Davis.] 20 [Nov. i6, 
rubbed over by a new adviuicc of tlie ice mid tlius deprived of 
tlieir more or less angular or water-woriillforins|andf^of tluMr more 
or less stratified arrangement, and given the features of true till ; 
and finally still further reduced in height by the melting away of 
the lower layer of old ice beneath ; and all this inside of a dis- 
tance of two miles. The present altitude of the summit of College 
Hill over the lowland to the north of it, whence the slates must 
have been dragged iip, is perhaps two hundred feet; and yet this 
is only a remnant of the altitude to which the fragments must have 
been raised in differential motion of the ice, and from which they 
must have been lowered by melting, washing, rubbing, and settling. 
The occurrence of numerous large boulders of amygdaloid in the 
till of a large drumlin in Brookline might be adduced as offering 
similar difficulties : the ledges of amygdaloid from which the 
boulders come are not far distant, and it is incredible that they 
should have been so greatly raised and lowered during so short 
a horizontal distance of transportation. Further than this, the 
presence of a large share of fine clay in the till of drumlins seems 
to me entirel}^ inconsistent with its supposed derivation from 
washed accumulations of superglacial drift : the clay would be 
mostly washed away during the slow discovery of the englacial 
drift as the ice melted. 
The process of verification is, therefore, so incomplete at present 
that I can only regard Mr. Upliam's explanation as a suggestion 
or tentative hypothesis, taking rank along with the several other 
tentative hypotheses previously invented by others in working on 
the same problem. Perhaps only one of these hypotheses is shown 
to be absolutely impossible ; certainly not one of them is fully 
verified as the com]>etent and chief process of drumlin formation. 
B\xt if 1 understand Mr. Ui)ham aright, he is not satisfied that 
his explanation should take its place simply as a possible process 
along with other possible processes. He regards all other ex- 
planations as entirely inapplicable. This seems to me an nnfoi-tu- 
nate position in the present stage of our inquiry. Drundins are 
accurmdations of till, of peculiar thickness and form : it is unsafe 
to assume that they were all made in a single way. All varieties 
(»f processes that contribute to such a result should be welcomed 
until a denvonstrative process of argument reduces their nund)er 
by exclusion. The particular pi-ocess that has seemed to me the 
most plausilde, although I am always overcome by its difficulties 
