OCCURRENCE OF SMALL DESERT TRACTS. is 
(ii.) The inability of the sediments to take up moisture 
to any considerable extent ; which, combined 
with their poor retentive powers, has led, during 
the hotter months of the year, to the almost entire 
depletion of their free water content. 
(iii.) The air spaces between the particles of the now 
perfectly dry soil resist the downward percolation 
of the rains when they fall ; while in the absence 
of a continuous film of wetted particles to establish 
surface tension (and thus bear the water down- 
wards) this resistance can hardly be overcome. 
By the gradual crumbling away of the red earth deposit and 
the consequent exposure of fresh sedimentary surfaces the 
deserts are slowly expanding. Year by year they encroach 
upon the red earth. 
Heat and the absence of water are the two chief factors in 
desert development.* Climate alone may account for desert 
conditions, and in this connection Corstorphine calls attention 
to the fact that certain sandstones and shales, which yield 
deep fertile soils in Southern Transvaal and in the Orange Free 
State, form desert areas in Cape Colony.f Here in Ceylon our 
small desert tracts are due, as we have seen, not so much to the 
arid nature of the climate as to the inability of the ground to 
hold water. 
Once a desert is started it tends to grow, and our small 
examples in Ceylon are no exception to the rule. By the 
gradual crumbling away of the red earth deposit, and the 
consequent exposure of fresh sedimentary surfaces, the deserts 
are slowly expanding. Year by year they encroach upon the 
red earth escarpment, and by imperceptible degrees the living 
forest gives place to the barren wilderness. 
Under the present conditions the sedimentary rocks of this 
type, and in this part of the country, must always give rise to 
arid tracts—unless the theory advanced for their origin be 
wrong. 
* For an account of the conditions which make for aridity see Prof. 
Macdougal’s Paper (Jour. R. G. S., Vol. XXXIX., No. 2, Feb., 1912, 
pp. 105-120). 
+ Proc. Geol. Soc. (S. A.), 1907, p. xix. 
