Notices of Memoirs—Desert Conditions in the Trias. 511 
IlI.—Dersert Conpitions AND THE ORIGIN oF THE Bririsu Trras.! 
By J. Lomas, A.R.C.S., F.G.S. 
T is now generally conceded that the Triassic rocks of our Islands 
are not of marine origin, but true continental deposits, and the 
view has of late gathered strength that the climate was arid when 
they were laid down. We can only hope to reach a safe conclusion 
by a critical examination of existing deserts. Let us confine ourselves 
to a consideration of the activities now at work in some typical sandy 
deserts, such as we suppose to haye existed in Triassic times. 
Sanpy Deserts. 
Rainfall—As a rule sandy deserts are characterised by a low 
rainfall, and their distribution is in general either on the leeward 
sides of mountain ranges or in valleys and plateaux between such 
ranges. In Northern Egypt the precipitation ranges from 0:27 inch 
at Cairo, to 1-lin. at Suez, 2°lin. at Ismailia, 3:4 in. at Port 
Said, and 8*lin. at Alexandria, In South California and West 
Arizona certain regions have a mean annual rainfall of less than 
2inches. In the Karoo the precipitation varies from 8°65 inches at 
Steotleville to 18°76 in. at Graaff-Reinet (Buchan). In the Kalahari 
desert from 3°79 in. at Pella to 41°10 in. at Pilgrims Rest. On the 
west coast of Africa from 2°11 in. at Port Nolloth to 8°37in. at Clan 
William. 
Examples need not be multiplied; those quoted are sufficient to 
show that the amount is generally small and varies very considerably 
in different places. An equally important consideration is the time 
during which rain falls. If the precipitation is confined to one 
season and for the rest of the year there is little or no rain, desert 
conditions may exist, although the annual rainfall is comparatively 
large, During the wet season the ground may be well watered, 
and streams of torrential size flow over the country, but the water 
is soon lost by percolation or evaporation and none is stored up to 
water the land in time of drought. 
Vegetation is either very scant or absent, and the lack of vegetable 
mould diminishes the power of the soil to retain moisture. Professor 
N. 8. Shaler (U.S. Geol. Sury. 12th Report) points out too that the 
absence of vegetable mould commonly causes the soil to present a dense 
baked surface which sheds rain like a roof. 
. Streams of Desert Regions.—In South Africa during the dry season 
we often come across deep watercourses excavated during the time 
of the seasonal rains but now perfectly dry. Their beds are covered 
with well-rounded boulders of such a size as to indicate that torrents 
must have flowed down the courses in order to move them. Towards 
the mountains the banks become steeper and loftier, while in the 
direction of the plain the valley merges into the general outlines of 
the veldt, and at the junction it is marked by a spread of pebbles. 
In the Libyan and Algerian deserts similar watercourses are found 
in the neighbourhood of their enclosing mountains. They are liable 
1 Proc. Liverpool Geol. Soc., vol. x, pt. 3, 1907, pp. 172-180; slightly abridged. 
