SOP PAR EICLES IOI 
other hand, are derived from silicates (compounds of 
silicon and oxygen with various metals such as 
aluminium, calcium, magnesium, potassium, sodium, 
or iron). These substances mostly disintegrate more 
completely into very small particles, which when wet 
cohere into a sticky mass and form clays. Along 
with the humus matter they include all the colloids of 
the soil. These latter bodies consist of the extremely 
minute—indeed, ultra-microscopic—particles, having 
in consequence of their small size a great total surface 
in proportion to their mass. In virtue of this, they 
function as the chief absorbents of the soil, holding 
water in enormous quantities, and abstracting and 
retaining till used by the plants the bases of the 
various substances applied as manures. Another 
constituent of the primitive crust was lime (oxide of 
calcium). Unlike the preceding substances, lime is 
readily soluble in acid water, and so is washed out of 
the rocks and carried in solution to the sea. Marine 
animals of many kinds—such as Molluscs, Corals, 
Foraminifera—extract the lime from the sea water 
and use it in large quantities to build up their shells 
or skeletons. This material slowly accumulates at the 
bottom of the ocean as generation after generation of 
animals passes away, becomes at length consolidated 
by heat and pressure, and through earth movements 
may eventually appear above the sea to form land, in 
the form of limestone or chalk. Exposed to the 
weather, it is once more slowly disintegrated; the 
lime passes off again in solution, the impurities being 
left behind; a limy soil results. 
On a great plain, devoid of hills or rivers, composed 
of different rocks, and subjected to the agents of 
