3844 =I. C. Russell—Subaérial Deposits of North America. 
the water that transported them takes place. During the long hot 
summers the water absorbed by the material filling enclosed basins’ 
is drawn to the surface by capillary attraction and evaporated. In 
this manner additional mineral matter is precipitated among the 
sedimentary deposits. It is evident, therefore, that the fine yellow 
earths filling enclosed basins owe their accumulation to both 
mechanical transportation and chemical precipitation. 
It may be suggested in this connection that the coherency of 
adobe, which enables it to stand in vertical escarpments for a long 
series of years, may be due to the partial cementation of its particles 
by the precipitation of mineral matter, principally calcium carbonate, 
among them. 
An examination of the fine silt in alluvial cones shows that it is 
identical with the silt forming the plains in the lower portions of 
the valleys in which they occur, except that it is not so evenly 
assorted, and in general is coarser. In both situations it is usually 
light yellow in colour, seemingly without reference to the character 
of the rocks from which it was derived. In addition to the material 
swept into the enclosed basins by ephemeral streams, there is 
another source of supply through aérial transportation. Dust is 
blown from the mountains overlooking the areas of accumulation 
and from neighbouring valleys, but the amount thus contributed is 
very small in comparison with the vast quantities of both coarse 
and fine material transported by water in the manner described 
above. This subject is again referred to on page 349. 
The fine dust blown into the air by volcanoes has also con- 
tributed to the filling of the valleys of the Arid Region. The total 
quantity of fine particles thus deposited must be very great, but it 
has only been observed near recent volcanoes in the neighbourhood 
of Mono Lake, Cal., and in Southern Utah. Interstratified with 
lacustrine sediments in the basin of Lake Lahontan’ there are 
deposits of white volcanic dust aggregating a thickness of perhaps 
six or eight feet, which was derived from the Mono craters two 
hundred miles away. ‘This indicates that very many of the drain- 
less valleys of Nevada and California must have received important 
contributions from the same source. 
Relation of Adobe to Playa Deposits——When an inclosed basin 
receives sufficient precipitation to form a lake, a portion of the fine 
debris swept down from neighbouring hills will be carried into it 
and deposited. In the Arid Region transient water bodies of this 
nature termed playa lakes are formed during every storm. These 
lakes are of all sizes up to hundreds of square miles in area, but are 
always extremely shallow, a depth of more than a few inches being 
rare. They usually disappear as quickly as they came, when the 
storms which supplied them pass away. On evaporating they leave 
a smooth plain or playa of light yellow mud, which is of almost 
impalpable fineness, and shows only very obscure, if any, lines of 
stratification. The last statement is based on a number of excavations 
which have been made in such deposits to a depth of six or eight 
1 Monograph No. 11, U.S. Geol. Sury., 1885, p. 146. 
