100 PLANT STRUCTURES 
material? The answer is to be found, in the first 
place, in the decay of rocks under the influence of 
natural agents. Heat and frost, rain and drought, by 
slow degrees break up the surface of the hard material 
of which the solid crust of the Earth is built up. The 
débris thus formed is washed into streams by rain, 
or scattered by wind. A stream flowing into the sea, 
and charged with the débris of the land, deposits the 
coarser material near its mouth, while the finer 
particles are carried farther. In dry regions wind 
plays a similar part. And so, while the materials 
which composed the surface layer of the cooling 
primitive Earth may have been tolerably uniform in 
composition, the débris derived from them has ever 
tended to get sorted out, as, for instance, into sand 
and mud at river mouths, or sand and dust in dry 
regions. In the course of ages the sorted materials, 
buried beneath subsequent deposits, have been formed 
through heat and pressure into rocks, which, when at 
length again brought to the surface by earth movement 
and exposed to the agents of disintegration, have been 
resolved once more into sands, clays, and so on. In 
the long history of the Earth this sorting process has 
been repeated till now large tracts of rocks and of 
soils are composed mainly of sand or mainly of clay. 
The prevalence of these two kinds of material arises 
from the abundance in the primitive crust of the 
substances of which they are composed. Silica (oxide 
of silicon), the material of which ordinary sand, as 
well as quartz, flint, etc., is composed, is of extreme 
hardness and insolubility, and its small crystals and 
fragments, disintegrated from the rocks, remain 
almost indestructible as grains of sand. Clays, on the 
