500 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 11, 



pointed crags of the higher ranges. As all these plains are the 

 lowest parts of the country, no sections are cut through them by 

 torrents, and only their surface can be examined ; but from their 

 extent it is palpable that the deposits of which they are composed 

 must be of great thickness ; otherwise the older rocks would crop 

 out from beneath more frequently than they do. 



But for inland seas and lakes to have occupied the interior of 

 Persia, and for large deposits to have formed in them, it is evident 

 that the climate must have been much damper than at present. In 

 recent times the rainfall has been insufficient to supply water either 

 to fill the basins, or, as we have seen, to wash down the detritus 

 which accumulates at the foot of the hills. Something is doubtless 

 due to increased evaporation, caused by the use of streams for irri- 

 gation ; but still this alone is insufficient to explain the drying-up 

 of the plains, because even flood- waters no longer reach beyond the 

 margins of the deserts. 



From the accounts given by ancient writers it appears highly 

 probable that the population of Persia was much greater, and the 

 cultivated land far more extensive, 2000 years ago than at present ; 

 and this may have been due to the country being more fertile in 

 consequence of the rainfall being greater. Some alteration may be 

 due to the extirpation of trees and bushes, the consequent destruc- 

 tion of soil and increased evaporation ; but this alone will scarcely 

 account for the change which has taken place. 



I cannot but think it probable that a gradual change in the 

 climate of Central Asia generally has taken place from the time 

 when the great plain north of Persia was under water, when the 

 Black, Caspian, and Aral seas were united, and when, as Loftus has 

 shown, the plains of Mesopotamia were a part of the Persian Gulf, 

 this gradual drying-up of the country being thus connected with 

 the elevation of the steppe-region of Central Asia, and of the 

 southern coasts of Persia. To this gradual reduction in the rain- 

 fall in modern times is probably to be attributed the circumstance 

 of the Oxus no longer reaching the Caspian, and the diminished 

 volume of that river ; for I cannot but suspect that the diversion of 

 the Oxus from the Caspian to the Aral sea in the sixth and again in 

 the sixteenth century* was but the last in a series of changes in the 

 course of a stream which once, in all probability, carried the surplus 

 waters of the Aral Sea to the Caspian. To the same cause is pro- 

 bably due the gradual diminution of the Caspian and Sea of Aral ; 

 and hence the disappearance of the lakes which once, I believe, 

 covered no small part of the interior of Persia. 



The probability of this theory will be at once seen by supposing 

 a reversal of the process and conceiving the results of a gradual in- 

 crease, from whatever cause, in the rainfall of Persia. First streams 

 would run in all the valleys, the desert plains would be filled with 

 water and converted into lakes, many of them brackish or salt, 

 whilst the gravels from the higher valleys and the edges of the 



* Proe. Koy. Geogr. Soc. 1867, vol. xi. pp. 114-118. 



