The drinkable quality of the water of Sistan is another of the qualities which many writers have deemed remarkable. At times of very high water, perhaps once in a dozen years, the lake possesses an outlet to the south which will presently be described. The amount of water passing through this, however, is a most minute fraction of the total which reaches the lake, and as it passes out at the time of high water, when the percentage of salt dissolved in the water is least, the amount of salt carried by it must be very small compared with the total amount brought in by all the tributaries. The amount thus brought to Sistan in a single year, or in a score of years, may be insignificant, but it must be large compared with that carried by streams in moist countries. Ferrier (pp. 40o ff.) describes the Dasht-i-Margo, through which the Helmund flows, as full of salt pools. Bellew (pp. 166, 168, 172, et al.) found the soil of the Helmund Valley highly charged with saline matter. In one place close to the river, a region some io miles long contained several thousand pits from which the people extract salt for commercial purposes. On the opposite side of Sistan, to the northwest, many of the tributary streams are strongly impregnated with salt. The Shor Rud (Salt River) of Durukh proved so saline that we could not drink it. The Gisha stream was drinkable in December, although our guide said that in summer, when the brook is reduced to a few pools, not even camels can drink the water. In the same way the Bendan stream and the other seeping brooklets which I saw along the northwest shore of the lake are all bordered by incrustations of salt ; and lastly, the unconsolidated strata of the bluffs and of the lacustrine plain on all sides of Sistan are frequently white with saline matter. Clearly the water of the lake is not fresh by reason of any lack of salt in the surrounding basin. The amount of saline matter brought in each year by the streams must be large.
It is possible that the freshness of the lake is due to its occasional overflow. If we suppose that a flood occurs every tenth year and carries out of the lake a tenth of the water which is that year discharged into it, the maximum salinity of the water would be one hundred times that of the tributary rivers. Under such conditions the water might be apparently fresh, although as to this we have no data. It is probable that the amount of water escaping from the lake is less than a tenth every tenth year, and the salinity should be two or three times as great as we have estimated. Another explanation of the lack of salt is that in comparatively recent times the lake stood so much higher than now that it overflowed permanently, and was flushed clean. Other lines of evidence, as we shall see, point to this conclusion. It is mentioned here to show that while the freshness of the lake proves nothing, it is strictly in accordance with the theory which will be considered later.
THE DELTA OF THE HELMUND.
The most important part of Sistan from a human standpoint is the arable plains which lie outside the central expanses of the lake and swamp. In certain places these represent a shore platform cut by the waves when the lake stood higher. Elsewhere they represent a portion of the general lake bottom, now laid bare by the