646 THE ANCIENT LAKES OF WESTERN AMERICA: 
Klamath and Pit Rivers, and all pointed to the same con- 
clusion. i 
In all this region I observed certain peculiarities of geo- 
logical structure that have been remarked by most of those 
who have traversed the interval between the Sierra Nevada 
and the Rocky Mountains. In the northern and middle por- 
tions of the great table lands the general surface is some- 
what thickly set by short and isolated mountain ranges, 
which have been denominated the <‘‘Lost Mountains.” 
These rise like islands above the level of the plain, and are 
composed of volcanic or metamorphic rocks. The spaces 
between these mountains are nearly level, desert surfaces, 
of which the underlying geological structure is often not 
easily observed. Toward the north and west, however, 
` wherever we come upon the tributaries of the Columbia, the 
Klamath or Pit Rivers, we find the plateaus more or less cut 
by these streams and their substructure revealed. 
Here the underlying rocks are nearly horizontal, and 
consist of a variety of deposits varying much in color and 
consistence. Some are coarse volcanic ash with fragments of 
pumice and scoria. Others I have in my notes denominated 
“concrete,” as they precisely resemble the old Roman cement 
and are composed of the same materials. In many localities 
these strata are as fine and white as chalk, and, though con- 
taining little or no carbonate of lime, they have been re- 
ferred to as “chalk-beds” by most travellers who have 
visited this region. Specimens of this chalk-like material 
gave me my first hint of the true history of these deposits. 
These, collected on the head waters of Pit River, the 
lamath, the Des Chutes, Columbia and elsewhere, were 
transmitted for examination to Professor Bailey, then our 
most skilled microscopist. Almost the last work he did be- 
fore his untimely death was to report to me the results of 
his observation on them. This report was as harmonious as 
it was unexpected. In every one of the chalk-like deposits 
to which I have referred he found fresh-water diatomacee. 
