17 
[Davis. 
at its maximum extension and volume by depression of the land 
on wliiclj it lay, and to have witnessed, during the retreat and 
removal of its load, a progressive re-elevation of the same area to 
its present height. 
Prof. W. M. Davis said : The most interesting question for 
the consideration of our members here present, most of whom are 
not actively engaged in geological study, is not so much hoAv 
drurnlins are made, but why should there be any difference of 
opinion regarding their origin in the face of a carefully argued 
explanation, such as has just been presented by Mr. Upham. 
In geometrical analysis, demonstrations have held true for 
thousands of years, because they are founded on accepted axioms 
and reached by a thoi'oughly logical sequence of deductions, from 
no one of which there is any alternative escape. In geological 
analysis, the postulates that serve us for axioms probably have 
somewhat different forms in different minds ; the sequence of 
argument by which conclusions are sought contains many alter- 
native possibilities, because it involves many matters not fully 
demonstrable and therefore susceptible of various interpretations. 
Geological arguments lead only to probable conclusions, and their 
safety must be measured not only by the logical secpience of the 
steps, on every one of which there might be several other turns 
taken besides the one that is followed, but also as by the degree of 
accordance found between the consequences reached through the 
argument and the facts with which they may be confronted. 
Beginning by agreeing with Mr. Upham in our general concep- 
tion of the former presence of an extended ice-sheet over the 
drift region of northern North America, we continue to agree 
essentially regarding the faster motion of the surface than 
of the bottom layers of the ice ; we then differ more or less 
regarding the effectiveness of an upward movement result- 
ing from the differential velocities of the ice, by means of which 
the su1)g]acial drift becomes englacial in greater or less amounts, 
and to greater or less heights ; we consequently differ still more 
regarding the amount of drift possibly accumulated on the surface 
of the ice during its melting, and regarding the subsequent con- 
centration of this surface drift upon various relatively small parts 
of the melting ice-sheet. Thus from beginning together, we come 
PROCEEDINGS B. S. N. H. VOL. XXVI. 2 JUXE, 1893. 
