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POPULAR SCIENCE MONTHLY. 



climb over the sharply angular fragments which form the wonder- 

 fully rugged surface of the obsidian flow is an interesting experience. 

 Great jagged points project up here and there from the broken sur- 

 face, 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 



Debbis Slope at Foot of Siebba Nevada Fault Scabp neab Owen's Lake. 



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 



