by the waves. The colour is mainly due to the presence of iron and manganese oxides. Mixed with these clays, especially in the South Pacific, great numbers of sharks' teeth and the hard compact ear-bones of whales were brought up by the dredge.

FIG. 175.-Diagram showing oceanic deposits at various depths.

The following table is derived from the Challenger Report on Deepsea Deposits:

Marine deposits are classified as follows :-

CHAPTER XVII.

OCEAN CURRENTS.

198. Causes of Currents. Great stream-like masses of water called currents are found passing along the surface of each great ocean from one part to another, the general effect being to transfer warm water from equatorial regions towards polar regions, and to cause an underflow of cold water from polar regions towards the equator. It is important to seek the cause of these ocean currents and what determines their direction. One cause, no doubt, is the difference of temperature between equatorial and polar regions, for water is expanded and made specifically lighter by heat, so that the warm surface-water of the tropics will tend to flow away towards the poles. This, however, is not the main cause of oceanic circulation, for it is not sufficiently great to account for the large currents observed, and another important agent plainly acts. The chief cause is

the direct frictional action of the constant winds on the surface of the water, since in each ocean the currents move in the same general direction as the winds. A wind sets the surface water in motion, and as the wind continues in the same direction, the motion is transferred by friction to lower and lower layers down to a depth of 400 or 500 fathoms. In the Indian Ocean the direction of the currents changes with the direction of the monsoons. Actual experiments also show wind to be an effective cause of currents. By the aid of a pair of bellows, and a vessel of water with sawdust floating on the surface, the production of surface currents can be easily shown. A model of the Atlantic Ocean has been constructed, and on imitating the action of the prevalent winds by air

Currents of the Atlantic Ocean.

243

currents blown from a number of tubes, a system of tiny currents corresponding to the real ones is set up.

In consequence of the operation of the trade-winds, therefore, there is a great westerly equatorial current in each great ocean. Owing to the obstacles presented by the east coast of South America, by the East Indian Archipelago, and by the coast of Africa, these equatorial currents are deflected both towards north and south temperate regions (see maps). Further, just as winds are deflected by the rotation of the earth on its axis from west to east, so also are currents; so that, in whatever direction the water or air moves on the surface of the earth, it is deflected towards the right in the northern hemisphere and towards the left in the southern hemisphere, the observer being supposed to be looking in the same direction as the current is flowing. All these causes combined lead to a huge whirl or eddy in the North Atlantic, the South Atlantic, the North Pacific, the South Pacific, and the Indian Oceans.

Much information on the direction and speed of ocean currents has been obtained by observations on the movements of wrecks and driftwood, and by actual experiments with floating bottles thrown out in certain positions, and picked up again by passing vessels or when cast ashore. Ocean currents carry enormous quantities of heat from tropical regions into higher latitudes, and thus modify climate. The immense influence of currents is well illustrated by the fact that Labrador, whose coast is washed by a cold Arctic current, is a bleak inhospitable land, while countries in the same latitude, on the opposite side of the Atlantic, tempered by the influence of the Gulf stream, enjoy an agreeable climate with a temperature nearly 20° F. higher than that naturally due to latitude. It should be noted that currents are named according to the direction towards which they flow-winds according to the direction from which they blow.

199. Currents of the Atlantic-The Gulf Stream.—The north-east and south-east trade winds of the Atlantic, combined with the rotation of the earth, produce equatorial currents that pass from the African to the South American coast. Near Cape St. Roque, in Brazil, the westerly equatorial current is

!

MAP VIII.

Currents of the Pacific Ocean.

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deflected, and divides into two branches, one flowing southwards and forming the Brazilian current, and another passing along the coast of Guiana. This latter branch, joining with the northern portion of the equatorial current, passes partly north-westwards outside the West Indian Islands, and partly into the Caribbean Sea, and thence into the Gulf of Mexico, to issue as the famous Gulf Stream. At the mouth of the gulf, termed the Strait of Florida, the Gulf Stream passes out as a strong current about 30 miles wide, with a speed of four miles an hour, a depth of 440 fathoms, a temperature above 80° F., and a deep blue colour, due to the high degree of saltness. Uniting with the portion that creeps along outside the West Indies, it flows north-east, increasing in width but diminishing in temperature, depth, and velocity. Off Newfoundland it is 320 miles wide, has a depth less than half the above, a velocity of only about one-third, and a temperature of 60° F. Near the middle of the Atlantic, about lat. 47° N., where its temperature is still 8° or 10° above the surrounding water, a division takes place. One portion flows on, or is carried by the prevailing south-westerly winds, as the Gulf Stream Drift, to the shores of the British Isles and North-west Europe. The other branch of the Gulf Stream turns southwards along the coast of Portugal and North-western Africa, to rejoin the northern part of the great equatorial current, thus completing a great oval whirl through the North Atlantic. In the midst of the calm waters of this whirl there is a vast area of seaweed and marine grasses, termed the Sargasso Sea.

A cold current from the Arctic Sea, formed by the union of branches from the east coast of Greenland and from Baffin Bay, known as the Labrador Current, passes southward between the Gulf Stream and the coast of North America, the line of separation between the two currents being known as the "Cold Wall." It disappears off Cape Hatteras, having partly sunk under the lighter water of the Gulf Stream, and having partly pierced the Gulf Stream in cold streaks.

200. Currents of the Pacific. The Kuro-Siwo.-From the Antarctic Ocean a cold current passes in a north-east direction to South America to form the Chili or Peru current. Gradually