Page:The Outline of History Vol 1.djvu/59

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are inclined to believe that the sun has blazed with a general steadfastness throughout geological time. It may have been cooling slowly, but, speaking upon the scale of things astronomical, it has certainly not cooled very much.


§ 3

A third great group of causes influencing climate are to be found in the forces within the world itself. Throughout the long history of the earth there has been a continuous wearing down of the hills and mountains by frost and rain and a carrying out of their material to become sedimentary rocks under the seas. There has been a continuous process of wearing down the land and filling up the seas, by which the seas, as they became shallower, must have spread more and more over the land. The reverse process, a process of crumpling and upheaval, has also been in progress, but less regularly. The forces of upheaval have been spasmodic; the forces of wearing down continuous. For long ages there has been comparatively little volcanic upheaval, and then have come periods in which vast mountain chains have been thrust up and the whole outline of land and sea changed. Such a time was the opening stage of the Cainozoic period, in which the Alps, the Himalayas, and the Andes were all thrust up from the sea-level to far beyond their present elevations, and the main outlines of the existing geography of the world were drawn.

Now, a time of high mountains and deep seas would mean a larger dry land surface for the world, and a more restricted sea surface, and a time of low lands would mean a time of wider and shallower seas. High mountains precipitate moisture from the atmosphere and hold it out of circulation as snow and glaciers, while smaller oceans mean a lesser area for surface evaporation. Other things being equal, lowland stages of the world's history would be ages of more general atmospheric moisture than periods of relatively greater height of the mountains and greater depth of the seas. But even small increases in the amount of moisture in the air have a powerful influence upon the transmission of radiant heat through that air. The sun's heat will pass much