160 The American Geologist. September, 1897 
rocks, of thermal and chemical action, of erosion and glaeiation. 
The park is situated in the northwestern corner of Wyo- 
ming, halfway between St. Paul and the Pacitic — a thousand 
miles from both. It embraces more than thirty-five hundred 
square miles — more land than is contained in Delaware and 
Rhode Island together. Its average elevation is eight thous- 
and feet above sea level and it is thus one of the domes of the 
United States. It is surrounded by mountains. On the south- 
west are the Tetons, containing some of the grandest and high- 
est peaks of the Rockies, rising nearly seven thousand feet 
above lakes at their base, and composed of Archaean gneisses 
and schists flanked by Paleozoic sediments on the north. On 
the southeast border the Wind River mountains which are 
composed largely of Cretaceous sandstone that protrudes in 
the park above the lava flows. The Absarokas, whose culmi- 
nating peaks are compact andesite and andesite breccia, ex- 
tend along the east ; and the Snowy mountains present their 
granitic and schistose faces along the north side. On the west 
for twenty miles stretch the Gallatin mountains which rise 
from two to four thousand feet above the park. Moisture- 
laden winds entering the park strike against the sides of the 
mountains and precipitate their snow and rain. Consequent- 
ly the rainfall is much greater and the temperature much low- 
er in the park than in any other place in that part of the 
country. It measures at Mammoth Hot Springs twenty-five 
inches yearly. Halleck gauged the Yellowstone and Lamar, 
the Firehole and Gibbon, and estimated the Gallatin, and 
found that the minimum discharge would equal that of a river 
Ave feet deep and one hundred feet wide flowing at the 
rate of three miles per hour. This large amount of water ex- 
plains the presence of the Yellowstone lake which is the larg- 
est body of water at that elevation known (being one hundred 
forty square miles in extent) and partly accounts for the 
rapid erosion by the rivers. The temperature being lowered 
by the evaporation of so much water explains the prodigious 
chiselling of the region by frost. While the country east of 
the park was suffering under the terrible heat of August 1896, 
we in the park were breaking the ice in our water pails. 
Thus the topography of the country influences the rainfall 
and the rainfall influences the topography. 
