1872,] | 101 [Perry. 
~ at lower elevations may have originated in the closing Glacial period, 
and thus would for the most part occur only in valleys, or depres- 
sions occupied by local ice-streams. Remembering that all the cle- 
vated pot-holes of this kind, which are in some cases of considerable 
depth, must have been eroded when the ice was immeusely thick, 
we may get some, though it be a very inadequate impression of the 
the probable duration of the Middle Glacial times.! 
The view of the matter just presented, if it be substantially cor- 
rect, reveals the non-sequitur of much that Professor Dana advances 
on this phase of the subject. Referring to pot-holes at a low level, 
he observes? “These pot-holes must have been made by torrents 
from the land. For the existence of such torrents the land [from 
which the torrents flowed] should have been above its present eleva- 
tion. . . The era of this higher level. . . may have been that of the 
ereat glacier, and the torrents sub-glacier streams.” Respecting 
these statements it may be said, in the light of what has been ad- 
vanced, first, that there is no such necessity, as the author supposes, 
for land-torrents in the usual sense of the expression; none for the 
asserted elevation of the region in order te the production of a tor- 
rential stream; that the work, since it is at a low level, was doubt- 
less wrought during the later Glacial times; that in some instances, 
in which there was a rapid descent of land, pot-holes were possibly 
worn by sub-glacial torrents; that in most cases, and probably those 
cited in the paper under review, this was effected, not by sub-glacier, 
but by super-glacier streams falling into breaks in the ice-mass. This 
aspect of the matter should clearly remove all the necessities implied 
in Professor Dana’s suppositions. So, according to the phase of the 
subject now presented, there appears to be no occasion for the 
1 As illustrative of the great duration of the Middle Glacial period, I may cite a 
single item of evidence. In Orange, N. H., on the summit ridge between the Con- 
necticut River and the valley of the Merrimac, more than 1200 feet above the 
ocean, there are the remains of a pot-hole, which, according to Professor Jack- 
son (feport on the Geology of N. H., p. 118), was eleven feet in depth. This pot- 
hole was clearly worn during the continuance of the main ice-mass, Post-glacial 
pot-holes may be seen in a similar granitic rock at Bellows Falls, Vt. Of these 
the deepest are only five feet in depth. Professor Jackson accordingly infers, 
“that if the pot-holes on the mountain top (in Orange) were excavated by a water- 
fall of the samme power as that of Bellows’ Falls,’’ (and if it were by a glacial 
stream, it was probably of far less power,) there must have been required for its 
formation ‘‘ more than twice the length of time that the present Connecticut has 
coe running.” See Proceedings of Am. Ass. Ad. of Science; 4th meeting, 
Dp. : 
* Paper cited, p. 62. 
