jo Distribution of Sediments. 



shore, we shall find that it is formed of innumerable 

 grains of quartz, and these grains are generally not 

 angular, but more or less rounded : their edges having 

 been worn off by the action of waves and tides moving 

 them backwards and forwards upon each other, till they 

 became grains, like water-worn pebbles in shape, only 

 much smaller. Such material when consolidated forms 

 sandstone. 



Finer-grained and more muddy deposits, in like 

 manner, are generally formed of the minutest grains of 

 sand, mixed with aluminous substances originally de- 

 rived from the waste perhaps of felspathic rocks. Such 

 material, when soft, forms clay ; when consolidated, 

 marl shale and slate. 



In this manner very large amounts of mechanical 

 sediments are forming and have been formed. The 

 daily sifting action of breakers, intensified during long- 

 continued heavy gales, the forcible ejection of muddy 

 waters, sometimes hundreds of miles out to sea, from 

 the mouths of great rivers like the Amazons, the power 

 of tidal and great ocean currents such as the Grulf 

 Stream, all contribute to scatter sediments abroad, and 

 by their rapid or more gradual subsidence, the bottoms 

 of vast submarine areas are being covered by mechanical 

 sediments, which must of necessity often be of great 

 thickness, and in which various kinds of strata may 

 alternate with each other. 



With sufficient time all land would, by these pro- 

 cesses of waste, be eventually degraded beneath the sea 

 (as was suggested by the naturalist Ray), were it not 

 that the loss is compensated by disturbance and eleva- 

 tion of land, always slowly taking place over portions of 

 the continents and islands of the world. Large areas 

 are also slowly depressed beneath the sea; but to 



