320 THE DESERT SANDSTONE. 



stratification formed dark bands from the presence of small 

 hornblende or augite crystals. 



The following passages from Russell's Geological History of a 

 part of North-Western Nevada,* so aptly illustrate the views 

 taken in the foregoing pages that no apology is necessary for 

 introducing them here. " Pumiceous dust. — In describing the 

 section of upper lacustral clays observed in the Humboldt, Truckee, 

 and Walker River canons, strata of fine siliceous 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, siliceous substance, homogenous in composition, and 

 having all the appearance of pure diatomaceous earth. When 

 examined under the microscope however, it is found to be composed 

 of small angular glassy flakes, of a uniform character, transparent 

 and without colour, but sometimes traversed by elongated cavities. 

 When examined with polarized light it is seen to be almost wholly 

 composed of glass with scarcely a trace of crystal or foreign matter. 

 On comparison with volcanic dust that fell in Norway in L875, 

 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 have the same physical 

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

 greater part of the recent dust examined, is composed of colourless 

 instead of brown or smoky glass. In the accompanying figures, 

 which we copy from Mr. J. S. Diller's instructive article on the 

 volcanic sand which fell at Unalaska, October 20th 1883, the 

 microscopic appearance of volcanic dust, from various localities 

 and of widely different geologic age, is shown with accuracy. The 

 peculiar concave edges and acute points of the shards of glass 

 render it evident that they were formed by the violent explosion 

 of the vesicles produced by the steam generated in the viscid 

 magma from which the glass was formed, and were not produced 

 by the mere attrition of the fragments during the process of 

 eruption. It is noteworthy that the dust erupted from Krakatoa 

 but yesterday is undistinguishable in its main characteristics from 

 the material of a similar origin which fell in the waters of Lake 

 Lahontan during the Quaternary, or from the dust thrown out by 

 some unknown and long extinct volcano in the vicinity of the 

 Atlantic coast, which fell near the site of Boston during pre- 

 Carboniferous or possibly in pre-Cambrian time. The volcanic 

 phenomena of to-day are governed by the same laws as obtained 



fc * U.S. Geological Survey. — Monographs, xi.,p. 146, Washington, 1885. 



