MOHAWK LAKE BEDS. 
389 
On Grey Eagle Creek, about one and a half miles nearly 
south of Mohawk Post Office, at an elevation of about 4,600 
feet, is another exposure of beds very similar to the older 
beds just described, with much clay, sand, and carbonaceous 
shale. 
In addition there was observed a fine white layer, a few 
inches in thickness, composed almost entirely of volcanic 
glass in angular fragments with fluted forms, very similar 
to some material described as volcanic dust by Mr. G. P. 
Merrill (XV). The most reasonable explanation of the 
homogeneity of the volcanic layer is to suppose that it was 
thrown out in its present fine condition, and falling in the 
water, was deposited as we now find it. It might also be 
supposed that it was eroded from an area of volcanic ash 
near by and re-deposited on the lake bottom. In either case 
it marks approximately the age of the eruption, since, if the 
result of the erosion, it could hardly fail, after a considerable 
period had elapsed, to be much mixed with other detrital 
material. The angular character of the dust is against its 
having undergone much transportation, though fine thin 
glass would doubtless be not greatly rounded, but rather 
broken into finer angular particles. The material presents 
all the appearance of being rhyolitic glass. This is further 
substantiated by a silica determination made by Dr. W. H. 
Melville, of the United States Geological Survey. He found 
the fine white powder from Grey Eagle Creek to contain 70.64 
per cent, of silica, while a rhyolite from near Mohawk Valley 
contains 71.14 per cent. 
Now the rhyolites of the Sierra Nevada in general under¬ 
lie the andesites. Indeed, in some places, there is proof that 
they were eroded somewhat before the andesitic eruptions. 
Evidence gathered by Mr. Waldemar Lindgren and my¬ 
self at a great many localities in the Sierra Nevada all 
points to the rhyolites being older than, at least, the horn¬ 
blende-andesites which form the great bulk of the andesitic 
eruptions. 
It is therefore likely that the Grey Eagle Creek and Mo¬ 
hawk Post Office beds belong to a lake that existed before the 
