MODERN GEOLOGICAL CHANGES. 349 



It has been already observed, that the action of the sea 

 is materially assisted by the percolation of rain, and the 

 operation of frosts. Where the cliffs, instead of being of a 

 homogeneous nature to their base, are composed, in their 

 lower portions, of an impermeable substance, such as clay, 

 the upper portions are increased in weight by the infil- 

 tration of water, while the lower are moistened and ren- 

 dered slippery, so that the upper masses glide down and 

 occasion a fall. Frosts prove a like important agent of de- 

 struction, fracturing various portions of the mass, by the 

 expansion of the contained water in the act of freezing, 

 which portions are subsequently detached by their own 

 gravity. The cliff thus attacked, becomes readily disin- 

 tegrated : and, from a due consideration of these causes, we 

 cease to wonder at the extensive ruin observable in all the 

 less resisting strata, from the coasts of Yorkshire to those 

 of Sussex, and thence, to a greater or less extent, to the 

 shores of Cheshire and the coasts of Scotland. 



To enumerate all the instances of the destructive effects 

 of aqueous agency, observable over the earth, would far ex- 

 ceed our limits ; it may be sufficient to state, that the same 

 principles universally prevail : the harder rocks stand out 

 as headlands, after the softer deposits have been worn into 

 bays, the action of the sea being modified by local causes, as 

 the form of the land, the course of tides, waves, breakers, 

 and currents. 



FLUVIATILE ACTION. Rivers are alike destructive of the 

 banks through which they flow ; especially in the case of 

 floods, torrents, and inundations occasioned by rain and the 

 melting of snow; the ravages they effect can hardly be 

 credited by those who have not directed their attention to 

 this class of agents. These phenomena are more strikingly 

 developed on the continents of Asia, Africa, and America, 

 the rivers of which constitute the drainage of great tracts of 

 land. We must refer the student to the works on this 

 branch of our subject, in which fluviatile action is discussed 

 in the most ample and satisfactory manner. 



ATMOSPHERIC AGENCY. The agency of the atmosphere 

 is both chemical and mechanical. The absorption of oxygen 

 and carbonic acid from the air, causes rocks of all classes 

 to disintegrate. The mechanical agency consists, in the first 



