THE DESERT SANDSTONE. 321 



at the dawn of geologic history More extnded 



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 

 wdth lacustral deposits, and were also found in the canons about 

 Bodie at a considerable elevation above the level of the Quaternary- 

 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 of some 10 or 12 miles long, 

 which extends south-eastward 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 grey 

 colour 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 Lake 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 found about Mono Lake and 

 scattered as superficial deposits over the neighbouring mountains, 

 are all accumulations of volcanic dust which was probably erupted 

 from the Mono Craters. The greatest distance from the supposed 

 place of eruption at which these deposits have been observed is 

 about 200 miles."* 



In the same region we have ash deposits like those of Sydney, 

 taking the form of loose sand duneswhich the author thus describes: 

 " The first acquaintance the explorer in the Great Basin usually 

 makes with the material forming these deposits is when it is in 

 motion, and fills the air with clouds of dust, sand, and gravel, 

 which are blinding and irritating, especially on account of the 



* I have to observe, with reference to this quotation, that the descrip- 

 tion here given of the volcanic dust from Krakatoa does not quite tally 

 with the specimens gathered by me. These were not wholly composed 

 of glass, and, small as they were, they were full of traces of crystal and 

 foreign matter, especially microliths of triclinic felspar and pyroxene. 

 I am not however contending that the Desert Sandstone is composed of 

 volcanic dust, but volcanic sand derived from ashes with which of course 

 dust is intermingled. 



U— November 7, 1888. 



