(4) The Ocean as a Storehouse of Minerals.—Sinee 3-} per cent of the weight of sea water consists of solid mineral matter in solution, the ocean serves as a storehouse of minerals. Every stream and river carries a small amount of such material in solution. When the water reaches the sea the liquid eventually is evaporated and goes back to the land, but the mineral matter remains. Thus the sea has slowly accumulated a vast amount of common salt, lime, potash, phosphorus, and many other materials. Even gold and silver are included, but in amounts so extremely small that they cannot be recovered at a profit.
(a) Salt.—The only dissolved material that man takes from the water in large quantities is common salt. On warm sunny seacoasts where the water is shallow, large ponds are often banked off by dykes.
Here the water evaporates until the salt crystallizes out. On the shores of the Mediterranean Sea near Smyrna, for example, and on the coasts of Central America, great piles of white salt crystals often form gleaming cones. Most of the world's salt, however, comes from an cient deposits like those at Syracuse and Stassfurt, and was laid down long ago in salt lakes whose waters very slowly dried up in the same way that the water of the enclosed ponds on the seashore now does.
(b) Limestone.—Aside from salt, the most valuable mineral in seawater is lime. Shellfish constantly use this for their shells. Some of the shells are thick and heavy like those of clams, oysters, and the great edible abalone of the Pacific coast. Others are beautifully branched like many corals. Still others are so small and thin that they cannot be seen by the naked eye. Such are those of the globigerina ooze, a soft mud which covers large areas of the sea floor, and which would form chalk if converted into stone. One or another of these kinds of shells has given rise to vast deposits of limestone. Since the sea once encroached far into what is now the continental interior, large deposits of limestone are found in most parts of the country. Without them we should be at a loss to make cement and concrete, to obtain lime for mortar and plaster, and to find the flux so essential in the smelting of iron.
(c and d) Potash and Phosphorus.—Certain other valuable mate rials, although present in quantities too small to be profitably ex tracted by man, are taken from the seawater by plants and animals.
One of these is potash. A certain alga or seaweed called kelp con tains so much potash that it is gathered by seacoast farmers as a fertilizer. According to the United States Department of Commerce the kelp crop on our Pacific coast would be worth $100,000,000 per year if properly harvested. Another valuable fertilizer, phosphorus, is taken from the seawater by fish, and is found in their bones and scales.
(5) The Ocean as a Source of Food: Marine Vegetation.—Except where waves and currents are too violent the sea floor from the level of high tide to a depth of about 600 feet is largely covered with plants, chiefly of the kind called algae. In deeper water plants cannot grow because there is no sunshine. Even in mid-ocean, however, as far down as the light penetrates, the water is full of microscopic one celled plants, small larvae and other minute animal forms. When the " plankton " dies much of it sinks so that even in mid-ocean minute bits of vegetable and animal matter fall constantly. The ocean vegetation is of little direct use to man, but it furnishes a vast supply of food for organisms like oysters, shrimps, and fish, which in turn are eaten by man.
How Man Utilizes the Food in the Ocean through Fisheries.—The presence of vegetation and hence of fish in the ocean gives the people of the sea coast an advantage because they can carry on fisheries as well as the ordinary occupations of the land. The word fisheries means not only the work of catching fish, but of gathering mollusks or shellfish like the oyster and clam, crustaceans like the lobster and crab, and even mammals like the whale and seal. The fisheries of the United States furnish an amount of food equal to nearly half the pork consumed in the country. In countries like Norway and Japan, where the mountains make farming difficult and where the deeply indented coasts are favorable to navigation, fish form the most im portant animal food. In Japan the traveler is surprised by the variety of ways in which they are served, for in addition to the ordinary dishes, he may be offered raw fish with salt and pepper, or a soup made of the water in which fish have been boiled. In our own country fish are used chiefly near the indented coasts of the rugged northeast and northwest, but form an important element of diet in most parts of the country.