512 Notices of Memoirs—J. Lomas— 
to floods of so sudden appearance and of such torrential violence that 
people engaged in gathering firewood, brought down by former floods, 
are sometimes overtaken and have no chance of escape. At one time 
the bed may be perfectly dry and a moment later it is filled with 
a rapid stream hundreds of feet wide. In India these sudden floods 
are sometimes miles in width; they have no defined channel, but flow 
like a sheet over the land. 
It is worth while at this stage to picture what kinds of deposits are 
being formed under the conditions just described. While the main 
portion will be composed of sand there will be included at various 
horizons patches of gravel, lenticular in shape and of limited extent. 
I have seen sections through such deposits in a dry donga near Tintas 
Kopje in the Vryheid district, and again in railway cuttings when 
traversing the Eastern desert of Egypt between Ismailia and Cairo. 
The pebble bands varied from a few inches to 4 or 5 feet in thickness, 
and in my notebook is an entry ‘‘exactly like our Bunter.” How- 
ever, we shall return to that later. I only wish at present to record 
the impression made on me at the time. | 
What becomes of the water 2—The water flowing over the sand may 
be disposed of in three ways. (1) It may percolate under the surface ; 
(2) it may be evaporated ; (3) it may lodge in pools or lakes in places 
which are below the general level of the country. 
(1) Percolation.—It has been known for a long time that under the 
dry baked sands of the desert there commonly exists a great store of 
water, which only needs to be tapped in order to be available for use. 
In the Sahara artesian wells were sunk by the Greeks and Romans. 
In Algeria a class of men called Meallem were formerly employed as 
water-finders, and another class—the R’tassin—were engaged in the 
construction and cleaning out of wells. Their methods were primitive 
and involved great dangers. Since the French occupation more 
scientific appliances have been introduced, and the fringe of fertile 
soil on the margin of the great Sahara is gradually beng extended 
southwards. In sinking the dry sand is penetrated until an im- 
permeable clay or pan is reached. On piercing this the water rises, 
sometimes with sufficient force to reach the surface. The depth of 
the impermeable layer varies from a few feet to hundreds of feet, 
and striking differences of level may be encountered within a short 
distance. 
The condition of the water below the impervious bands must 
necessarily be largely a matter of inference. We naturally want to 
know whether it is stagnant or flowing, and whether it is carrying 
substances in solution and depositing these round the sand grains and 
thus cementing them into compact rock. If the water is flowing it 
must trend towards an outflow, and if this is restricted im area there 
must be definite lines along which the water runs. The stream lines 
will, under these circumstances, be convergent towards the outfall, 
and the water outside the cone of flowing water may be stagnant. 
That subterranean streams of fresh water do flow into the Red Sea 
from the desert is certain, and opposite their mouths the coral growth 
round the coast is interrupted. Some of these streams have been 
traced underground for great distances almost to the base of the 
