Review and Comment. 
345 
rock floor not as a rigid plane but as a flexible rasp scratching 
it with the coarser and polishing it with the finer material, at 
the same time detaching and bearing away rock fragments 
which in turn score the surface beyond. ” 
Of the effects of this action on pre-existing topography he 
writes as follows : 7 
“Two general stages may, however, usually be well distin¬ 
guished, the first, in which the pre-glacial features are entirely 
dominant and the effect of drift erosion and covering is merely 
to soften and subdue the earlier surface expression; the second 
in which the pre-glacial rock contour has been thoroughly sub¬ 
dued (1) by cutting off projecting peaks, shoulders and spurs, 
and the filing of the whole down to a system conformable with 
the demands of glacial flowage . . . and (2) by the filling up 
of the valleys in various degrees according to situation and atti¬ 
tude toward the glacial movement; so that while the main 
ridges and valleys remain the same as in pre-glacial times and 
even many minor features still find expression, the whole as¬ 
pect is very markedly changed and has assumed a parallel lin¬ 
ear arrangement and a softened expression quite in contrast to 
the dendritic arrangement of ridges, and valleys, and the rough 
abrupt contour ]ines of the pre-glacial erosion type.” 
There is nothing here indicating a belief in the excavation of 
rock basins by ice, yet Prof. Chamberlin is sometimes claimed 
by the believers in that hypothesis as favoring that view. 
L. P. Gratacap 8 9 notes that in a fresh advance of the glacier 
of La Brenna in 1831 it attacked a promontory in its path with 
such vigor as to shatter it with fissures and compel the removal 
of a chapel upon its crest. This would be strong evidence if 
we knew that previous to the advance of the ice, the promon¬ 
tory was not already fissured by other agencies, and so ready 
to fall an easy prey to the advancing glacier. No such state¬ 
ment is made by this author and we cannot therefore attach 
much weight to this evidonce. 
The same writer says that lake Wakatiper in New Zealand by the 
most indisputable proofs has been dug out of the rock by ice to 
the depth of fourteen hundred feet. It is unfortunate that he did 
not state the evidence constituting this indisputable proof. 
7 Third Annual Report U. S. Geol. Survey, p. 305. 
8 Popular Science Monthly, Nov., 1878. 
9 Popular Science Review, 1879. 
