240 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
first steps in the solution of such a problem should be the attempt 
at restoration of the geographical conditions of the time. The rocks 
of all three basins are now extensively folded and faulted and often 
dip at high angles. Originally they must have been nearly horizontal, 
or at least ahil slightly inclined, though in consequence of coincident 
subsidence of a floor of deposition, or because of varied conditions 
of sedimentation, the lower beds may have been of steeper inclination 
than the upper. If the present deformed floors of the several basins 
could be straightened out, the overlying strata would form a great 
mass, probably extending across the granitic areas now separating the 
basins and attaining a length of at least 60 miles and a breadth of 
30 miles. The figures for thickness given above show that there 
would be under these conditions a diminution in the elevation of the 
restored mass northward from 12,000 feet to perhaps 5,000 feet or less. 
Since it is certain that great erosion has taken place over the entire 
area subsequent to the disturbance of the strata, it is probable that 
the limits of the restored mass would greatly exceed those above 
outlined. 
The apparent northward diminution in thickness may be due 
either to original difference in the depth of the deposits or to erosion, 
for the floor of the northern sediments may not have been carried so 
far beneath the base-level of erosion as in the case of the southern 
sediments and a consequently greater proportion of the northern 
deposits may have been removed. ‘The more marked development 
of overthrust phenomena in the northern basin, however, lends sup- 
port to the idea that the sediments in that region never attained the 
thickness reached by the southern strata; for: Willis has shown that 
in the Appalachian region the greatest faulting has occurred where 
the thickness of strata is not so great (Willis, a, p. 269). 
Any restoration of the strata must include the restoration of the 
neighboring land areas from which their materials were derived. 
It has been shown (page 169) that the increasing coarseness of 
the Dighton or Purgatory Conglomerate toward the south, together 
with the increase in the percentage of quartzite pebbles in that direc- 
tion, indicate the probable occurrence of land at the south, no longer 
extant, while the increase in quantity of muscovite-granite pebbles 
toward the north or northwest indicates that the muscovite rocks of 
the crystalline area northwest of the Boston Basin were exposed 
to erosion at the time of the deposition of the upper conglomerate. 
The occurrence of granite pebbles eight or ten inches in diameter 
at Attleboro within the Narragansett Basin, and twenty or thirty miles 
from present occurrences of muscovite granite, indicates that the 
