128 THE FUTURE OF ARID LANDS 
rift shows that in the saline areas the beds of sand and gravel 
interbedded with thick clays also yield saline water (14). 
In some countries, as in northern Africa, bodies of shallow 
saline water are sometimes due to the evaporation of ground 
waters of usable quality, and there is room for further investiga- 
tion into the occurrence and use of such ground waters. 
For these and other reasons related to the nature and structure 
of rocks and the relative freedom of movement of waters in them, 
usable and unusable waters are frequently found only short dis- 
tances apart, so that an area yielding unusable supplies should 
not be given up too hurriedly before the possibilities of finding 
local fresh waters have been exhausted. 
A recent survey carried out by Bosazza (1) in the low rainfall 
and former desert area of the Sul Do Save in Mozambique shows 
that over an area of about 2,500 square kilometers, out of all the 
boreholes drilled to 30 to 40 meters, 24% are of low enough sa- 
linity for human consumption and a further 24% good enough 
for stock. Thus 48% of the boreholes are usable, a percentage 
that is very high for southern Africa; and these conclusions are 
drawn from work on boreholes which have been drilled at inter- 
vals throughout the area without any geophysical work to assist. 
Shotton (25) has recorded that ideas on the standard of water 
acceptable to man for drinking have changed considerably in 
recent years. It may now be taken as a fact that water with a 
salinity of 3,000 parts by weight of sodium chloride per million of 
water can be drunk regularly by human beings in a desert climate, 
that a figure of 4,000 unaccompanied by important quantities 
of other salts 1s acceptable, and that for short periods even a 
figure of 5,000 is endurable. Domestic animals are often more 
tolerant of dissolved constituents than man though there is no 
close agreement on the worst limits of quality. Much investiga- 
tion on this question has been carried out in Australia. Jack (15) 
in South Australia states that horses will thrive on water with 
I ounce of sodium chloride per gallon (6,260 parts per million) 
and sets upper limits for living as 7,800 for horses, 9,400 for cattle, 
and 15,600 for sheep—unless magnesium sulfate is present, 
when the figures must be lowered. Edgeworth-David and Browne 
