Shaler.] 
260 
[March 4 
The finely sculptured drumlins and the frail kames while 
overlaid by several feet of wet and spongy decaying vegetable 
matter would feel none of the influences of the frost, and so far 
as they were composed of siliceous material would be practically 
exempt from chemical decay. It is not reasonable to compare 
the action of atmospheric agents on these surfaces when cleared 
by the artifice of man with those which prevailed during the 
times before the settlement of the country by Europeans. There 
are good reasons to believe that many of our mountain tops and 
other elevated districts, which are now bare of vegetation, have 
thus been exposed within two centuries, particularly through the 
spread of forest fires. Thirty years ago the summits of the 
Mt. Desert mountains, which are now generally bare rock 
surfaces, were still covered with a mat of vegetation which has 
mostly disappeared by the frequent fires which have ravaged that 
district. Considerable areas now exhibiting glacial scratches 
were thus protected by a thick mantle of vegetable growth. 
It is easy to see that all these physical measures which have 
been applied to the lapse of time since the close of the glacial 
period are liable to give us gravely erroneous measurements for 
they all depend upon the result of exceedingly intermittent or 
variable actions. 
On reviewing the field whence information as to the lapse of 
time since the close of the glacial period can be obtained, it ap- 
pears to me that in addition to the bases of computation already 
noted, which alone have been made the subject of inquiry, there 
are two neglected classes of facts which on examination appear 
likely to afford more satisfactory measures. These are the work 
of the sea along our shores which has taken place since the disap- 
pearance of the ice and the migration of the vegetation of the 
continent from the regions of the south, where it survived the de- 
struction which the ice coating brought upon the northern part of 
the continent. 
At first sight it may appear as if the shore-line phenomena of 
the coast were not likely to afford any better basis for the com- 
putation than the ancient coasts of the lakes which have disap- 
peared by the withdrawal of the ice dams or dessication of the 
area in which they lie ; but on closer inquiry we perceive that 
there are certain reasons why the record of the duration made by 
the work done on the sea margins is on the whole more trust- 
