Lacustrine deposits, showing approximately the actual limits of the ancient Fayûm lake, can be traced over wide areas of now barren desert; these will be more fully dealt with later. The present lake-level is still continually sinking owing to an improved system of irrigation, by which a constantly decreasing amount of waste water drains into the lake. Its average annual fall has, during the last decade, been nearly half a metre,[12] and the slope of the land being very gradual, large areas have been reclaimed during the last few years, though whether the advantages derived from this constant lowering of the lake are not more than balanced by certain drawbacks is somewhat doubtful.[13] With the new areas now being brought under cultivation the amount of drainage water finding its way into the lake will increase and the fall be checked. At the beginning of 1904 the level was markedly higher than in the previous winter, and a difference of even half a metre alters the shore line to a considerable extent, owing to the flatness of the land by which the lake is for the most part bounded. Although under the present desert conditions practically no material from the surrounding desert is washed into the lake, doubtless a considerable amount of fine dust and sand is carried into it by the wind, especially during the violent sandstorms which occur frequently in the locality. The high cliffs which bound the northern shore of the lake throughout a portion of its length probably have the effect of checking the velocity of both north and south winds, thus causing a considerable amount of sand, which would otherwise be carried across, to be dropped on its surface. This material, together with the fine mud brought down by the canals on the cultivation sides, must have an appreciable effect in raising the level of the bed of the lake. The phenomenon of the extraordinary freshness of the water of the Birket el Qurûn has been commented on by Schweinfurth, who shows that the degree of concentration of salt in a lake whose volume has been continually reduced, and to which salt has constantly been added, should be many times greater than the actual existing amount. An analysis[14] of the water at the west end of the lake (where the concentration is greatest, owing to the distance from the feeder canals) showed that the total salts amounted to only 1·34%, of which 0·92% was sodium chloride. Dr. Schweinfurth[15] concludes that the lake has a subterranean outlet, which alone would enable it to maintain its comparative freshness.[16] In this connection it is interesting to note the existence of distinct currents, which may possibly be caused by such outlets, in certain localities on the north side of the lake; and it is just possible that a careful survey of the lake itself would not only prove the existence, but show the exact position, of such underground outlets.