R. C. Hills—Extinct Glaciers of Colorado. 391 
Art. XLV.— Extinct Glaciers of the San Juan Mountains, Colo- 
rado; by R. CO. H1xts.* 
THAT portion of the Rocky Mountain range which, for the 
es of this paper, will be considered as the San Juan 
Mountains, includes the whole of the elevated region embraced 
in the Counties of Hinsdale, San Juan, Ouray, San Miguel, 
Dolores, Rio Grande and La Plata. . 5 
‘he drainage east of the Continental Divide constitutes the 
source of the main branch of the Rio Grande, while that to 
the westward includes the principal tributaries of the San Juan 
and the southern tributaries of the Grand and Gunnison. The 
volume of water conveyed into the Rio Grande is small com- 
pared with that which, flowing westward, furnishes more than 
half the total volume of the Colorado River. [In fact the vol- 
ume of the Rio Grande from this source only does not exceed 
that of either the Dolores or Animas, streams which are simply 
tributaries of the Grand and San Juan. see 
The extent of glacial action in the past seems to have been, in 
@ great measure, proportional to the magnitude of the existing 
river systems, and it is found that not only was there a greater 
thickness of ice on the western slope but the area of glaciation 
was many times more extensive. Evidence of the former 
existence of glaciers in the Rio Grande drainage area is most 
decided in the region lying west of Wagon Wheel Gap. Fora 
* A paper read before the Colorado Scientific Society, October 1, 1883. 
= _ AM. Jour. Sct.—Turrp Serres, Vou. XXVII, No. 161.—May, 1884. 
ae 26 
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