38 WASTE AND BECONSTEUCTION. 



and frosts may wear away the mountain-cliff, slowly as the 

 river may deepen its channel, gradually as the delta may 

 advance upon the estuary, and little by little as the volcano 

 may pile up its scoriae and lava, yet after the lapse of ages 

 the mountain will be worn down, the river-channel will be 

 eroded into a valley, the estuary converted into an alluvial 

 plain, and the volcano rear its cold and silent dome into the 

 higher atmosphere. All that is necessary is time, and this 

 is an element to which we can see no limit in the future, any 

 more than we can discover a beginning to it in the past. To 

 render these incessant mutations thoroughly intelligible, how- 

 ever, to the ordinary observer, it will be necessary to describe 

 the agents by which they are effected, and at the same time the 

 varying power of these agents according to the latitudes and 

 altitudes within which they operate. These agencies may 

 be conveniently arranged under five great categories ; name- 

 ly, 1, The Meteoric, or those like winds, rains, and frosts 

 depending upon the atmosphere; 2, The Aqueous, or those 

 like rivers, waves, and tides arising from the action of water ; 

 3, The Chemical, or those resulting from chemical actions and 

 reactions ; 4, The Organic, or those like peat-mosses and 

 coral-reefs depending on the growth and decay of plants 

 and animals ; and 5, The Igneous, or those like the volcano 

 and earthquake connected with the manifestations of heat 

 within the interior of our planet. Each of these agencies 

 has its own mode of working some chiefly wearing and 

 degrading, some degrading and at the same time accumulat- 

 ing, and others solely reconstructing. Let us now glance 

 at them in detail : 



The principal effect of the Meteoric or Atmospheric agen- 

 cies is to weather and wear away. Slowly but surely the 

 gases and moisture of the atmosphere eat into every ex- 

 posed rock-surface. The disintegrated matter is washed 

 down by the rains, taken up by the runnels and streams, 



