138 GEOLOGICAL SUEVEY OF THE TERKITOEIES. 



I ascended one of the liigbest, though not the highest, and found it 10,950 

 feet. The general average of these peaks is about 10,000 feet. The 

 summits of these high peaks are all close, comijact trachyte, while all 

 around the sides are built up walls of stratified conglomerate. It is plain 

 that all of them are the nuclei of old volcanoes. .The trachyte may 

 sometimes be concealed by the conglomerates, but I am inclined to think 

 that each one has formed a center of effusion. Large quantities of sili- 

 cified wood are found among the conglomerates, mostly inclosed in the 

 volcanic cement, evidently thrown out of the active craters with the 

 fragments of basalt. My impression is, that when the old volcanoes 

 disgorged their contents into the great lake of waters around, they 

 threw out also portions from the sedimentary formations, and thus the 

 silicifled wood comes from the Tertiary or Cretaceous beds, which may 

 have formed the upper part of the walls of the crater. At any rate, these 

 woods belong to the Coal Series of the West, and they are scattered pro- 

 fusely among the conglomerates. Interlaced among tbe massive beds of 

 volcanic conglomerates, are some layers of a light-gray or whitisb, sandy 

 clay, which show that the whole breccia or conglomerates, with the inter- 

 calated layers of clay or sand, were deposited in water like any sedi- 

 mentary water rocks. 



Upon the east branch are a few interesting ruins of springs. There 

 is one very curious mammiform mound, about forty feet high, built 

 up by overlai)ping layers, like the "Cap of Liberty" on Gardiner's 

 Biver, only by much less hydrostatic force. The material is principally 

 calcareous. This cone is a complete ruin, l^o water issues from it at 

 the present time, and none of the springs in the vicinity are above the 

 ordinary temperature of brook- water; sulphur, alum, and other chemical 

 deposits are abundant. This old ruin is a fine example of the tendency 

 of the cone to close up its summit in its dying stages. The top of 

 the cone is somewhat broken ; but it is 18 feet in diameter at this time, 

 and near the center there is a hole or chimney 2 inches in diameter, 

 plainly a steam-vent. This marks the closing history of this spring. 

 The inner portions of this small chimney are lined with white enamel, 

 thickly coated with sulphur, which gives it a sulphur-yellow hue. The 

 base upon which the cone rests varies in thickness. On the east side 

 huge masses have been broken off, exposing a vertical wall 20 feet high, 

 built up of thin horizontal laminse of limestone. On the west side the wall 

 is not quite as high, perhaps eight or ten feet. It would seem, therefore, 

 that it was at first an overflowing spring, depositing thin horizontal layers, 

 until it built up a broad base ten to twenty feet in height; then it gradu- 

 ally became a spouting spring, building up with overlapping layers like 

 the thatch on a house, until it closed itself at the top and ceased. 



Wemay inquire again in regard to theorigin of the lime in this cone. Not 

 over a mile below the spring, the Carboniferous limestone comes to the 

 surface, and as we follow the river down toward its juncture with the 

 main Yellowstone, it soon becomes 400 feet in thickness; hence we 

 know that these limestones extend under the valley of this east braiwph, 

 and that the waters passed up through them, and thus we have a pre- 

 dominance of lime instead of silica, as is the case at Gardiner's Eiver. 

 Over this limestone the basaltic rocks have been poured, rising to the 

 height of 2,000 or 2,500 feet above the valley. Immense quantities of 

 the broken fragments of basalt have fallen dov/n on the sides of the 

 mountains, and, by their bright black color, look like heaps of anthracite 

 coal in the distance. About five miles below the junction of the two 

 branches of the East Fork, the mountains on the east side become quite 

 rounded and grass-covered, instead of the bald, black, rugged character 



