366 J. LeConte on the great Lava-flood of the West, 
reach, presented a singular appearance, as if thickly broken out 
wit i 
tions were marked only by long hill-side furrows ‘and ridges, 
evidently produced by surface erosion; and between these €X- 
tremes every stage of gradation could be trace 
No one, I think, can ride over those thirty miles and observe 
closely, without being convinced that these mounds are wholly 
the result of surface-erosion acting under peculiar conditions. 
These conditions are a treeless country and a drift-soi/, consis0ng 
of two layers, a finer and more movable one above aud a 
coarser and less movable one below.* Surface-erosion cuts 
through the finer superficial layer into the pebble-layer beneath, 
leaving, however, portions of the superficial layer as mounds. 
The size of the mounds depends upon the thickness of the super 
ficial layer; the shape of the mounds depends much upon the 
slope of the surface. The process once started, small shrubs “ 
weeds take possession of the mounds as the better soil, and hol 
them by their roots, and thus increase their size by preventing 
or retarding erosion in these spots. The treelessness of the 
country in eastern Oregon has been produced gradually, since 
* The necessary conditi ieve, is t ness of the surface 
soil, as compared oar eee ibed: Wake ay fo the nas of the & aro 
