The earth being chiefly covered with water, and the air partly freed from watery vapour, then commenced the great creation of organised beings. The air, although to a certain extent, free from vapour, must yet have contained an enormous amount of carbonic acid; this, being less easily condensed than any of the matters which had gone through that process, would still remain there as a gas, and the effect of this superabundance would be to saturate all the water covering the earth with it; this solution of carbonic acid, being capable of dissolving lime, would (as it percolated and rushed in currents through the rocks and inequalities of the surface) become converted into a solution of percarbonate of lime, taking up a large quantity of this earth from the broken-up granite, &c., then this carbonate of lime would be deposited at the bottom of the water in one of two ways--either from pressure, diminution of carbonic acid in the air, and heat of the surface, it crystallised at the bottom in the form of marble (that the water does thus become saturated with carbonic acid and take up lime, is constantly shown by the stalagmites which form on the floors of caverns, where the water thus loaded with percarbonate of lime, evaporating, deposits gradually the carbonate in all sorts of fantastic shapes, as in fig. 9), or the myriads of crinoidians and brachiopodous molluscs (fig. 10) which were about this time created, absorbed it into their systems and at their death deposited their shells, which are made of this earth, at the bottom of the sea; these shells, accumulating through ages into strata, became hardened and partly crystallised by heat, thus forming the limestones, which were the first containing carbonate of lime. That such a collection, from such causes, is quite possible, will be presently shown in describing the chalk formation. Ages of comparative quiet now appear to have succeeded the first great contraction of the earth's crust, probably millions of years, during which time the tides and currents of the ocean had to wash and wear down all the thousands of projecting rocks or inequalities and dissolve (as before described) all the lime, depositing the sand and clay in those immense strata which form the "transition series;" this appears to have taken place over nearly the whole world at that time, and ages upon ages must have elapsed to form such deposits as the sandstone, claystone, and limestone, in alternation, forming the "Llandilo," "Caradoc," and "Wenlock" strata, more than a mile in thickness; these are by some geologists reckoned among the primary series (by some called the "transition rocks"), and in England form the "Cambrian" and "Silurian" systems. [Illustration: FIG. 11--TRILOBITE.]