Desert Conditions in the Trias. 513 
Abyssinian mountains. The wells in the oases, too, must be fed from 
the same source. 
Similar lines of flow of percolating water can be traced in our local 
Triassic rocks. Sometimes they show themselves by a deeper staining, 
and in extreme cases the grains of sand have become so heavily charged 
with interstitial matter that they have formed pipes, the interior of 
which is dark brown or even black, while the sand surrounding is 
almost white. 
(2) Evaporation.—Another way in which the water flowing from 
the high grounds is disposed of is by evaporation. The amount of 
evaporation is largely dependent on the relative humidity of the air. 
Over the oceans the air may be approximately saturated, but in 
deserts the amount is commonly as low as 20 per cent. of the total 
capacity of the air to hold moisture. At Assuan it is 38 per cent. for 
the year, varying from 29 per cent. in Summer to 51 per cent. in 
Winter. At Wadi Halfa the average for the year is 32 per cent. 
When the air is not saturated, there is a constant exchange of 
water from the land to the atmosphere, and this results in evaporation 
at the surface and a creep of water from below owing to capillary 
action. Mineral substances contained in solution are disengaged from 
the water on evaporation at the surface, and the soils in course of time 
become charged with saline matter. In this way the ‘sour’ or 
‘alkaline’ soils have their origin. 
The nature of the encrustation will, of course, depend on the 
composition and the amount of salts which the water holds in solution, 
and these again are dependent on the nature of the rocks over which 
the water passes. Potash and soda salts are common encrustations in 
some regions, and they are often associated with the carbonates and 
sulphates of the alkaline earths. The former may be redissolved and 
partly removed in the rainy season, but the salts of the alkaline 
earths will be effected only to a very slight extent and will tend 
to accumulate. A very instructive example of this occurs in the dry 
donga near Vryheid mentioned above. Near the surface the sands are 
coated with carbonate of lime. Sometimes there appear embedded in 
the sand roundish balls, ranging from the size of a marble to that of 
a man’s head. In other places limestone of a compact texture and 
not enclosing sand forms a continuous layer on the surface of the 
ground. 
At Croft, near Leicester, a calcareous crust exactly resembling that 
found in deserts occurs on the surface of the underlying igneous rock, 
ah the covering Keuper marls are strongly impregnated with lime 
salts. 
(3) Desert Pools and Lakes. — Looking across a vast expanse of 
desert the monotony of the scene tends to dwarf the sense of vertical 
height. With no outstanding features to rivet the attention of the 
eye the rolling dunes or naked rock give the impression of a level 
landscape. As a matter of fact the desert has its hills and its hollows, 
its crags and its ravines. In some of these hollows, enclosed in a net- 
work of dunes or in rocky basins, water tends to accumulate and form 
pools or lakes. They may be fed by rain, by water percolating 
through the surface sand, or by stteams flowing from the hills. 
DECADE Y.—VOL. IV.—NO. XI. 33 
