Page:Popular Science Monthly Volume 52.djvu/638

618 climb over the sharply angular fragments which form the wonderfully rugged surface of the obsidian flow is an interesting experience. Great jagged points project up here and there from the broken surface, while the total lack of soil and the bright, clean surface of the brecciated glass makes it appear as though it were but yesterday when the seething mass welled up, overspread the surface, and cooled. Several circular depressions in the valley adjacent to the lava attract the attention. They appear like craters, but it is clear



that no lava ever flowed from them, and they were probably formed by some explosive action from beneath. The greatest body of ash, lapilli, and obsidian flows lies immediately south of the lake and constitutes the Mono craters. They present a weird appearance, rising in their nakedness from the sandy sagebrush plains with the dark background of pines to the south. The Mono craters can not be more than a few hundred years old. They certainly date from post-glacial times.

The two islands in Mono Lake are possessed of great interest, and are well worthy of a careful examination. They are largely volcanic, and present some exceptional geological features. The larger, known as Pa-o-ha, is about two miles long, and consists in part of stratified lake deposits raised up by some convulsion, while