484 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 cur- 

 rents 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 distrib- 

 utes, 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 dissi- 

 pated, 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 dis- 

 tances 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 influ- 

 ence ; 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. 



(V) Uniform depths. — Changes in depth of water affect the 

 velocity of a current and thus modify its power to distribute sed- 

 iment. Narrowing channel or shallowing water may cause a 

 "Scottish Geogr. Mag., Vol. V. No. 8, Aug. 1889. 



