298 GEOLOGY. 



Over areas much greater than those occupied by lakes in Pliocene 

 times, and over tracts which never formed parts of definite flood plains, 

 broad aprons of detritus brought from the higher slopes are accumu- 

 lating now, and similar accumulations were quite surely making in 

 Pliocene times. Such accumulations are most considerable on the 

 flanks of mountain ranges where precipitous slopes join plains of 

 low gradient. Particularly is this the case where the climate is sub- 

 arid, and the rain falls in sudden and copious showers, largely 

 concentrated on the mountain heights, while the thirsty plains below, 

 covered with porous wash, quickly drink up the sudden mountain 

 floods and strand the detritus which they brought down in their 

 swift descent. Most of the western mountains of America are flanked 

 by such deposits, which sometimes spread far out upon the adjacent 

 plains. A portion of these deposits are of Pliocene age, and a por- 

 tion are still younger. In basins occupied by lakes, these sibaerial 

 sediments merge into lacustrine deposits, and, as a consequence of 

 the fluctuations of the lakes, are more or less interstratifled with them. 

 They also merge so insensibly into true flood-plain deposits that they 

 cannot be systematically separated from them; nor should they be, 

 since they are of the same essential nature. If slopes are suitable, 

 deposits on plains free from standing water are likely to be more extensive 

 than lacustrine deposits, for the whole plain is then open to subaerial 

 aggradation free from competia^e lacustrine catchment. It is prob- 

 ably safe to affirm that Pliocene deposits of this type lie concealed 

 beneath later accumulations of a similar sort in nearly all the large 

 basins, and at the bases of nearty all the steep slopes in the western 

 mountain region. Positive proof of their presence is difficult, both 

 because of the difficulty of distinguishing them from later deposits 

 physically, and because of the paucity of fossils. The Pliocene deposits 

 of this sort which have been identified are probably but a small frac- 

 tion of all that exist. 



On the whole it would appear that erosion was the dominant 

 process in the Cordilleran region during this period, but that a not 

 inconsiderable part of the eroded material was left in basins and 

 valleys and on plains, not far from its source. 



Among the formations which have been described, -usually as lacustrine, 

 from the area west of the Rocky mountains, are those of the Great basin 1 and 

 1 King, Geol. Expl. of the 40th Parallel, Vol. I, pp. 525-543. 



