THEIR DEPOSITS AND DRAINAGE. 649 
formed strata which were, in some instances ten or twenty 
feet in thickness. At other times the voleanie aetion was 
still more intense, and floods of lava were poured out which 
formed continuous sheets, hundreds of miles in extent, pen- 
etrating far into the lake-basins, and giving to their bottoms 
floors of solid basalt. When these cataclysms had passed, 
quiet was again restored, forests again covered the land, 
herds dotted its pastures, fishes peopled the waters, and fine 
sediments abounding in forms of life accumulated in new 
sheets above the strata of cooled lava. The banks of the 
Des Chutes River and Columbia afford splendid sections of 
these lake deposits, where the history I have so hastily 
sketched may be read as from an open book. 
But, it will be said that there are portions of the great 
central plateau which have not been drained in the manner I 
have described. For, here are basins which have no outlets, . 
and which still hold sheets of water of greater or less area, 
such as those of Pyramid Lake, Salt Lake, etc. The history 
of these basins is very different from that of those already 
mentioned but not less interesting nor easily read. By the 
complete drainage of the northern and southern thirds of the 
plateau through the channels of the Columbia and Colorado, 
the water surface of this great area was reduced to the tenth 
or one-hundredth part of the space it previously occupied. 
Hence, the moisture suspended in the atmosphere was di- 
minished in like degree, and the dry hot air, sweeping over 
the plains, licked up the water from the undrained lakes 
until they were reduced to their present dimensions. Now, 
4s formerly, they receive the constant flow of the streams 
that drain into them from the mountains on the east and 
west, but the evaporation is so rapid that their dimensions 
are not only not increased thereby, but are steadily dimin- 
ishing from year to year. Around many of these lakes, as 
Salt Lake, for example, just as around the margins of the 
old drained lakes, we can trace former shore lines and meas- 
ure the depression of the water level. Many of these lakes 
AMER. NATURALIST, VOL. IV. 
