52 GLACIAL AND SURFACE GEOLOGY OF THE PACIFIC COAST. 



tion of the Sierra, a constant theme of remark. Not less striking was the 

 polish of the rock surfaces, which often, when struck by the sunlight at the 

 right angle, glittered in the distance like mirrors. In crossing from Tenaya 

 Lake to Soda Springs the rocks for long stretches were bare, and so smooth 

 that the mules could with difficulty keep their footing. The fact was re- 

 peatedly noticed, all through this region, that where a great thickness of ice 

 had formerly rested on the granite, which is almost the only formation of 

 this glaciated region, the surface of the rock seems to have undergone a 

 molecular change, a sort of crust having been formed, as if by the crushing 

 and compacting together of the crystalline minerals of which the granite is 

 made up. This crust appears to be more durable than the body of the rock ; 

 and, in the process of weathering, the exterior sometimes scales off in plates 

 or flakes, half an inch thick or more, of which one side is as nicely polished 

 as if the Avork had been done only yesterday. Other portions of the granite 

 which have not been subjected to this pressure seem to be eroded with com- 

 parative rapidity, and their surfaces are often quite irregularly worn out in 

 cavities. Professor Brewer thought that he observed this peculiar change 

 of the crust of the granite to be strikingly developed in certain portions of 

 the Mono Pass, where the glacier had been forced to make its way through 

 narrow gorges, and where, in consequence, the pressure exercised on the 

 enclosing rocky walls must have been enormous. 



The locality farthest north in the Sierx\a where any extensive proofs of 

 former glaciation have been discovered, -^ so for as known to the writer, — 

 on the eastern slope of the range, is at a point about ten miles northwest of 

 Mono Lake. Here, at the head of Walker Piiver, and between Castle Peak 

 and the Tower Peak Range, is a depression in the Sierra over which crosses 

 the so-called Virginia Trail, once considerably used as connecting the Upper 

 Tuolumne and Merced rivers with Aurora and the mining districts of 

 Nevada. 



This trail crosses to the southeast of Castle Peak ; but Green River, which 

 heads at the summit of the pass, finds its way down the eastern slope on the 

 northwest side of this mountain, which is nearly 12,000 feet high. In the 

 canon of this stream a very large glacier once descended almost to the level 

 of the Big Meadows. The moraines left by it are very large and distinct, 

 forming two parallel ridges, some 350 feet in height, made up of the usual 

 angular debris of the range above. It was noticed by the writer, how much 

 these moraines resembled, both in form and size, the detrital accumulations 



