WATER AS A MECHANICAL AGENT. 225 



vessel pursued the same course. Again, an anchor with 10 fathoms of cliain attached, 

 from a brig of 200 tons wrecl^ed on Cape Cod near Truro, was drifted a mile and a half to 

 the north in three weeks. These facts are from papers by Lieutenant C. H. Davis (1849, 

 1851). Such transportation is beyond the power of any currents ; it is the work of the 

 dashing, lifting, and propelling waves. 



In the following example, the change of position 

 is connected with a change in the seasons. J. D. 

 Hague states that at Baker Island (of coral) , in the 

 Pacific (0° 15' N., 176° 22' W.), this fact is well ex- 

 hibited. In Fig. 197, I, I, I is the southwest point of 

 the island, and R, R, R, the outline of the coral-reef 

 platform, mostly a little above low-tide level ; its 

 width, cd, 100 yards. In the summer season, when 

 the wind is from the southeast, the beach has the 

 outline s, s, s ; during the winter months, when the 

 wind is northeast, the material is transferred around 

 the point, and has the position w, w, w, having a 

 width at ab of 200 feet. A vessel wrecked in sum- 

 mer, and stranded at V, was transferred to V in the course of the month of November. 

 (J. D. Hague, '62.) 



6. Sand-bars at the entrances of harbors or mouths of tidal rivers. — The 

 material of the sand-bars wliich obstruct the entrances of harbors has two 

 main sources : an inner, and an outer ; the former fluvial, the latter the wave- 

 and-current driftings of the coast, which contribute so largely to sand-barriers. 

 The positions of the bars depend much on the strength of the river current ; 

 but also on the direction, form, and supplies of the wave-and-current move- 

 ment produced by the storm-winds. A small stream is often blocked entirely 

 by a sand-bar across its mouths, so that the waters reach the ocean only by 

 percolation through the beach. But large streams make distant sand-reefs 

 or barriers through the aid of the outflow, and keep open channels even 

 in the face of the ocean. 



The depth of water over the sand-bars at the mouth of a large river 

 or bay is, in great part, only 3 to 10 feet : a remarkable fact, considering the 

 opposing forces at work — the tidal outflow and inflow, and the plunge of the 

 storm-made waves over the mobile sands. The sands lie along the area of 

 rest between the contesting movements. New York Bay (map, page 211) 

 affords an example. The contributions of river sediment come from the 

 Hudson Eiver and from small ]S[ew Jersey rivers ; and the Hudson is mod- 

 erate in its supplies, considering its length and size, because it has almost no 

 tributaries for 60 miles, and small ones for 100 niiles, owing to the westward 

 dip of the Catskill strata and the barrier of the Palisades in the southern 

 part. The wave-and-current supplies come from the direction of the Long 

 Island and the New Jersey coasts ; for New York Bay is exceptional in 

 lying to the leeivard of both coasts. Under these circumstances, Sandy 

 Hook, the sand-bars, and the barriers of the Long Island coast adjoining, 

 have been accumulated. The outlining of the bars, and the positions of the 

 three channels through them, are mainly due to the tidal outflow, which 



DANA'S MANUAL — 15 



