LAE VMOLARIMGAN AND ITS SIGNIFICANCE 531 
in their upper parts the two are separated by the glacier erosion. 
The upper canyons were occupied by glaciers, and the compari- 
son of these upper with the lower canyons shows that while their 
forms were modified, and perhaps their depths somewhat 
increased, they were not made by thisagency. This was mainly 
the work of the post-Tertiary but pre-Glacial times, z.¢., of the 
Ozarkian. But the glaciation serves to show how much has 
been done by water since the Glacial,z. ¢., during the present 
times; and we find it very insignificant. 
In conclusion, no one who sees and reflects upon the prodi- 
gious work done in the Sierra since Tertiary times can resist the 
conviction that even making all due allowance for exceptional 
energy and rapid work, determined by high slope and also for 
other causes of rapid work explained in a previous paper,’ the 
time necessary must have been enormous—many times as great 
as any reasonable estimate of the duration of the Glacial epoch. 
II. SOME MODIFICATIONS OF MY PREVIOUS VIEWS. 
In the article already alluded to, on the “Relation of Land 
Elevation and Ice Accumulation” I attempted to reconcile the 
two antagonistic views in regard to the attitude of land during the 
Glacial epoch. According to some writers the land in high latitude 
regions was greatly elevated ; according to others it was on the con- 
trary depressed. According to the one, the intense cold and ice 
accumulation was the direct result of the elevation, and a subse- 
quent depression produced a moderation of the climate and a 
melting and final disappearance of the ice ; according to the other, 
the ice accumulation was not coincident with the elevation and, 
therefore, there must have been some other cause for the cold and 
ice accumulation. In the paper referred to I showed that all the 
phenomena might be satisfactorily explained by supposing that 
the elevation was the cause of the cold—the cold the cause of 
the ice accumulation— the weight of the accumulated ice the 
cause of the depression—the depression the cause of returning 
* Origin of Transverse Mountain Valleys, Univ. Chronicle, Vol. I, No. 6, p. 179, 
1898. 
