MUSEUM OF COMPARATIVE ZOOLOGY, 97 
been referred to, and this diminution is accompanied by a decrease in 
the number of boulders. It is common to see two boulders side by side 
which evidently were once parts of the same boulder, the fragments 
having been separated from each other by the action of frost. The 
fissure by which the frost entered was probably started by the shattering 
action of the ice-sheet, while the mass still formed part of the bed-rock, 
and undoubtedly many, if not all, of the fragments torn off by the ice 
contained incipient fissures, which were afterwards developed by the 
crushing force of the glacier. This being the case, the fragments would 
be crushed finer as they receded from their source, and the diminution 
in size, which has been alluded to, would be brought about. 
Under the influence of gravity, the imbedded fragments would be con- 
stantly working down towards the under surface of the ice-sheet, and 
thus, the farther from the source the greater the amount of material 
lost from the trains and added to the material destined to be ground up 
beneath the ice. 
On looking at the map, it will be observed that the general direction 
of the ridges and valleys is N. N. E. and S. S. W. It has been sug- 
gested to the writer by Professor Shaler that, although the stratigraphi- 
cal relations of the limestones and schists have, in the main, determined 
the topographical features of the region, there are peeuliarities in the 
present contour of the surface which cannot be accounted for by strati- 
graphical and ordinary erosive agencies, and that the mountain ranges 
and valleys give evidence of having been largely molded by glacial 
action. Professor Shaler considers that the direction of glaciation at 
that time was much the same as the present course of the ridges, namely, 
from N. N. E. to S. S. W., and thinks it likely that the shape of these 
valleys may indicate that in glacial periods anterior to the Post-Pliocene 
age the form of the continent was such that the ice found its outlet 
towards the region lying to the west of south, rather than towards the 
southeast, as in the last glacial period. 
The perfection of the principal train indicates that it, at least, was 
deposited at the time of the melting away of the last general ice-shect 
which covered the region, since any glacial sheet moving over the 
region after the deposition of this train would have obliterated it. 
There may have been local glaciers in the subordinate valleys at a 
subsequent period, though no evidence of them was observed. As 
remarked by Professor Shaler, such glaciers, had they existed, would 
not have interfered with the continuity of the principal train, since it 
crosses the main valleys upon “cols,” where it would have beon exposed 
