14t) GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



in the specimen of dust, and the fact that it has been exposed to the action 

 of solvents much more than the rock remaining in the crater walls) strongly 

 favors the assumption that they had a common origin. 



More extended operations in the field revealed that beds like those 

 described above are not confined to the Lahontan basin, but are found as 

 superficial deposits above the Lahontan beach at many localities and at 

 points far distant from the old lake margins. Accumulations of the same 

 nature occur in the Mono Lake basin, interstratified with lacustral deposits, 

 and wei'e also found in the cations about Bodie at a considerable elevation 

 above the level of the Quaternar)^ lake that formerly occupied Mono Valley. 

 About Mono Lake these deposits are frequently of a coarser texture than 

 those found farther northward, and, at times, graduate into strata which 

 reveal to the eye the fact that they are composed of angular flakes of 

 obsidian. 



The Mono Craters form a range some 10 or 12 miles long, which 

 extends southeastward from the southern shore of Mono Lake, and in two 

 instances attains an elevation of nearly 3,000 feet above the lake. A few 

 coulees of dense, black obsidian have flowed from them, but the great 

 mass of the cones is formed of the pumiceous obsidian which occurs both 

 as lava-flows and ejected fragments, the latter forming a light lapilli which 

 gives a soft gray color to the outer slopes of the craters. Fragmental 

 material of the same nature has been widely scattered over the mountains 

 and on the ancient moraines that occur in the Mono basin, while fine dust, 

 unquestionably derived from the same source, may be traced to a still 

 greater distance. 



From the evidence given above we conclude that the strata of fine, 

 siliceous, dust-like material occurring in the Lahontan sections, as well as 

 the similar beds fox;nd about Mono Lake and scattered as superficial 

 ■deposits over the neighboring mountains, are all accumulations of volcanic 

 <lust, which was probaly erupted from the Mono Craters.^" The greatest 



*'This material could uot have been erupted from the craters iu which the Soda Lakes, near 

 Ragtown, are situated, as these volcanoes are formed of quite different and more heterogeneous ma- 

 terial. The fragments of scoria ejected from these vents are composed of basalt in which grains of 

 ■olivine are conspicuous. 



