When we remember that the Mesozoic aeon was preceded by the long Palæozoic, and earlier by the Eozoic,—whose aeon has been laid down at eleven million years, with a strata at least 50,000 feet thick of hardest rock,—what could have been the cause of the submerging of the lands where these gases and coal lay? For millions of years the Eozoic strata had upheld the oceans that were two or more miles in depth, and of enormous weight. If the earth was cooling all those years the crust must have been hardening, and what added weight could have been at so late a period to cause the submerging of those lands? It has been thought that in the glacial period when the accumulation of ice over a portion of earth’s surface lay 5,000 feet thick that its weight might have depressed the terrestrial crust.[13] But ice being lighter than water, even though it were of that thickness, would not have had the weight of water a mile in depth; whereas more than twice that depth of water rested upon three-fourths of the globe. Moreover the ice-period was many millions of years later than the Palæozoic and Eozoic ages, and had the earth all this time been growing colder its crust must have been thickening, making it capable of enduring almost any pressure. Any crust, as ice upon water or the shell upon an egg, will bear more than its whole weight before it will sink into the substance upon which it rests. It might be thought that earth, while cooling, would shrink beneath its crust and leave a vacuum, as ice sometimes does upon a small surface. This could not happen as the vast surface of extended crust, by the enormous pressure of air upon it, would be held closely pressed to whatever was beneath. For, although earth’s surface is convex, it is still 25,000 miles around it, a distance so great that its crust would have almost the same pressure as if the surface were level. FOOTNOTES: [12] Winchell, “Walks and Talks in the Geological Field,” p. 99. [13] Winchell, “Walks and Talks in the Geological Field,” p. 275. CHAPTER VI. THE EARTH’S HEAT.