660 DYNAMICAL GEOLOGY. 



the first portions would reach bottom, and would be distributed 

 along for 20 miles ; the corresponding numbers for the others would 

 be— (2) 225 and 25 ; (3) 360 and 40 ; (4) 450 and 50. Thus, four kinds 

 of deposits would be formed from the same stream, at different dis- 

 tances from its mouth. 



2. Formations on Soundings and along Coasts. 



1. Origin of the material. — The material of sea-shore formations 

 is derived from two sources : (1) the detritus of rivers ; (2) the wear 

 of coasts. 



All the rivers entering an ocean bring in more or less detritus, 

 especially during freshets. The quantity from the Mississippi is 

 stated on page 644. The amount thus contributed to the ocean 

 depends on the geographical extent of the river-systems bordering 

 it, and the annual amount of rain, snow, etc. In both these respects, 

 North and South America exceed the other continents ; and the 

 ocean which receives the detritus is the Atlantic. 



2. Distribution and accumulation. — The distribution and accumula- 

 tion of the material may take place (1) from the action of waves 

 alone; (2) from waves, and tidal or wind currents; (3) from the 

 waves, the shore-currents, and the currents of rivers. 



(1.) The accumulations made by waves are either in the form of 

 beaches or off-shore deposits of detritus. As the plunge of the 

 wave is analogous to that of a torrent, its waters, while grinding the 

 material upon which they act, wash out the finer portion, and carry 

 it away by means of the under-tow. The beach consequently 

 consists of more or less coarse material, according to the strength 

 of the waves : it may be sand, pebbles, or even large stones, if the 

 rocks of the coast are of a nature to afford them. In sheltered 

 bays, where the waves are small, trituration is gentle, and the mate- 

 rial of the beach may be a fine mud or silt. 



The height of a beach depends on the height of the tides and the 

 strength of the waves. The sands thrown beyond the farthest 

 reach of the waves are often accumulated into higher ridges, and 

 make the wind-drifts and dunes described on page 629. 



(2.) The tidal and wind currents give direction to the material taken 

 up by the waters. This material may be the sands, stones, etc. of a 

 beach, or the finer material from the bottom, or the mud stirred up 

 from greater depths, down even to 100 feet, by the heavy waves of 

 storms. The currents, their general course being otherwise deter- 

 mined, flow where they find the freest and deepest passage, and 

 drop their detritus wherever there is a diminution of velocity. This 

 precipitation takes place in the waters thrown off either side of the 



