Review of Recent Geological Literature. 53 
In explanation of the manner in which these hundreds of thousands 
of square miles were covered with alluvial deposit whose surface was 
an almost perfect plain at the time of its making, the author discusses 
the work of streams in an arid climate, particularly those flowing from 
mountains to plains. Such streams aggrade their channels, Their load 
is progressively dropped, not because of decreased slope, but because of 
decreased volume due mainly to percolation. Such streams yield their 
water to the ground instead of being supplied from the ground water? 
Unlike streams ot humid regions which flow persistently along a cer- 
tain course, these streams habitually change their courses to adopt new 
ones at lower levels. If all the water of all the streams which built 
the great plains had issued from -.one canyon in the mountains, the 
plains would have the form of a very flat alluvial fan, With the waters 
issuing from the mountains at small intervals for hundreds of miles, 
the many incipient fans have coalesced into one great aggradation plain. 
While this great plain was in process of building, the streams which 
brought the sediment, were ramified over the surface in a multitude of 
interlacing channels, in a pattern whose record is now left in the net- 
work of gravel courses belonging to any one horizon. 
The history of the plains has been somewhat as follows. The plateau 
of stratified rock which forms the body of the great plains was first 
eroded into “considerable relief” by the streams which crossed it from 
the mountains. A change of climate to greater aridity then gave to the 
streams their desert habit as noted above and their deposits built the 
great plains to a higher level than that of the original surface. At pres- 
ent a greater degree of humidity again enables the streams to cross the 
plains and to degrade their channels. It is believed that this present 
degradation stage dates “from the opening of that period of cli- 
matic oscillations in the Pleistoeene” which has been correlated with the 
lakes of the great basin. There have been minor oscillations, but in the 
main, the building of the plains is regarded as having been completed 
in the Tertiary. 
Mr. Johnson does not find it necessary to invoke crustal move- 
ments, either to bring about alternations of aggrading and erosion, 
as has been suggested by Dr. Gilbert, or to provide for the varying 
coarseness of the material of the plains, as was done by professor Ha- 
worth, On the contrary, he points to the closely parallel shores of lake 
Bonneville as evidence of remarkable stability during at least a part of 
the time for which crustal oscillations have been presumed to affect the 
great plains. In general the very broad graded slope and uniform con- 
stitution of the plains are taken to indicate “long-enduring stability 
of climate” and freedom from earth movements. Nor does he agree 
with professor Haworth in considering a large rainfall necessary to 
bring from the mountains so large a mass of waste. The building 
was presumably done by streams which did not, in the main, reach the 
sea. The climate is*supposed to have been more arid than at the pres- 
ent time. The torrential habit of rains and streams in arid regions is 
assumed to be sufficient explanation for the presence of the coarsest 
gravels in the plains. 
