I. C. Russell—Subaérial Deposits of North America. 348 
shells are frequently present, but their number, so far as observed, 
is always small in comparison with the associated land-shells. 
Samples of adobe collected near Fort Wingate, N.M., contained 
the following fossils, as has been kindly determined for me by 
William H. Dall, who states that they all belong to living species 
and occur from Alaska to Mexico. 
Lanp-SHELLS. FRESHWATER- SHELLS. 
Succinea salleana ?, Pfr. Bythinella tenwipes, Couper. 
Hyalina (Conulus) chersina, Say. Limnea desidosa? sei 
Hyalina conspecta, Bland. Pisidium virginicum ? Bourg. 
Hyalina arborea, Say. Planorbis ae Say. 
Pupilla fallax, Say. Anodonta? (in small fragments). 
Vertigo ovata, Say. CRUSTACEAN CASES. 
Pupa muscorum, Linn. Cypris. 
Near Caiion City, Col., I saw detached bones, apparently of the 
Bison, which were obtained at a depth of twenty feet in adobe. 
Other similar finds are reported in the same region, but the bones 
obtained have not been preserved. In one instance, near the same 
locality, a tooth of an Elephant was obtained at a depth of forty 
feet in the adobe deposit. These bones are always detached, and are 
true fossils belonging to the deposit in which they occur. 
The only marine shells found in the adobe are fossils that have 
been derived from older rocks. An instance of this came under my 
notice near Caiion City, where fragments of an Inoceramus were 
found in this deposit, which could be readily traced to Cretaceous 
outcrops near at hand. These fossils should be considered merely as 
pebbles so far as they relate to the character and origin of the adobe. 
Mode of Formation.—That adobe is a subaérial deposit derived 
from the waste of surrounding mountain slopes, does not seem open 
to question. Its accumulation is now in process and may be studied 
in all its details at thousands of localities; the most instructive 
being the drainless and lakeless valleys of Utah, Nevada and neigh- 
bouring areas. The action of ephemeral streams and of the general 
surface wash in transporting debris from the uplands to the valleys 
may be observed in such basins at any time when rain falls. The 
thousands of little streams and rills, born in the hills of the 
Arid Region from the waters of passing showers, flow rapidly 
down the steep slopes, loaded with coarse sediment and turbid with 
fine yellow silt. On leaving the mouths of the rocky gorges, and 
entering one or more of the channels that lead from them down the 
alluvial slopes, the coarse material is deposited, and when the 
showers are transient, the water disappears by evaporation and by 
percolation through the material of the alluvial cones. During this 
process fine silt is deposited in the interspaces of the alluvial cones, 
and serves to make them more and more impervious. When the 
rain is copious, the streams may continue to the plain where the 
waters spread out in’ sheets among the desert shrubs, and deposit 
their silt. 
The particles of silt deposited by the ephemeral streams absorb 
the precipitates which are thrown down when the evaporation of 
