GEOLOGICAL SURVEY OF THE TERRITOEIES. 



51 



posed of compact trachyte, but surrounded with stratified breccia or 

 coDglomerate, jutting" up agaiust the sides and reaching nearly or quite 

 to the summits. Indeed, some of the highest peaks are apparently made 

 up of the conglomerate. We may conclude, not only that the carving 

 out of the channel of the Grand Oauon was a very modern event, but 

 that the deposition of the entire material which forms the caiion is, in a 

 geological sense, quite a modern occurrence. I'he drainage of the country 

 commenced long before the excavation of the present water-courses, but 

 it is difficult to answer the question how this great drainage was brought 

 about, unless we account for it by a general elevation of the entire 

 country, gradually sending this immense body of water, which must have 

 prevailed all over the ISTorthwest at least, perhaps all over the Eocky 

 Mountain region, westward into the Pacific and eastward into the 

 Atlantic. As the waters slowly subsided they were separated into 

 lakes of greater or less size, and then came the excavation of the Grand 

 Canon, which slowly drained the great lake-basin above the falls, so 

 that now we have only the comparati\ely small remnant, the Yellowstone 

 Lake. (Fig. 10.) Other small fragments are scattered about in the vicin- 

 ity, which now form reservoirs for the local drainage. Undoubtedly the 

 same series of remarkable physical events occurred in Oregon and in 

 California and in Idaho and-Washington Territories, and, perhaps, far 

 southward into Mexico, judging from the published reports. The Hot 

 Springs, which are now slowly dying out, are, of course, the last of this 

 series of events. The evidence seems clear that all over the West dur- 

 ing this great period of volcanic activity the hot springs and perhaps 

 even geysers were very numerous. We everywhere find the remains or 

 deposits in all the States and Territories west of the Mississippi, and 

 now and then a warm or hot spring remains to indicate the story of their 

 former power. 



Fig. II. 



WALL OF BASALT, YELLOWSTONE RIVER, AT MUD SPRINGS 



Fig. 11 is intended to illustrate the probal>le avenues through which the 

 meteoric waters pa«s down througli the rocks to the heated portions. At 

 Mud Springs, about six miles below the Yellowstone Lake, there is a 

 vertical wall of basalt, about 50 feet high. This wall is only a remnant 



