
SOILS AND SUBSOILS 369 
by epigene action. Again, while dealing with the structural 
phenomena of derivative rocks, we have briefly considered 
the various origin of sedimentary deposits. The remarks 
that follow, therefore, may be taken as a kind of summary 
of much that has already been advanced on the subject of 
rock-disintegration, illustrated by special reference to the 
geological origin of soils and subsoils. 
Insolation and Deflation.—Among the various agencies 
that tend to disintegrate rocks, and to form a soil-cap, are 
changes of temperature. Rock-surfaces are heated by day 
and in summer time—cooled at night and during winter. 
They thus alternately expand and contract, and this leads 
to disintegration, for their constituent minerals often yield 
unequally to strain or tension. Such is notably the case 
with rocks composed of minerals differing in colour, density, 
and expansibility, such as granite, gneiss, diorite, etc. Even 
in the case of homogeneous rocks, it is obvious that alternate 
heating and cooling of the surface must give rise to strain 
and tension. In countries like our own, where there is no 
great diurnal range of temperature, any rock-changes due 
to this cause alone are hardly noticeable, since they are 
masked or obscured by the action of other and more potent 
agents. But in the rocky deserts of tropical and subtropical 
regions, bare of verdure and practically rainless, the effects 
produced by alternate heating and cooling, or “insolation” 
as the process is termed, are very marked. The rocks are 
cracked and shattered to a depth of several inches; the 
surfaces peel off and are rapidly disintegrated and pulverised. 
Wind then catches up the loose material and sweeps it away, 
leaving fresh surfaces exposed to the same destructive 
action. More than this, the grit, sand, and dust carried off 
by the wind are used as a sand-blast to abrade and erode 
the rocks against which they strike. In this manner cliffs 
and projecting rocks are undermined, and masses from time 
to time give way and fall to the ground, where, subject to 
the same grinding action, especially towards the base, they 
tend to assume the appearance of irregular blocks supported 
upon pedestals. “Deflation,” or the transporting action of 
the wind, goes on without ceasing, with the general result. 
that the whole surface of a rainless region tends to be 
Ps 
