3846 86. OC. Russell— Subaérial Deposits of North America. 
That the flood-plain deposits of an arid country should be of the 
same character as the adobe deposited on gentle slopes and in 
inclosed valleys by ephemeral streams, is not surprising, since the 
process in each instance is essentially the same and the material 
handled is identical. The ephemeral streams spread out their 
waters on reaching gentle slopes, and deposit their sediment quietly 
among the scanty desert vegetation. In the case of perennial 
streams which overflow their banks the process is similar, that is, 
they form sheets of comparatively still water along their borders, 
and deposit silt about the vegetation which obstructs their flow. In 
both instances it is to be expected that casts of the roots of plants 
would remain in the soil. Evaporation takes place in both instances, 
thus contributing precipitated mineral matter to the deposits, but 
occurs most rapidly in the case of the ephemeral streams. 
In flood-plain deposits the shells of the freshwater molluscs occur, 
while land-shells may or may not be present. In the deposits 
of ephemeral streams the shells of land molluscs predominate, not 
to the exclusion of freshwater-shells, however, since the deposits 
are laid down by streams which in their upper course may survive 
throughout the summer and be inhabited by molluscs. The bones 
of land-mammals may be buried during either mode of accumulation. 
The geological interest of adobe and allied deposits centres not 
only in the manner in which they are formed, but also in their 
extent and depth when accumulated under favourable conditions. 
It is evident that the subaérial filling of an inclosed basin with fine 
material might continue in the manner described, until the depression 
was filled or the source of supply exhausted, so long as the climate 
conditions remain favourable. A decrease in precipitation would 
retard the filling or perhaps check it altogether, while an increase 
would favour the extension of playa lakes, or transform them into 
permanent water bodies. Should the increase be sufficient, the per- 
manent lakes would overflow and cut down their channels of dis- 
charge until their basins were drained to the bottom. It appears, 
therefore, that the maximum thickness which subaérial deposits may 
attain in an arid region is very great, especially in the case of fault 
valleys, which may have their borders raised at the same time that 
their bottoms are being filled. 
The thickness which playa lake deposits may attain is also very 
great; for, like the subaérial deposits just considered, they depend 
for their accumulation, the supply continuing, on a combination of 
climatic and topographic conditions which may remain favourable 
for a long period of time. 
Of the three methods described above, by which fine silt deposits 
may be accumulated in arid regions, the least important in a 
geological point of view is the one dependent on the overflow of 
streams. In the formation of flood-plains the conditions favouring 
accumulation depend not only on climatic and topographic con- 
ditions, but more definitely than in the other instances on the 
supply of suitable material The supply of sediment not only 
determines whether a stream shall deposit or not, but whether the 
