THE GREAT SIERRA NEVADA FAULT SCARP. 617 



Many of the lakes are due to the presence of the terminal moraines, 

 while others fill rock basins. 



Russell has described the terraces or gigantic stone steps by 

 which many of the canons rise, and the deep-walled cirques at their 

 heads. About Mount Whitney these features are illustrated on a 

 grand scale. The vast rocky amphitheaters at the heads of the 

 canons are bounded by precipitous walls one thousand to two thou- 

 sand feet high, while their bottoms are generally occupied by lakes 

 in basins of solid rock. 



Lake Mono is at present gradually rising, thus differing from 

 Owen's Lake. The history of these sheets of water has been a 

 checkered one. Periods of high water have alternated with those 

 of desiccation. The last high-water stage seems to have been con- 

 temporaneous with the glaciation. Well-defined terraces appear 

 about Mono Lake, indicating the various stages of the water. In 

 this region the opportunities for the study of the glacial phenomena 

 are excellent, for only a small part of it has as yet been investigated. 



What is perhaps of more interest than the glaciers is the recent 

 volcanic action so remarkably exemplified about Mono Lake. Fol- 

 lowing the andesitic flows which took place near the close of the 

 Miocene and in the early Pliocene, there were numerous eruptions 

 of basalt, although some of the basalts of this region may be con- 

 siderably older. South of Owen's Lake the basalt flows are numer- 

 ous and extensive, volcanic action in that region being most strongly 

 marked in the Coso Mountains. The most recent eruptions of all 

 have, however, taken place in the vicinity of Mono Lake. Here 

 the surface of the flows is often so fresh and free from soil that it 

 seems as if they had but just cooled. Prom the lake southward 

 for twenty miles there is a line of volcanic cones known as the Mono 

 craters, which are connected with the most recent eruptions and pos- 

 sess great interest. The cones have been built up on extensive 

 obsidian flows, the whole forming a considerable mountainous eleva- 

 tion. The glassy eruptions came first, then the cones were built up 

 of lapilli and scoriaceous material, while the volcanic ash which 

 covers the country for miles around was probably the last to be 

 thrown out. These recent glassy lavas were very viscid and cooled 

 in thick masses instead of thin sheets. This is finely shown in the 

 southernmost of the eruptions, where the glistening, glassy mass 

 which broke into fragments as it cooled rises in almost precipitous 

 crags fifty to seventy-five feet. The bottom of the valley between two 

 different coulees is occupied by a small stream which runs over an 

 older lava. From the surface of the latter, which is covered with 

 soil, spring immense pine trees which must have sprouted since the 

 glass cooled, for no vegetation could have withstood the heat. A 



