Page:South African Geology - Schwarz - 1912.djvu/51

 the floor of the ocean within 200 ml. of the coast. They are known as the marine sediments, and when hardened and consolidated are known as sandstones and shales, which form the bulk of the sedimentary rocks. Besides these mechanical sediments, as we may call them, rivers carry substances which they have dissolved out of the rocks; all river water will, on analysis, yield quite perceptible amounts of various salts. We can find the salts in river water, and by measuring the discharge of the river we can estimate the amount of salts removed annually from the land drained by the river. In round numbers the average is as follows: —

AMOUNT OF SUBSTANCES REMOVED ANNUALLY IN SOLUTION FROM THE LAND SURFACES OF THE GLOBE (Mellard Reade)

Silica and iron are not found in sea water, though both must exist in minute traces; but the rest are found somewhat changed in the salt evaporated from sea water.

COMPOSITION OF SEA-SALT (Dittmar)

The proportion of sodium chloride to the salts of lime is 8 to 70 in the river waters, but in the sea it 77 to 4.