I. O. Russell—Subaérial Deposits of North America. 347 
deposits once laid down shall remain. In the formation of adobe 
and playa deposits, however, there is substantially no waste. 
While adobe and playa deposits depend on aridity of climate for 
their formation, the flood-plain deposits are not so limited, but may 
go on apparently with the same results in an arid as in a humid 
climate, provided a periodic variation in the volume of the streams 
takes place, sufficient to cause them to overflow their banks during 
the flood stage. There seems to be no reason to suppose that the 
character of flood-plain deposits in arid and in humid regions should 
be distinct, unless it be that the fine material washed into streams in 
dry countries is largely supplied from subaérial deposits which have 
already been assorted, and perhaps have a different chemical 
character from the surface debris of humid regions. 
It is beyond the scope of the present paper to discuss the processes 
by which flood-plain deposits are formed, especially as accumulations 
of this character are comparatively inconspicuous in the Arid Region, 
and also because the constructive power of streams has been exten- 
sively studied elsewhere. 
The accumulation of adobe and of eolian and volcanic dust in the 
central portions of the valleys of the Arid Region, together with the 
formation of talus slopes and alluvial cones about their borders, 
promotes the levelling of the hills and the filling of the intervening 
depressions. The tendency of this twofold process is to reduce the 
country to a plain, but not to bring it to sea-level. An exception 
here exists to the nearly universal law of base-level erosion, an 
exception, however, that is transient when considered in the way 
that geologists are forced to reckon time. A long continuation of 
existing climatic conditions, together with an absence of orographic 
movements, in the Great Basin portion of the Arid Region, would 
result in the formation of a broad high-level plain with rocky crests 
projecting here and there to mark the sites of buried mountain- 
ranges. A change to more humid conditions after this process was 
far advanced would initiate drainage systems which would make 
rapid changes in the configuration of the region. One of the first 
results of an increased rainfall would be the cutting of deep canons, 
especially in the unconsolidated adobe and playa deposits, thus 
exhibiting sections of these peculiar formations and revealing their 
great thickness. 
In connection with a description of the occurrence of adobe it is 
proper to state that nothing similar to the red soils formed by the 
residual clays of the South Atlantic States, or the red earth of 
Bermuda and the West Indies, or the terra rossa of Southern Hurope, 
or the laterite of India, is to be seen anywhere in the Arid Region. 
Rock disintegration is there active, but rock decomposition is 
retarded. The absence of residual clays from the comparatively 
rainless portion of this country is of interest as tending to show 
that such deposits are formed in humid and not in arid countries." 
1 The subaérial deposits of humid regions have been discussed at some length by 
the present writer in Bulletin No. 52 of the U.S. Geological Survey (1889), to which 
this paper may be considered as a supplement. 
