GENERAL MEETING. 19 
of white, unconsolidated, dust-like material, which is undistin- 
guishable in general appearance from pure diatomaceous earth. 
Beds of this material, varying in thickness from a fraction of an 
inch to four or five feet, were observed at a number of localities 
in the sides of the cafions that have been carved in lacustrine 
strata of Lahontan age by the Humboldt, Truckee, Carson, and 
Walker rivers. Deposits identical with those of the Lahontan 
sections were observed at a number of localities among the moun- 
tains of Nevada and California at an elevation of several hundred 
feet above the former level of Lake Lahontan and at a distance of 
forty or fifty miles from its borders, thus showing that the deposits 
were both sub-aerial and sub-aqueous in their mode of accumu- 
lation. Further exploration revealed the fact.that similar beds 
occur abundantly in Mono Lake Valley, where they may be seen 
to pass into well-characterized fragmental deposits of pumice and 
obsidian, thus suggesting that the finer material was also of volcanic 
origin. Experiment confirmed this hypothesis. Under the micro- 
scope the dust from a number of widely separated localities was 
found to consist almost wholly of angular flakes of transparent 
glass, with scarcely a trace of crystallized matter. When a sam- 
ple of pumice from near Mono Lake was reduced to a fine powder, 
it was found to present the same physical and optical properties 
as the dust in question, with which it also agreed closely in chem- 
ical composition, as shown by analyses made by Dr. Chatard, of 
the Geological Survey. 
The Mono Craters, from which this dust is supposed to have been 
erupted, form a group of cones about fifteen miles in length, situ- 
ated in the southeastern part of the Mono Lake Valley, California. 
These extinct volcanoes are composed almost entirely of pumice 
and obsidian, in the condition both of coulées and lapilli, the latter 
constituting cones of great symmetry and beauty, the grandest of 
which have an elevation of nearly three thousand feet above Mono 
Lake. Some of these craters were in eruption during Quaternary 
times, while others were active after the ancient lakes and glaciers 
of the region had passed away. Many times during their history 
vast quantities of lapilli and dust were thrown out. As the 
volcanic dust interstratified with the sediments of Lake Lahontan 
is undistinguishable from that deposited in the Mono Basin, there 
is little room for doubting that they had a common origin. The 
