498 PEOCEEDWGS OF THE GEOLOGICAL SOCIETY. [June 11, 



instance noticed, not far from Karman, the strata have been much 

 disturbed. "Whatever may be the mode of origin of these higher 

 deposits, it is, I think, probable that they must be more ancient 

 than the superficial accumulations of the valleys. 



Valleys near Shirdz. — Towards Shiraz the slopes of loose detritus 

 on the sides of valleys are much less extensive, and in places, as in 

 the valley of the Bandamir, above Persepolis, entirely wanting, the 

 flat alluvium of the valley extending to the limestone ranges on 

 each side. This may be due to a former extension of the existing 

 salt lakes far into the valleys of Shiraz and Persepolis, and to the 

 deposition of silt in the lakes in sufficient quantities to conceal any 

 accumulation of detritus near the sides of the valley ; but there 

 appeared to me to be a similar deficiency of gravel slopes on the 

 sides of the higher valleys containing running streams, and I am 

 much inclined to believe that their absence is connected with the 

 heavier rainfall. Passing from the Shiraz country northwards to- 

 wards Isfahan and Tehran, or eastwards towards Karman, there is a 

 well-marked increase in the mass of the gravel-deposits and in the 

 extent of country covered by them ; at the same time the country 

 to the north and east becomes much more arid from the climate 

 being drier and the rainfall less. 



Probable Origin of Gravel accumulations. — This gives a clue to the 

 origin of these immense spreads of recent or subrecent deposits ; and 

 in connexion with the last observations I may mention that usually, 

 in Southern Asia, so far as I have seen, it is the drier tracts in 

 which accumulations of gravel attain their greatest dimensions. 

 They are far more extensive in Sind and Baluchistan than in those 

 parts of India in which the rainfall is much greater. Bearing in 

 mind that all accumulations of detrital matter are due to arrest of 

 motion, whether partial or total, in the transporting agent, we can 

 easily understand that the rainfall on the Persian hills may suffice 

 to wash down as far as the sides of the valleys those fragments 

 which, by chemical agency or the action of frost, are loosened from 

 the hill-sides ; but when once the momentum given by the steepness 

 of the incline is at an end, the quantity of water drained from the 

 surface is insufficient to transport the debris to a lower level ; all 

 that it can do is to leave the detritus in a long slope, the surface of 

 which is arranged by the wash of rain. 



Probable Origin of Desert Plains. — But although the above hypo- 

 thesis may account for the slopes on the sides of the valleys and 

 desert plains, we have yet to explain the origin of the vast deposits 

 Avhich fill the plains themselves ; and the only probable explanation 

 appears to be that these extensive basins were formerly lakes, most 

 of them probably brackish or salt, like the Caspian and Aral Seas, 

 lakes of Van, Urumiah, Mriz, &c, the fine soil of the plains consisting 

 of silt deposited in such lakes (see Map, p. 499). On the southern 

 margin of the Sistiin desert near Jalk the resemblance to an old sea- 

 or lake-shore is most striking, the lowest spurs of the hills, here 

 formed of vertical shales and sandstones, being rounded in outline, 

 as if worn by the sea, and contrasting strongly with the sharply 



