THE OCEAN. 667 



taining this detritus were 100 feet deep at mouth, and entered a sea 

 having a uniform depth of 1,000 feet, and a rate of motion of two 

 miles an hour, the first kind would be carried 1 80 miles, before 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 distances 

 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, which is at 

 present the principal one, though not so in Paleozoic time ; (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 648. 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 most detritus is the Atlantic. 



2. Distribution and Accumulation. — The distribution and accumu- 

 lation 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 in the form either of 

 beaches or of 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 material of the 

 beach may be a fine mud or silt. 



The material added by the waters is deposited partly over the 

 sloping surface, and partly at the top of the beach, where thrown by 

 the toss of the waves, especially in storms. The former is necessarily 

 bedded or laminated parallel to the beach surface ; and the bedding 

 has consequently its slope, or ordinarily 5° to 8°. 



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 



