1872.] 87 [Perry. 
must slowly melt, until at last there would be left only local ice- 
streams, and these for the most part in the more elevated portions of 
the country. This process gradually going on, the long winter slowly 
retiring to more boreal regions, and its accompaniments disappearing 
at about the same rate, though with protracted pauses followed by 
sudden starts, finally even the minor valleys of New England lying 
high up on the flanks of the mountains would come to be free from 
ice. And itis this wasting of the ice which, as it occupied the clos- 
ing part, constitutes the third stage of the glacial period. 
§ 9. The Extension of the Ice-Sheet, 
Both superficial and vertical, is a matter as yet by no means fully 
settled. While not alittle has been said and written upon this pomnt— 
much perhaps that has been to no good purpose,—a great deal re- 
mains to be done in the way of careful observation of facts, of accu- 
rate estimates and critical deductions. There is meanwhile con- 
stantly accumulating evidence that the ice was probably of much 
greater extent than most observers even conjectured, only a few 
years ago. That some such agency as the glacial has wrought on the 
surface of all New England, a few isolated summits of the White 
Mountains alone excepted, now seems to be clear beyond a doubt 
to those who advocate the glacier hypothesis. Prof. Dana indicates 
that the entire continent north of about the 40° of latitude must 
have been affected by it during some portion of the drift period. As 
to whether it extended further southward, New England, so far as 
we yet know, furnishes no evidence except that implied in the esti- 
mated thickness of the ice-sheet. Had it been in great force in the 
southern portion of the Eastern States, or say in the vicinity of New 
York city, the probability is that it must not only have stretched 
along the Alleghanies, but also spread widely over the country on 
both sides of the range, and far to the south, even in the Mississippi 
Valley. : 
As to its actual thickness in New England there is, up to this time, 
considerable uncertainty. Granting that the work done in this re- 
gion was effected by glacial agency, we cannot doubt that the mass 
of ice covered almost the entire surface ; but very different estimates 
have been made as to its vertical extent. Supposed traces of glacial 
action may be now seen on the White Mountains, at points some 
5,500 feet above the existing level of the ocean. Indeed, up to that 
