Page:South African Geology - Schwarz - 1912.djvu/55

 salts in the ocean is possible except by organic agency. The magnesium salts, however, compromise by uniting with the calcium salts after the animals die, so that half the latter are set free again to be used by the living animals. This process of combination of calcium and magnesium salts is known as dolomitization, and the chalk or coral rock, instead of remaining a pure limestone, becomes a dolomite. All natural limestones contain a certain proportion of magnesium. This process is continued, after the sediment is hardened into rock, by the adding of magnesium by water circulating from the surface downwards.

In some places the surface of the ocean, owing to the cold or from other unfavourable circumstances, is devoid of life, and then the floor receives no deposit, except the very finest dust, which results from the burning meteorites (cosmic dust) and volcanic dust.

The floor of the ocean being for the most part covered with sediment of some kind shore, deep sea, or abyssal is more or less flat; but in reality it is simply a sunken continent, the islands being the tops of mountains; and the valleys can to a certain extent be recognized by soundings in spite of the covering of sediment. Very careful surveys of the ocean floor have been made by the cable companies, and we know now that the ocean floor is far from being a featureless plain. Round the borders of the continents there are shelves beneath the sea, from which evidence can be obtained, in the form of river boulders, and continuations of the river gorges, that they were once dry land. The most noteworthy of these is the 100-fathom plateau on which the British Isles stand, and a ledge submerged to the same extent is also found on the other side of the Atlantic. In South