Perry.] 1 14 [February 28, 
simply of New England, to say nothing of the addition required, on 
account of the extension of the great ice-sheet far to the north, and 
perhaps much further southward. I would not intimate that this view 
affords any exact measure of the single portion of the Ice period now 
in question, it simply suggests one of the many elements which are 
to help us in our study of their relative duration. If now it be granted, 
as I think it should, that the presumed rate at which the ice-sheet 
moved is perhaps far too great, that its actual movement was possibly 
no more than one fifth as rapid ; that, as many suppose, the great ice- 
mass must have been at least 10,000 instead of 6,000 feet thick; that 
thus the closing Glacial period was 85 or 40, or even only 30 thousand 
years in leneth: that the opening portion was-as long as the closing, 
and that the middle equalled, or exceeded both the others, —it is evi- 
dent, after making every necessary deduction, that the Later Glacial 
times, in New England, must have been of very considerable duration; 
and that the facts, as adequately explained, are likely to require a far 
greater lapse of time than has been ordinarily admitted or supposed. 
The location of perched rocks may be next considered. This point 
is deserving of attention, both as requiring an explanation, and as 
furnishing another element in the computation of time. We have al- 
ready seen that huge boulders are often found perched on the sum- 
mits of isolated hills; and that, in many such instances, they are in 
far greater number than in other localities of equal area. How came 
they to be in this position, and why do they occur in such profusion? 
Not being able to explain these seeming anomalies by reference to any 
known agency of icebergs, we are to look for another cause. Without 
dismissing the action of ice altogether, I would ask (since it seems to 
have played an important part during this period) whether there he 
any other form, under which it could have led to such an aecumula- 
tion and deposition of erratics? 
Most have doubtless noticed, as they have traversed bogey grounds 
in the spring of the year, that the ice thaws soonest in the spots which 
have underlying hummocks. The rays of the sun are attracted to 
these points; indeed, they seem to concentrate their main force in 
such places: the heat-rays, as transmitted by.the transparent medium, 
are absorbed by the dark subjacent mould, and thus a speedy melting 
of the overlying ice ensues. There is accordingly an earlier melting 
in some spots than in others. Let us now suppose the whole country 
covered by a glacial sheet: the hill-tops and mountain peaks would be 
like uprising hummocks beneath the surface of the wintry garment. 
