PHYSICAL FEATURES. 189 



has been dealt with very fully by Mr. Oldham in the Records of the 

 Geological Survey. 1 



Occasionally an unusually heavy shower will cause a flood, carry- 

 ing many of the boulders into the plain below. 



Stony desert or " dasht." „,,.-, , . . , i v. r 



These floods, which, no doubt, were more fre- 

 quent in former times, have spread the pebbles over large areas in 

 the desert, giving rise to the stony plains known by the name of 

 "dasht." The outer surface of nearly all the pebbles is coloured 

 black through the oxidation of iron compounds; and this dark tinge 

 still further increases the desolate appearance of these dreary plains, 

 which occupy immense areas not only in Baluchistdn, but through- 

 out Persia. In the latter country they have formed the subject of 

 a special study by Mr. Bl an ford. 2 



Even in places where, not infrequently to this day, after an 

 unusually heavy shower, large bodies of water do reach the plain 

 without sinking into the ground or being evaporated, the water does 

 not gather sufficient strength to give rise to a definite channel, 

 except in a very few cases where a stream whose head-waters drain 

 a considerable mountain region has preserved sufficient importance 

 to excavate a channel through the plain. Such exceptional cases 

 are those of the Lora which drains a large area of high ground in 

 the neighbourhood of Quetta ? and whose course can be followed 

 up to the great dried-up lake called after its name the " Lora 

 Hamun." Such is the Mashkhel river which gives its name to 

 another shallow lake-basin, the fi Hamun-i-Mashkhel." Such, again, 

 is the Tahlab river whose course forms the boundary between Persia 

 and Baluchistan. But in most cases the water is ponded back by 

 irregularities of the ground and spreads into shallow pools which 

 may cover a large area, but become dried up in a few days, sometimes 



1 " Sub-recent and Recent Deposits of the Valley Plains of Quetta, Pishm, and the Dasht- 

 i-Bedaolat; with appendices on the Chamans of Quetta; and the Artesian water-supply of 

 Quetta and Pishin," by R. D. Oldham. Rec. Geol. Surv. Ind., Vol. XXV (1892), 

 PP. 36-53. 



3 " On the Nature and Probable Origin of the Superficial Deposits in the Valleys and 

 Deserts of Central Persia ," by W. T. Blanford. Quart. Journ. Geol. Soc, Vol. XXIX 

 0&73)>PP. 493-503. 



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