146 GEOLOGICAL HISTORY OF LAKE LAHoNTAN 



EXCEPTIONAL SEDIMENTARY DEPOSITS, 



PUMICEOLS DUST. 



In describing the section of upper lacustral clays observed in the 

 Humboldt, Truckee, and Walker River canons, strata of fine silicious 

 material, varying in thickness from a fraction of an inch to five or six feet, 

 were noted at a number of localities; it is now our intention to describe 

 these abnormal deposits more fully. 



In all the exposures of this material the same characteristics were 

 observed. The beds are composed of a white, unconsolidated, dust-like, 

 silicious substance, homogeneous in composition, and having all the general 

 appearance of pure, diatomaceous earth. When examined under the mi- 

 croscope, however, it is found to be composed of small, angular glassy 

 flakes, of a uniform character, transparent and without color, but sometimes 

 traversed by elongated cavities. When examined with polarized light, it is 

 seen to be almost wholly composed of fragments of glass, with scarcely a 



1. Volcauic dual which IVU in Norway, March 29 and 30, 1875. 



2. Volcanic du&t emptied fr<.>m Krakatoa, August 27, 1S83. 



3. Volcanic dust from the Truclcee Eiver, Nevada. Quaternar.y. 



4. Volcanic dust from Brakleast-Bill in Saugus, Mass., pre-Carboniferous. 



Fig. 23. --Volcanic dust. 



trace of crystal or of foreign matter. On comparison with volcanic dust 

 that fell in Norway in 1875, derived from an eruption in Iceland, with the 

 dust erupted in Java in 1864, and the similar material ejected in such 

 quantities from Krakatoa in 1883, it is found to liave the same physical 

 characteristics; but it is much more homogeneous, and, unlike the greater 

 part of the recent dust examined, is composed of colorless instead of 

 brown or smoky glass. In tlie following figure, which we copy from 



