Desert Conilitions in the Trias. 515) (i 
individual spheroids resting on these surfaces resemble perched blocks.. 
At the foot of a cliff screes of angular flakes tend to accumulate. 
These still further break up under similar influences and finally form 
sand. They may accumulate until the rock is buried under its own 
fragments, or in the rainy season the loose material may be carried’ 
away to form sandy deltas, and the surface of the solid rock may be 
kept open for further flaking. Sands resulting from the disintegration 
of granite and other rocks in the desert are in striking contrast to 
those formed in our own country in so much as chemical action takes 
no part in the breaking down of the rock. The fragments are 
chemically unaltered, and the flaked surfaces consist of perfectly fresh 
minerals. Of course, in the neighbourhood of mountains sufficiently 
high and conveniently situated to intercept moisture-laden winds, 
both chemical and mechanical disintegration may take place and 
streams may flow into the lowlands, mingling water-borne material 
with that of desert origin. 
It is difficult to apply the tests of sun-flaking and the characteristics: 
of desert sand to our Triassic deposits, as so few places exist where 
they are seen to lie on the rocks which gave them their origin. 
However, in the neighbourhood of Leicester, in Charnwood Forest, on 
Mount Sorrel at Croft, and at other places where the Keuper rests on 
the igneous rocks we get the very best conditions for making the 
comparison. Professor W. W. Watts has shown that in this district. 
the Keuper fills up hollows in the older rocks, and denudation is 
now uncovering the old pre-Triassic landscape. The older rocks, 
particularly at Mount Sorrel, present curved flaked surfaces exactly 
like those described from the desert. Loose blocks are flaked into 
spheroids and le tumbled in the Keuper Marls, which must have been 
accumulating at the time the blocks fell from the cliffs above. Screes. 
of angular flakes rest against the ancient slopes, and the sand is 
composed almost exclusively of material from the adjacent rocks. 
Moreover, the surfaces of the rock, the scree material, and the sand 
are all chemically fresh. 
The absence of decomposition products is well seen, too, in the 
Keuper sandstones of our own neighbourhood in places where infil- 
tration has not taken place. When a clay band occurs in a rock face, 
the felspars in the top portion are usually kaolinised by the percolation 
of surface waters, but below the impervious layer they are remarkably 
fresh. This conclusively demonstrates that when they were laid down 
they were not subject to chemical changes. 
Work of the Wind.—During the dry season the wind does its own 
work in carrying, sifting, rounding, and etching. 
(1) Carrying.—In open ground driven sand tends to accumulate in 
ridge-shaped masses, the long crests lying transversely to the wind. 
When a dune of this kind has been originated the wind blows grains 
up the exposed slope and over the summit, where they drop on the 
leeside. The slope to windward is gentle, and on the leeside steeper 
and slightly concave. The concavity is due to a backward eddy 
which sets in as the wind passes over the crest, and can easily be 
demonstrated in our local sand-hills by putting a piece of paper or any 
light object against the leeward slope when a stiff wind is blowing. 
The paper will be seen to move up the slope against the wind. 
