135 
is, however, a sufficient indication that outflows of lava and, 
very probably, the deposition of conglomerate had already 
begun in the deeper parts of the basin; and we may fairly 
suppose that both conglomerates and lavas older than any 
exposed in southern Nantasket underlie at a great depth the 
northern part of the peninsula. Eventually the slow subsidence 
of the land then in progress carried the shore-line over and 
beyond the Nantasket area, and the so-called basal conglom- 
erate was formed upon the uneven and fissured surface ofthe 
granite. This conglomerate had attained a maximum thickness 
of perhaps 50 feet, when the first flow of melaphyr was spread 
over it. This was probably originally an ordinary compact, 
black lava; but through subsequent alterations it has become 
greenish and purplish, compact and jaspery. The lava-flow 
being submarine, the sedimentary process was uninterrupted ; 
and the uneven and scoriaceous upper surface of the melaphyr 
was slowly covered by the second conglomerate, which is 
largely composed of debris derived from the melaphyr, and like 
this melaphyr is especially characterized by the segregations of 
red jasper. When the second conglomerate had attained 
approximately the same thickness as the basal conglomerate, 
a flood of more acid lava (porphyrite) was spread over the sea- 
bottom to a depth of from 50 to possibly more than 100 feet ; and 
all the porphyrite in the Nantasket area probably belongs to 
what was once one continuous flow, increasing in thickness 
astward, or toward the vent from which it issued. Over 
the porphyrite was gradually accumulated the third conglom- 
erate to a thickness of from 50 to 100 feet or more, but termi- 
nated at last by a comparatively thin flow of highly basie and 
vesicular basaltic lava, which is recognized now as a typical 
green and amygdaloidal melaphyr. The fourth conglomerate, 
with a thickness of from 20 to 30 feet, separates this second 
melaphyr from the very similar third melaphyr. The latter 
consists, however, of two flows, having an aggregate thickness of 
from 40 to 50 feet. Once more the beach conditions prevailed, 
