CLIMATE AND TERRESTRIAL DEPOSITS 379 



in angle between the stable grades under the two conditions and the 

 volume of sediments required to be shifted to cause one grade to pass 

 into the other. 



These deductive statements find perhaps their best illustration in 

 Persia, a region which, while so arid as to have no outflowing drainage, 

 yet is known from several lines of evidence to have undergone marked 

 climatic desiccation during historical times. Blanford in 1873 called 

 attention to the superficial gravels, sands, and clays of the valleys and 

 deserts of Persia.^ He notes that the margins of the valleys usually 

 consist of a long slope composed of gravel and bowlders, and with a 

 surface inclination of from 1° to 3°. Such slopes often extend for a 

 distance of from 5 to 10 miles from the base of the hills bounding the 

 plains, the difference in level between the top and bottom of the 

 incline being frequently from 1000 to 2000 feet, or even more. The 

 peculiarity of these slopes in Persia consists in their great breadth and 

 in the enormous mass of detrital deposits which they contain. In 

 the central portions of the valleys the surface usually consists of very 

 fine, pale-colored, rather sandy earth, not unfrequently impregnated 

 with salts. ^ In discussing the origin of these gravel deposits Blanford 

 notes that it is usually the drier tracts in which accumulations of 

 gravel attain their greatest dimensions while 



Toward 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 side 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 



Probable origin oj gravel accumulations. — This gives a clue to the origin of 

 these immense spreads of recent or sub-recent deposits; and in connection 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 Bearing in mind that all accumulations of detrital matter 



1 W. T. Blanford, "On the Nature and Probable Origin of the Superficial Deposits 

 in the Valleys and Deserts of Central Persia," Quarterly Journal oj the Geological 

 Society of London, Vol. XXIX, 1873, pp. 493, 503. 



2 Loc. cit., pp. 495, 496. 



