22 THE CRUST WE DWELL UPON. 



as certainly as waste and degradation are going on from 

 without, so the fiery forces the volcano and earthquake 

 are as incessantly operating from within, upheaving new 

 lands and mountains, and conferring on the whole new irre- 

 gularity and diversity of surface. The earth's crust is thus 

 held in equilibrium between these two opposing forces, fire 

 and water between waste and degradation on the one 

 hand, and reconstruction and upheaval on the other. In 

 this way former lands have been wasted and worn down, 

 and former estuaries and seas filled with the sediments; 

 old continents and islands submerged beneath the waters, 

 and the sea-bed upheaved into newer lands. The rocks of 

 the earth's crust are the only memorials of these repeated 

 changes ; and if geology is earth-history, it must endeavour 

 not only to decipher the changes they record, but to arrange 

 them in chronological sequence and connection. 



When we look, then, at the changes now taking place 

 on the earth's crust, and the new rocks that are in process 

 of formation, we behold in them the exact counterparts 

 of what must have taken place during all former periods. 

 Winds, frosts, rains springs, streams, rivers waves, tides, 

 and ocean -currents are ever weathering and wasting the 

 rock-matter of the globe; and the matter worn down is 

 borne by rivers to lakes and estuaries and seas, and there 

 deposited in layers of mud and clay and sand and "gravel, 

 or further reasserted by the tides and currents of the 

 ocean. Coral-reefs, shell-beds, and other masses of animal 

 origin, are also accumulating in various parts of the ocean ; 

 while peat -bogs, swamps, and forest -'growths are adding 

 analogous masses of vegetable origin to the land. Hot- 

 springs and mineral -springs are also carrying matters in 

 solution from the earth's interior, and depositing these along 

 their courses ; while volcanoes are ever throwing out froni 

 the same interior showers of dust and ashes, and masses of 



