Page:Popular Science Monthly Volume 54.djvu/409

Rh interior. Originally the whole mass was fluid, but the surface has cooled more rapidly than the interior, and so a firm crust has been formed. As the central mass cooled, it contracted, and the crust became wrinkled and folded, as does the skin of an apple as its pulp dries, and, by this folding, great ridges were thrown up in some places and vast depressions formed in others. When the crust became cool enough for water to remain on it, most of the depressions were filled by it, and the "dry land appeared," not only on the crests of the ridges, but on the elevated plateaus about them, and thus oceans and continents were formed.

Had one of us seen the earth at that time he would have been loath to select it as a residence. Rugged, rocky ranges of precipitous mountains surrounded by stretches of naked rock made the landscape. Dense clouds from the tepid oceans dashed against the icy peaks, and torrents of water rushed back to the sea. Where the slopes permitted, the glaciers spread over wide areas, for no vegetation checked the rapid radiation of heat, and night brought bitter cold. The crust waved and fluctuated over the liquid interior as does thin ice under a daring skater, and as it fell the sea rushed over the land, only to flow elsewhere as the depressed area rose again. The freezing and thawing and the effects of wind and water in time produced a change. The rocks were riven and broken to powder, their nearly vertical slopes became less steep, and instead of bare rock the earth showed dreary morasses and stretches of sand.

Over these marshes vegetation began to thrive. In the sea there lived then, as now, a teeming population, animal, vegetable, and living beings that can with difficulty be assigned to either of these classes. Each of them, however, contained carbon, and many had built lime, phosphorus, nitrogen, and other valuable substances into their bodies. Where food was abundant these grew in vast numbers, and though many are infinitely small singly, their aggregate mass is enormous. Among the tiny organisms is one called the Globigerina, a being so small as to require a microscope to study it, but in the past, as now, growing in great numbers in the sea. The animal is soft and jellylike, but it forms an outside skeleton of shell of carbonate of calcium or chalk, a structure that protects it living, but entombs it dead. When death comes, the little Globigerina sinks to the bottom, and its tiny shell helps to cover the sea floor.

In the days of long ago these lived as now, and when some convulsion of Nature lifted the bottom of prehistoric seas, the Globigerina ooze was lifted as well, and thus the "limestone" formed. In our land a bed of this kind extends from Alabama to Newfoundland; thence, as the "telegraphic plateau," it passes under the Atlantic, rising into the chalk downs and cliffs of England; then, again