MUSEUM OF COMPARATIVE ZOOLOGY. 37 



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 shatteriug 

 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 X. 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 peculiarities 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. 



Tlie perfection of the principal train indicates that it, at least, was 

 deposited at the time of the melting away of the last general ice-sheet 

 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 been exposed 



