xi The Bed of the Oceans 199 



continental shelf nearly to sea -level for a considerable 

 distance, as, for example, along the east coast of India. 

 Sandbanks or bars, sometimes locking in lagoons of salt 

 water and forming a lace-like margin to the land, are pro- 

 duced where river deposits are brought down to the coast 

 for instance, on the south-east coast of North America, and in 

 the vast mangrove-grown mudbanks of many parts of South 

 America and Africa. Where the land is high and rocky the 

 broken-off stones are rolled and rounded by the waves and 

 used as battering-rams to break away the land ; finally they 

 are swept out to sea and spread in sheets over the bottom, 

 the level of which is raised and the slope reduced. In this 

 way a beach is formed, the upper part of it being quarried 

 out of the solid land, and forming a notch or ledge (ABC, 

 Fig. 39) on which the sea is always encroaching, while the 

 lower part forms an embankment (CDE) built up of the ex- 

 cavated material which 

 is laid down in flat beds 

 one above another. The 

 name Beach is restricted 

 to the strip of land 

 covered and laid bare by 



the tides. On a typical FIG. 39. Formation of a Beach. AD, original 

 slope of land ; ABC. notch cut out by wave 



beach large Stones are action; CDE, embankment of sand, etc. 

 Usually found heaped Up (worn-down rock); BC, gravel resting on 



near high-water mark ; 



smaller pebbles, rounded by the sea, form a steeply sloping 

 bank at a lower elevation, and are rattled to and fro, ground 

 against each other, reduced in size to. fine shingle, and 

 raked nearer the sea by every tide. Next there is a wide 

 stretch of sand, which usually consists of quartz grains, 

 resulting from the breaking down of the pebbles, the quartz 

 being the densest and hardest ingredient of the rocks. 

 Nearest the sea, and often only uncovered at the lowest 

 spring ebbs, there are banks of mud formed of the softest 

 ingredients of the rocks, which were ground to the finest 

 powder and carried to the greatest distance. Sometimes 

 perpendicular cliffs occur, at the base of which the rushing 

 tides permit no fragments to accumulate. 



