84 ON EROSIONS OF THE EARTH'S SURFACE. 



stratification and lamination, but also through the mass of most rocks. This it 

 does chiefly through capillary attraction : and few rocks or minerals, long immersed 

 in water, escape its all pervading influence. 



By means of these agents of chemical change in the atmosphere and the waters 

 (some others of minor influence might be mentioned), we sometimes find the sur- 

 face of rocks, to the depth of 10 to 15 feet, so thoroughly disintegrated that they 

 can easily be removed by the shovel. This fact may be observed in many rocky 

 regions south of Pennsylvania. The drift agency further north, has swept off the 

 disintegrated mass, so that in New England we get but a feeble idea of its extent. 



In this way are the rocks prepared to be acted upon mechanically by erosive 

 agencies. These too are chiefly water in some form. 



3. Water acts mechanically, first, as breakers or waves, tides, and oceanic cur- 

 rents. These all act conjointly, for the most part, and it cannot be doubted that 

 nearly all the materials of which the sedimentary rocks, consolidated and uncon- 

 solidated, are composed, have been accumulated and deposited by this joint action. 

 To be sure, waves, tides, and currents act with the most important results upon 

 loose detritus : but if we suppose a continent gradually rising or falling, every 

 part of its surface will be brought under the denuding agency ; and the projecting 

 naked rocks, subject to the ceaseless action, cannot but yield to its force. Indeed, 

 of all the causes operating to wear down the surface, waves, tides, and currents, 

 have been the most efficient, and have done most to give our present continents 

 their form and outline. 



Secondly, as fresh water currents, chiefly in the form of rivers, which drain the 

 land. They would have but little effect upon the rocks were not the latter 

 softened and disintegrated. But loosened materials it can sweep off, and dis- 

 tribute, according to its velocity. And when once it has set detritus in motion, 

 that will tear away projecting fragments, which the water alone could not remove. 

 In some mountain slides, such as that described in my paper on Terraces, as pro- 

 duced on Mount La Fayette, by a powerful shower, the work of erosion accom- 

 plished by the water and detritus is almost equal to that of glaciers. 



Thirdly, the expansive force of water, when freezing, is one of the mightiest of 

 all known agencies for lifting rocks out of their beds. If water finds its way into 

 cracks and cavities in the rock, and then freezes as solid as we know it may do in 

 northern regions, it will exert a power which even gunpowder could not equal. 

 Thus would the fissures be widened, giving an opportunity for a larger quantity of 

 water to freeze in them the subsequent winter, with a still stronger force and 

 wider effect. Thus, in time, are the most solid and deep-seated masses so heaved 

 out of their original beds, that ice floods, or other agencies converf them into boul- 

 ders and roll them along the surface. 



Fourthly, Glaciers, Icebergs, Ice Floes, and Ice Floods, form agents of erosion of 

 tremendous power. In these cases, blocks of stone, gravel, and sand are frequently 

 frozen'into the bottom of the ice, so as to act like enormous rasps upon the sur- 

 face, over which they move with almost irresistible power. I need not go into 

 details on this subject. For any one who has read the works of arctic and antarc- 

 tic voyagers in latter times, and the histories of Alpine glaciers, must be impressed 



