Page:Popular Science Monthly Volume 52.djvu/639

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THE GREAT SIERRA NEVADA FAULT SCARP.

the smaller, known as Negit, or Black Island, is wholly of volcanic origin. The lake beds on the larger island are faulted and generally much disturbed, but are probably not older than the Quaternary. 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 distances 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 earthquake 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 phenomena more strikingly than almost anything else in the United States. The fact that the volcanic action has been so recent adds