Relation of Secular Decay of Roch^. — Tarr. 33 
The general effect of such conditions in arid regions of mod- 
erate slope is to produce great deposits of loose gravels extend- 
ing far out from the mountain bases. These are not properly 
talus deposits, nor are they of the cone delta type, but are inter- 
mediate between the two and with some of the characters of each. 
They are found in the arid regions of the w^est about the base of 
mountains, buttes and mesas and often merge into cone delta 
deposits. 1 have seen walls of recent construction partly buried 
beneath this wash in the mesa region of eastern New Mexico. 
Blanford* describes remarkable deposits of this character in 
Persia. Great gravel slopes in that country extend from the base 
of the mountains far out into the desert, often five or ten miles 
from their source. The foot hills are often quite buried beneath 
these gravels which bear some resemblance to lake deposits, but 
do not appear to be of this character. The angle of slope is from 
one degree to three degrees, there l)eing sometimes a dift'erence 
in elevation of from one to two thousand feet between the lower 
and upper ends. Much of the region is thus covered, and the 
thickness of the deposit is often several hundred feet. Near the 
source it is a coarse conglomerate, changing progressively dowai 
the slope to a gravel, and then to a sand, the latter covering the 
great desert flats and probalily being in large measure trans- 
ported thither by the wind. Blanford explains the gravel slope 
as a surface wash, the result of the conditions of aridity, and 
rare, concentrated rains, in the manner described aliove. 
Similar deposits occur in India, and it is not impossible that 
the great gravel slopes of Patagonia ascribed 1)}' Darwinf to ma- 
rine action may be in part of the same origin. The same condi- 
tions are imitated in a small way, in moist regions, on ploughed 
fields on hill sides. Instances of the downcreeping of the soil 
in these places are very common, and often the total result of a 
few years of such creeping is quite appreciable. 
By a change in climatic conditions, and the development of 
river systems in these desert regions, much prepared load ready 
for transportation, will be found in these deposits; and the en- 
croachment of the sea upon the gravel slopes would admit of a 
rapid deposition of extensive deposits by the working over of 
*Superficial Deposits in the Valleys and Deserts of Central Persia, 
Quart. .T. Geol. Soc, 1873, xxix, 493-508. 
■fDarwin, Geological Observations. 
