It will be readily seen that the saltness of the sea will be, generally speaking, not only more uniform, but also some what greater at certain depths than at the surface. For the surface waters of the sea are nearly everywhere more or less subject to admixture with large quantities of fresh water from rivers, rainfall, or melting icebergs. And as the fresh water thus obtained is specifically lighter than the salt water on which it falls, or over which it flows, it will float on the sur face for some time. This comparative freshness of the surface water is, of course, greater in certain localities and under cer tain conditions, as at the mouths of large rivers, or immediately after excessive rainfall, or, in the polar seas, after extensive liquefaction of enormous icebergs. In fact, water fit for use has been often " skimmed " from the surface of the sea several miles out; and it is a proved fact that the fresher waters of the Amazon may be perceived even at a distance of two hun dred miles out in the open sea. The regions of minimum saltness are'—the Gulf of Guinea, North Sea, a narrow belt along the American coast from Newfoundland to Florida, in the Atlantic ; about north and south of the Equator in the Indian Ocean ; about 10° north and south of the Tropic of Cancer in the Pacific ; and south of lat. 40° S. in the Southern
Ocean.
Besides the saline matters chemically dissolved in the waters of the ocean, other ingredients are temporarily me chanically suspended in its waters—sand, mud, and other debris being nearly always present, especially near the coast, but never in any very considerable quantities. These sub stances are brought down by rivers, or washed off the coast by wind-waves and tides ; and although the finer particles are probably transported to great distances, the greater portion are deposited on the bottom of the sea in the immediate neigh bourhood of the coast. This transport and deposition of sand and other debris form the shoals and banks so dangerous to navigation. In most cases there is an incessant change in the position or extent of these shallows, rendering frequent surveys absolutely necessary.