22 
tary soil such as may now be observed in low latitudes — was 
rapidly worked over by the advancing tide to form extensive 
beds of gravel and sand on the beaches and near the shore, 
while the clayey constituent of the soil was carried far out into 
the deeper waters of the bay. 
Not long, apparently, after the formation of these strata 
began, and as a result, probably, of the same agency that gave 
rise to the subsidence, volcanic phenomena were instituted in 
different parts of the Boston Basin ; and for a long time Mas- 
sachusetts Bay must have borne some resemblance to the mod- 
ern Bay of Naples. But since, as already stated, the eruptions 
were largely submarine, this region doubtless exhibited a still 
more perfect combination of aqueous and igneous activity. 
Sheets of basic and sub-acid lavas of greatly varying thickness 
and extent were poured out over the newly formed beds of gravel 
and sand and were in turn covered by these ever-growing de- 
posits, the overlying gravel enclosing many fragments worn 
from the lava itself. The different flows of lava represented 
in the same vertical section are usually separated by considera- 
ble beds of conglomerate ; but sometimes the eruptions suc- 
ceeded each other so rapidly as to produce composite beds of 
lava, or lava and tuff, several hundred feet thick without any 
interlarded sediments. Some dikes of porphyrite and at least two 
of melaphyr probably date from this period and may, perhaps, 
be regarded as channels through which the lava reached the sur- 
face. But the great majority of the dikes traversing the conglom- 
erate and melaphyr are diabase and appear to have been formed 
long subsequently, when, after the upper conglomerate and a 
great thickness of slate had been quietly deposited over this area, 
the strata were tilted up and extensively faulted. The geologic 
revolution marking the close of this period in the history of the 
Boston Basin was not attended by severe plication in the Nan- 
tasket area; but faulting is by far the more prominent structural 
feature. The subsequent history of this area, during all the 
long ages down to the glacial epoch, is recorded only in the 
quiet, erosion which has swept away the great slate series and, 
at some points, the entire thickness of the conglomerate and 
interbedded lavas. 
