THE OCEAN. 679 



are stratified mainly by the " undertow," and the beds have therefore 

 a dip corresponding with that of the beach, which is usually about 7° 

 to 8° with a low tide, but often 15° to 20° when the tides are high and 

 the bottom outside drops off rather rapidly. 



The deposits making the surface of a beach are more or less coarse, 

 varying from stones to fine sand, according to the force of the waves ; 

 and the coarser kinds fail to be present only in regions where stones 

 are not to be had along the shores. The plunging breakers and the 

 powerful uudertow sweep away the finer material. 



Accumulations of iron sand or garnet sand are often made toward 

 high-water mark, these materials being left behind because of their 

 higher specific gravity. The finer material, which is constantly made 

 by abrasion chiefly of the feldspar and other softer grains of the beach 

 sands, but also of its silicious grains, is carried out by the undertow 

 into deeper water, to add to the accumulations of mud in progress 

 over the sea bottom. When corals and shells abound in seas these 

 become the material that is ground up by the waves into beach sands. 

 See p. 619. 



Such seashore action thus tends to assort out the siliceous portion 

 of the material exposed to the breakers, and make siliceous accumula- 

 tions, and it has always been a common source of siliceous gravel de- 

 posits. But it is sure also to make mud deposits in the deeper waters ; 

 so that the two belong together, and are results of the same agency. 

 Coarse sand-beds, or gravel beds, can never have been made by beach ac- 

 tion without the making also of mud-beds in regions not very far dis- 

 tant. Moreover, not a year has passed since granite and gneiss first 

 existed in which quartzose sand-beds were not somewhere in progress. 



4. Eartltquake Waves. — Earthquake waves commence their work 

 at an unusual depth, through the retreat of the waters (p. 674), and 

 tear up earth, rocks, and strata, with marine shells and relics, from 

 regions that are at other times under the protection of the waters ; 

 and then the waters advance to an unwonted height, and make depos- 

 its of what they have gathered at varying distances inland, according 

 to their gravity, besides devastating the country they cover. At the 

 Lisbon earthquake of 1755, the oceanic wave had a height along the 

 Tagus of 40 feet, at Cadiz, of 60 feet, on the shores of Madeira, of 18 

 feet, and 8 to 10 on the coast of Cornwall. The earthquake of 1746, 

 on the coast of Peru carried a frigate several miles inland, besides del- 

 uging the seaport Callao, and the city of Lima seven miles inland. But 

 the geological effects of such waves are small compared with those of 

 ordinary waves ; for the latter are in incessant action on all shores. 



The tidal wave, as it swells over the land and covers the great tidal 

 flats, often carries in little besides the weeds and sticks floating on its 



