680 DYNAMICAL GEOLOGY. 



surface. But in other places the movement is rapid, and, besides, a 

 river or a cliff contributes sand and silt for transportation. This ac- 

 tion is treated of in connection with the next subject, — that of tidal 

 and wind-made currents. 



5. Tidal and Wind-made Currents. — Tidal currents work in the 

 same direction through the year, with only the variations that attend 

 each tide. Wind-made currents vary in direction and force with the 

 winds. But those of some directions are more prevalent than others 

 in all seas ; often the direction is constant for half a year or more ; 

 and in the line of the trades, it is uniform for the larger part of the 

 year. 



The materials transported and deposited are derived (1) from the 

 degradation of coasts ; and (2) from the detritus brought down by 

 rivers. The latter is now the principal source ; but the former was so 

 in early times when the lands, and therefore streams, were small. 



The tidal waves and currents often drift the detritus that is within 

 reach along the coast, in the direction of the movement and thus make 

 (1) lines of banks or sand flats parallel with the shores ; (2) fill up 

 bays or obstruct their entrances and widen the seashore flats ; (3) 

 cover the outside region of soundings with deposits. The wind-waves 

 often aid much in this work ; but when from an opposing direction, the 

 combined action tends to plough into and sweep away the beach. 



The coasts of Long Island afford a good example of the action 

 where rivers have taken almost no part. On the map on page 422, the 

 south side is seen to have for more than forty miles a nearly straight 

 and narrow sand reef as the outer limit of as long a bay. There are en- 

 trances through it ; but they are very small breaks in the line of reef. 

 This reef has been made partly by the waves rolling in from the At- 

 lantic, but more from the sands drifted along the coast by tidal and 

 current action. The high bluffs of stratified gravel and sand beds, 

 which make the shores toward Montauk Point, give way before the 

 heavy surges ; and besides making a very coarse, stony seashore, con- 

 tribute sands for the reefs to the west. The inner shore of the long 

 shallow ba3 7 s is full of indentations ; and these are evidence as to the 

 character of the coast line as it was left at the last elevation of the 

 land ; while the straight-edged reef shows the effects of straight-line 

 drifting from the debris of the projecting bluffs westward along the 

 shores. The entrances through the reef are outlets for the tides by 

 whose outflow they are kept open. After a time the waves throw 

 into them so much sand as to obstruct them ; and then at an unusually 

 high tide or great storm, they open at a new point, or wherever there 

 is least obstruction. Again, on the north side of Long Island, where 

 also the tide moves westward, there are similar high cliffs of sand and 



