston Basin, 
which had previously been a land surface, experienced a gradual 
but profound subsidence, during the progress of which the sea 
slowly encroached upon what is now the Nantasket area. The 
abundant detritus which must have resulted from the sub-aerial 
chemical decay of the granitic and other crystalline rocks of this 
region during long preceding ages — a thick sheet of seden- 
