ROCKS 57 
between rain and frost; while in hot and dry deserts, rocks 
crumble away mainly under the influence of insolation—they 
expand when heated up during the day, and contract more 
or less rapidly at night, and thus their constituent ingredients 
lose cohesion and the rocks become disintegrated. Some 
or all of these operations, it is obvious, may be carried 
on concurrently, almost anywhere, so that it is usual to 
include them under the general term of weathering. Of 
subaérial and zolian rocks, the most important are the 
following :— 
Soil and Subsoil.—The particular origin of these will 
be fully discussed in Chapter XXIV., and they need only be 
shortly defined at present. Sudsoz/is an unconsolidated hetero- 
geneous aggregate of disintegrated rock-material; while Soz/ 
is essentially the same, with the addition of organic matter. 
Rock-rubble is the general term applied to collections of 
angular fragments which owe their origin chiefly to the action 
of frost in high northern and temperate regions, and mainly 
to insolation in low latitudes. Familiar examples are the 
taluses of stony débris which gather at the base of precipice 
and scaur, and the sheets of angular rock-rubbish which 
curtain the hill-slopes of our mountain areas. A rubble of 
angular fragments formed in this way, if cemented together, 
would be called Scree-breccta. [There are many kinds of 
breccia—this term being qualified in each case by some 
adjective descriptive of its origin or the character of its 
dominant components. | 
Rain-wash, Brick-earth, etc.—In this and other temper- 
ate regions the finer grained material derived from the 
disintegration of rocks by weathering is gradually washed 
down by rain from higher to lower levels, and tends to 
accumulate on gentle slopes and in hollows. This rain-wash 
is occasionally sufficiently fine-grained and plastic to serve 
for brick-making purposes (d77ck-earth). But rain-wash may, 
on the other hand, consist of very coarse materials. In some 
countries where the rainfall is crowded into a short space of 
time, sudden torrential rains sweep the steeper declivities of 
the land bare of rock-débris, and spread the materials over 
the low-lying tracts that extend outwards from the hills and 
mountains. The stones included in such accumulations are 
