SPECIFIC GRAVITY. 65 



how the breakers of the ocean eat away the shore, 

 by referring to the North of Germany, where it has 

 swallowed up large tracts of the continent and 

 islands. Of twenty-four islands of East Friesland, 

 which existed at the time of the birth of Christ, 

 only sixteen at present remain. In like manner we 

 have examples of the opposite effect of water, show- 

 ing that at least a portion of that which it has taken 

 from the land in one place is deposited in another ; 

 — in the delta of the Nile, which is almost half as 

 large again as the kingdom of Saxony ; in the delta 

 of the Ganges, twice as large ; in the south coasts 

 of France and Italy, where a band of solid land, 

 from one to two leagues wide, has been formed by 

 alluvial deposits since the time of the Romans ; in 

 the Low Countries, in the Coral Islands, etc." In 

 the polar regions immense bodies of ice are often 

 detached from the land, bearing large quantities 

 of earth with them, and often transport it great 

 distances. Sometimes glaciers separate from the 

 mountain-top on the approach of summer, and, in 

 sliding toward the valley, wear oif the rock, and 

 push the heaps of gravel before them to the plain. 



" Rain, frost, and gravity," says Dr. Hitchcock, 

 " are the three yjrincipal causes which operate to 

 degrade the hills and fill up the valleys." 



Let us now consider the properties of soils. 

 Specific gravity. Those soils which weigh the 

 6* 



