I. C. Russell—Subaérial Deposits of North America. . 345 
feet; but, as natural sections are wanting, the thickness of these 
deposits as well as their internal structure is not well known. 
The lack of stratification in playa deposits may perhaps be due to 
the peculiar conditions under which they accumulate. The material 
composing them is fine throughout, but the waters continue turbid 
so long as they remain unevaporated, and the last addition made to 
the filling of the basin, in part a chemical precipitate, is of exceed- 
ing fineness. When the basins are refilled, this mud is quickly 
saturated, and may be again taken in suspension and in part redis- 
solved. ‘The deposits of various rainy periods may in this way be 
mingled so intimately that no lines of stratification would appear. 
Playa lakes are sometimes alkaline and saline, and on evaporating 
deposit salts of various kinds. Commonly, however, especially in 
the smaller basins, no salt appears at the surface, but the playas 
present a smooth, even expanse of cream-coloured mud, which 
becomes hard and divided into a net-work of shrinkage cracks 
when it dries. 
The fine sediment carried down the sides of an inclosed basin 
holding a playa lake is deposited in part before reaching the lake, 
and in part, as we have seen, in the lake itself. In both instances 
the accumulation is a yellow, finely divided earth, practically with- 
out stratification. In both instances the fine silt may have worn or 
angular pebbles and stones mingled with it, but in large valleys the 
accumulations, whether subaérial or subaqueous, are entirely of 
homogeneous yellow earth. The only marked difference in these 
- deposits is that the playa are much more saline than the subaérial 
earths. The sediments of playa lakes are sometimes obscurely 
vesicular, as if small gas bubbles had been formed in them. In 
subaérial deposits this has not been observed. The subaérial 
deposits, on the other hand, are traversed, at least in a number of 
instances, by small vertical tubes that branch downward; in the 
playa deposits these are not present. 
Relation to Stream Deposits.—Wherever the streams of the Arid 
Region overflow during high water and submerge their flood plains, 
a fine deposit is thrown down which in many instances is to all 
appearance identical with the adobe formed in drainless valleys. 
The deposit made by a stream in its immediate channel, at least 
in the vicinity of mountains, is coarse, and is frequently composed 
of well-worn boulders and gravel. As the stream sways from side 
to side of its general course during a succession of years, a sheet of 
gravel is spread out as a flooring over an entire valley. The 
material deposited on the flood-plain however, and superimposed on 
the gravelly stream-bed previously laid down, is usually a fine silt, 
which in many instances does not show lines of stratification. 
Along the streams of the Arid Region the flood plain deposits are 
frequently composed of fine, grey adobe, which breaks in vertical 
walls when undermined, and is without stratification. In some 
instances it is penetrated by vertical tubes apparently made by 
rootlets. In all these characteristics the stream deposit agrees with 
the subaérial deposit. 
