126 



GEOLOGICAL SUEVEY OF THE TEKEITOEIES. 



or local drift deposits, and thirdly the crust of silica which forms a floor of 

 greater or less thickness for the entire valley. 



The mountains which form the divide between the sources of the 

 Madison and the Yellowstone are very high and steep. After traveling 

 about 8 miles, we came to the nearly vertical sides of the main divide, 

 which is composed of trachytic basalt. Immense quantities of broken 

 rocks had fallen down at the bottom of the ridges. Little lakes occur 

 every mile or so, nestled among the pines 9,000 and 10,000 feet above 

 the sea. At the head of Fire-Hole we ascended a steep ridge, with al- 

 most vertical sides, with just room to travel, to the summit of the divide. 



Fig. 62. » 



OLD FAITHFUL, UPPER GEYSER BASIN, FIRE-HOLE RIVER. 



From this point we could look back and obtain a full view of the Madi- 

 son Yalley with its branches, and the high volcanic mountains that 

 inclose it. The mountains are gashed with deep gorges, and on the 

 sides are immense quantities of the fragments of trachyte and obsid- 

 ian. The pines grow upon the declivities of the mountains where they are 

 so steep that it would be impossible for a man ever to ascend. The ele- 

 vation of what appeared to be the highest point of our route was 9,500 feet, 

 but the general elevation of the mountain summits is about 10,000 feet. 

 It is only in exceptional cases that isolated peaks rise above that eleva- 

 tion. 

 As we descended the mountains on the east side, we saw through the 



