THE ROCKS OF NANTASKET. 
GENERAL RELATIONS AND ORIGIN. 
Although the surface exposures of the rocks of the Nantasket 
area are completely isolated by drift deposits and the sea, these 
strata are probably continuous to the north and west with the 
great body of sediments occupying the Doston Dasin. And it 
is certain, as will appear later, that they are terminated on the 
south by profound dislocations; so that the sharply defined 
boundary between the Nantasket sediments and the broad area 
of granite can not be regarded as marking the true original 
border or maximum extension of the Boston Basin in this direc- 
tion. On the contrary, the facts point very plainly to the con- 
clusion that the basin rocks formerly extended a considerable 
but undetermined distance beyond this line; being here, how- 
ever, on the upthrow side of the great faults, they were lifted 
above the present plane of erosion. We may, nevertheless, en- 
tertain the hope that future investigation will reveal upon the 
granitic plateau outlying remnants of the sedimentary series, 
and thus indicate more exactly the original limits of the basin. 
The Nantasket rocks, above the fundamental granite, and 
omitting the dikes, consist chiefly of the conglomerate (Roxbury 
pudding stone) and the interbedded lavas and tuffs. The 
incompleteness of the Nantasket section is plainly shown in the 
entire absence of the great slate series, which elsewhere in the 
Boston Basin overlies the conglomerate ; and it is probable, as 
will appear when the facts are presented, that the upper members 
of the conglomerate series are also wanting. The student will, 
however, find compensation for these deficiencies in the magnifi- 
cent development of the basal beds of the conglomerate. At no 
other point are the floor of the Boston Basin and the strata rest- 
