What, then, were the agents of denudation and transportation which operated in the formation of these great depressions? Under the existing arid conditions the surface rocks, unprotected by vegetation, are rapidly disintegrated or weathered as the result of the great diurnal variations of temperature to which they are subjected (insolation). The weathered material, however, does not accumulate and form a protective soil-cap, but is carried away by the wind (deflation), the heavier siliceous grains effecting an immense amount of abrasion of the exposed rock-surfaces over which they are swept. Changes of temperature, sand, and wind are, indeed, the chief agents of erosion and transportation at the present day, and, given a sufficiency of time and a continuance of favourable conditions, we can confidently admit the combination to be capable of effecting a vast amount of earth-sculpture. But the formation, in this way, of huge hollows 300 to 400 metres deep, and the removal of material amounting to hundreds of cubic kilometres, would necessitate the assumption that the present rigorous desert conditions have obtained for a very considerable period. Taking all the available evidence of which we are cognizant into consideration, we do not feel justified in assuming this to have been the case, especially when we recollect the frequent presence on the escarpments of thick deposits of calcareous tufa, which it is evident must have been laid down after the depression had been carved out to a considerable depth. These tufas are almost certainly of Pleistocene age, though whether they date from the early or late part of that period has not been determined. In some localities they occur as thick, horizontally-stratified beds, and were evidently deposited on the bottoms of lakes; in other places they appear as fan-like cakes spread over the face of the cliff, and may have been formed by springs situated near the summits of the escarpments. The tufas frequently contain large numbers of fresh-water shells and an abundance of fossil vegetation, and, from the presence of casts of the leaves of such trees as the oak, one is led to refer the deposit to the more humid period which preceded the incoming of the modern desert conditions.