STRATA OF VOLCANIC DUST. 



147 



Mr. J. S. Ciller's instructive article on the volcanic sand which fell at 

 Unalaska, October 20, 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 since extinct volcano in the vicinity of the Atlantic 

 coast, which fell near the site of Boston during pre-Carboniferous or pos- 

 sibly in pre-Cambrian time. The volcanic phenomena of to-day are gov- 

 erned by the same laws as obtained at the dawn of geologic history. 



Farther study revealed that even the finest of the dust obtained from 

 the basin of Lake Lahontan has identically the same physical properties as 

 pumiceous rhyolite forming the Mono Craters, ground in a mortar to a cor- 

 responding fineness ; under the microscope the two powders were very similar. 



The dust deposits are rich in silica, as shown by the following analysis, 

 by Dr. T. M. Chatard, of a sample collected in the bank of the Truckee 

 River near Pyramid Lake ; for comparison we give also an analysis, by the 

 same chemist, of a specimen of pumiceous rhyolite from the Mono Craters : 



Constitaents, etc. 



Loss by ignition (water) 



Silica (SiOz) 



Alumina (AI2O3) and iron (FeaOa) 



Lime (CaO) 



Majaie&ia (MgO) 



Magnesia (MnO) 



Potash (K2O) 



Soda(Na20) 



The striking similarity in the composition of the above samples 

 (especially when allowance is made for the greater percentage of moisture 



« Science, Vol. Ill, p. 652. 



