Page:South African Geology - Schwarz - 1912.djvu/27

 globe, but is curved upwards towards the continents. In other words, were the attraction of the continents on the water of the ocean to be done away with, the sea round the coasts would sink several hundred feet, and the continents would stand so much higher above sea level.

An alternative view has been held, which supposes the water in the ocean basins to be kept in place by denser rock forming the floor of the oceans; the rock forming the continents is about 2.6 times as heavy as water, whereas the sub-oceanic crust has been supposed to be three times as heavy. This necessitates that the oceans have always occupied more or less the same place. This has been disproved, and it is now certain that just as the continents have been covered repeatedly by the ocean, so the ocean floor as often has been exposed as dry land. The Permanence of Ocean Basin Theory, therefore, is no longer tenable, though it was powerfully advocated by Dana and Wallace.

The Atmosphere. — Surrounding the globe is an envelope of gas consisting principally of oxygen and nitrogen, but carrying also carbon dioxide and water vapour in notable proportions. This atmosphere enables plants and animals to live, but it has also a profound influence on rocks, and its action is called "weathering". On the surface of the moon, which has no atmosphere, the rocks stand up clear and unaltered, though they have been exposed for countless ages; a slight alteration is noticeable in some of the mountains of the moon, which is ascribable to the splitting of the rocks under the influence of the alternating heat and cold of the moon's day and night. Our day is much shorter than the moon's day, and the variations of temperature are consequently so much less, yet