172 SPOLIA ZEYLANICA. 
of spherical particles all of the same size, the empty spaces 
between these particles would amount to about 47 per cent. of 
the volume with the loosest packing, and to nearly 26 per cent. 
with the closest packing. The total empty space would be the 
same, whatever the size of the particles. If the interspaces with 
the closest packing were occupied by another set of smaller 
spheres, they would be reduced to 6°7 per cent. of the volume. 
If this process was again repeated, they would become 1:7 per 
cent. With loose packing the proportion of interspaces would, 
’ 
in all cases, be much larger ...... * In many subsoils the 
ingredients are of various sizes and irregular shapes, as in @ 
mixture of gravel and sand, or in boulder clay, with a conse- 
quent reduction of pore-spaces. In many loams, clays, and marls, 
as well as sands, the materials are fairly uniform.* 
It will be seen therefore that the character of the desert 
floor (recorded above) is such that it is unable to take up as 
much water as the red earth; and we learn, moreover, that 
its retentive powers are less, for as Woodward (speaking 
on the authority of Warington) says :— 
The amount of water retained by @ soil depends on the 
extent of the surfaces of the particles to which the water adheres, 
not on the volume of the interspaces; hence, the smaller the 
particles, if they are not excessively fine, the greater is the 
amount of the water held by capillary attraction.’ } 
Add to this the fact that rain falling on a dry soil sinks but 
slowly, while in a moist soil it is freely absorbed, and the 
apparently anomalous juxtaposition of forest and desert 
becomes easy of explanation. It would seem that most of 
the rain which falls on the barren tracts flows rapidly away, 
and the remainder is abstracted by the process of evaporation, 
which, in the almost entire absence of vegetable growth, is 
unimpeded. 
These, then, appear to be the conditions which have given 
rise to the small desert tracts which are described in these 
notes :— 
(i.) The exposure of the sedimentary beds, possibly at a 
time when the rainfall was greater—or more evenly 
distributed throughout the year—than it is at 
present. 

*H. B. Woodward: ‘‘ The Geology of Soils and Substrata,” 1912. 
} Op. cit. 
