(Uncial Erosion. — I'arr. 151 
planes. Here also the upper portions move along more rapidly 
than those below, and it is conceivable that, in certain areas 
where the drift was thick, the bed rock may have been scarcely 
eroded, being protected by the mantle of drift. 
For this reason it is questionable whether the rate of motion 
observed in the Greenland glacier at its margin can be used as 
a basis for comparison since this is the motion of the upper 
part and may not even approximately represent the rate of 
motion of the lower portion where erosion is in progress. 
Movements measured along the margin of a continental 
glacier are of hardly greater value since they may represent 
the exagerated rate of flow of an ice mass crowded into a 
limited area, hence giving a more rapid rate than is normal 
for the main ice sheet. If the measurements are made upon 
an elevation the rate may be too slow. It ma}' in reality require 
an entirely different amount of time for a rock fragment held 
in the base of an ice-sheet to move over a certain distance 
than one would be led to infer from a study of the Muir 
glacier or the Greenland glaciers. 
These various complex factors make a time estimate of 
little value ; } T et, although I find myself unable to acccept his 
line of argument and conclusions, I believe Mr. Uphani's es- 
timate is much nearer the truth than those which call for a 
long period of time. When the time for the departure of the 
ice-sheet arrived, it had not removed the material from the 
region of greatest activity, and had accumulated a mass of 
drift in the peripheral region, having arrived at no greater 
topographical age than that of youth or early maturity, or 
what we may call the stage of adolescence, to use another of 
the terms employed by Prof. Davis in his classification of 
rivers. Much work was still to be done when the glacier de- 
parted. So far as there are indications of value, it seem- to 
the writer that they point to a slower rate of erosion than 
that assumed by Mr. I'pham in his estimate quoted above 
and to a smaller total. Accepting the rate of erosion to be 
that of the Muir glacier, about three-fourths of an inch yearly, 
in 10,000 years the continental ice-sheet would have removed, 
on an average, four hundred feet of rock in the zone of 
greatest activity which he assumes to be near the ice margin. 
Hut the perfection of preglacial topography, even in minor 
