338 J. LeConte—Ancient Glaciers of the Sierras. 
that, favored by the great altitude of the Sierras, the universal 
ice-sheet itself was extended along this chain at least to Mid- 
dle California, lat. 37°, and probably even to Southern Cali- 
fornia, lat. 33° or 34°. The flow of the ice-sheet in the Sier- 
ras was probably determined by the slope of this great chain, 
rather than by the arctic elevation; it was eastward and west- 
ward from the crest, rather than southward alung the range. 
This general ice-sheet stretched out finger-like projections east- 
ward and westward in the form of separate glaciers, far down 
the lower valleys. 
he former existence of such a flowing ice-sheet on the Sier- 
ras is, I believe, proved by the domes and dome-like forms so 
abundant on the higher slopes of the Sierras, especially about 
Yosemite. These are the roches moutonnées of an ice-sheet 
their forms. ile the great mass of this enormously thick 
sheet flowed in one general direction with a steady current, 1ts 
lower portions, or portions in contact with the earth, doubtless 
conformed more or less to the greater valleys. 
The actual forms of a glaciated surface are determined not 
only by the nature of the eroding agent, but also by the char- 
acter of the material eroded. This latter factor is extremely 
variable. Sometimes it is the dip of strata or of cleavage, 
sometimes relative hardness in different parts, sometimes 1t 1S 
other kinds of structure, which have the controlling influence. 
The slopes of the Sierras, where the general erosion cannot be 
a 
oS cleavage. The former is beautifully seen in_ the 
out taking vee ponte structure into account. The origin 0 
Yosemite we wi 
centric 
- would 
would do 80 more perfectly, both because it is a more powerf 
