ORIGINAL COMPOSITION OF TIIE EARTH'S CRUST.
Geologists believe that there was a period in the history of the earth when its entire surface was composed of crystalline rocks like those described and that this was so because the whole earth was then highly heated and the atmosphere was many times denser than now. The con ditions then were exactly those that make silicic acid more powerful than the other acids and consequently it was able to take possession of all the bases and so form crystalline (granitoid) rocks which are aggregations of silicates.
These conditions probably did not reach to any great depth, possibly some tens of thousands of feet, and below that the bases are probably either uncombined or exist in combination with each other. Let us then think of the earth at this early time as being covered with a mantle some tens of thousands of feet thick made up entirely of silicates and that this mantle then contained all the chemical elements with which we are familiar, except possibly the gases of the atmosphere, and to some extent these also, locked up in the form of silicates. As the earth gradually lost its heat, let us think of these silicates as passing from one form into another under the compulsion of changing conditions of heat, pressure, etc., and so come to look upon these outer layers of the earth not as something that is fixed, stable, immutable, the symbol of all that is untransformable and enduring, but as a busy workship in which the various chemical elements are always trying to adjust themselves to everchanging conditions and are never quite able to reach their goal.
After millions of years of this activity, the conditions at the surface of the earth came to be markedly different from those which have been described. During all this time the earth had been absorbing the gases of the atmosphere and its pressure had been reduced to approximately what it now is. The absorption of gases made the atmosphere more transparent to dark heatt and so permitted the earth's surface to cool more rapidly, and this in turn allowed the water, most of which had up to this time been suspended in the atmosphere, to be precipitated and remain upon its surface.
The cooling of the earth's surface and the reduction of atmospheric pressure gradually destroyed the conditions which gave to silicic acid the power to keep possession of the bases in spite of the presence of other acids, consequently these acids feeling their newly acquired strength be gan to assert themselves, to crowd out the silicic acid, and to unite with its bases. Among these acids carbonic acid (carbon dioxide + water] easily takes first place, principally on account of its abundance in earth-water.