THE GREAT SIERRA NEVADA FAULT SCARP. 619 



the smaller, known as Negit, or Black Island, is wholly of volcanic 

 origin. The lake beds on the larger island are faulted and gen- 

 erally much disturbed, but are probably not older than the Quater- 

 nary. The eastern and northern parts consist of a black basaltic 

 lava,, very fine-grained or even glassy. Much of it is younger than 

 the sedimentary beds. The lava extends beneath the water on the 

 northern side of the island in such a manner as to indicate that it 

 flowed out during a stage of low water. At the southeast side hot 

 springs issue from the lava just beneath the surface of the water. 

 The bare rocks above are seamed and decomposed, and from holes 

 and crevices there issue jets of steam which are visible for long dis- 

 tances in the cold weather. At the eastern end there are several 

 peculiar craterlike depressions in the lava. One or two of them lie 

 partly beneath the surface of the lake, and are being destroyed by 

 the action of the waves. Several are, however, very perfect and 

 symmetrical, about fifty feet deep, and as they extend beneath the 

 lake level their bottoms are filled with water. No melted rock ever 

 appears to have come from them, and there are several reasons for 

 concluding that their origin is due to the same causes as those on the 

 head of the North Fork of Owen's River already mentioned. This 

 was the explosive action of gases beneath, completely shattering 

 the massive lava and blowing out the material once occupying the 

 depressions. It does not appear that any of this material could have 

 been fused, for the fragments scattered around over the surrounding 

 surface are all angular. Volcanic ash is strewn about and may have 

 followed the explosions. A similar craterlike depression occurs in 

 the sedimentary beds on the southern side of the island, and it can 

 only be accounted for by a similar cause, or possibly by the falling 

 out of the bottom. 



Negit Island presents from a distance a dark, forbidding aspect. 

 It is long and low, with a broad, truncated cone at its western end. 

 A close examination shows this cone to be a mass of different colored 

 lavas and scoria fissured and thrown into all kinds of shapes by earth- 

 quake movements. It is so shattered that it seems ready to crumble 

 away and sink in the lake. The whole island has the appearance of 

 having been elevated from beneath the water in very recent times, 

 and shattered to its very center. On the eastern side the massive flows 

 have evidently been lifted upward in such a manner as to leave great 

 open fissures four to six feet wide and descending to unknown depths. 

 From the level of the lake they are filled with water. These islands 

 possess a most unique character, and, taken in connection with the 

 Mono craters with their flows of acid lavas, illustrate volcanic phe- 

 nomena more strikingly than almost anything else in the United 

 States. The fact that the volcanic action has been so recent adds 



