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THE JOURNAL OF GEOLOGY.

the south shore of Long Island, and the formation of barriers of wave-washed sand.

(b) Continuous currents.—Distribution by continuous currents is the condition usually assumed as having controlled the arrangement of sediments in seas of past geologic periods. In consequence of the sorting which results from different rates of settling clay is carried beyond sand, and silt is distributed more widely than clay. The prevailing current, which thus distributes, is under-tow more or less checked and assisted by tides. If the submarine slope descends from the shore steeply into oceanic depths, the force of undertow must rapidly be dissipated, but pebbles and sand move easily down the steep incline, and form a sequence of continually smaller particles, which is usually not very extended. This is the case on the western coast of South America. If, on the other hand, the seaward slope is very gentle, undertow loses force more gradually and fine sands may occur to great distances from the shore, with clay and silt deposited beyond them. This is the case off the Atlantic coast of the United States where tides probably form a powerful alternating influence; there the continental plateau is covered with sand to its outer rim, as is shown by soundings by the Coast Survey. But the force of undertow is determined in the first place by the force of waves, and it can be effective in distributing only where waves are powerful. It fails in limited seas except in a very narrow zone along shore.

Ocean currents also distribute sediments very widely. The terrigenous deposits of the Bay of Bengal and Arabian sea, mapped by Murray,[1] covering 1,600,000 square miles, owe their wide spread distribution apparently to the ocean currents which circulate east and west alternately with the changes of season in these great bays.

(c) Uniform depths.—Changes in depth of water affect the velocity of a current and thus modify its power to distribute sediment. Narrowing channel or shallowing water may cause a

  1. ↑ Scottish Geogr. Mag., Vol. V. No. 8, Aug. 1889.
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