J. LeConte—Ancient Glaciers of the Sierras. 339 
agent and because it tends, under all circumstances, to deter- 
mine forms of this kind. 
s the glacial period waned, the universal snow sheet 
retreated to the summits, and the slopes of the Sierras were 
now occupied, east and west, by great but separade glaciers, sev- 
eral of which I have attempted to describe. ese in their 
turn retreated upward and took refuge in the snow fields, nes- 
tled amongst the peaks and amphitheaters of the highest sum- 
mits. There, their still remaining but feeble existence has 
escaped observation, until revealed by the indefatigable indus- 
try and keen scrutiny of Mr. Muir. 
During the fullness of the glacial period, I suppose glacial or 
moutonnéed forms covered every portion of the Sierras, even to 
€ summits. These forms have not been retained on the 
summits, because they have been subsequently modified by 
continuance of snow in this region. The evidences of univer- 
sul glaciation, therefore, must be looked for only on the higher 
slopes. 
tous just before reaching the slate. w general this is I do 
not know, but the fact that all these cafions, with nearly verti- 
cal walls, have been occupied by glaciers, makes it almost cer- 
tain that they have all been formed by this agency. If Yose- 
mite were unique, we might suppose that it was formed by vio- 
lent cataclysm: but Yosemite is nol unique in se and there- 
fore probably not in origin. There are many Yosemites. It is 
more philosophic to account for them by the regular operation 
of known causes. I must believe that all these deep perpen- 
dicular slots have been sawn out by the action of glaciers; the 
peculiar verticality of the walls having been determined by the per- 
pendicular cleavage structure before spoken of. 
* Geol. Survey, vol. i, p. 421. Yosemite Guide Book, p. 74. 
