WATEfl AS A MECHANICAL AGENT. 



209 



188. 



Plicated clayey layer. Vanuxera. 



a layer thus plicated, from the Quaternary of Booneville, N.Y. Vanuxem 

 illustrates the facts there observed by him, with this and other figures 

 (N. Y. Geological Report), and attributes the plications to lateral pressure 

 while the layer was in a softer state than those contiguous. 



In parts of the shores of western Patagonia, 

 where the soil is always wet, the soil-cap is 

 always slipping downward over the basement 

 rock ; and it carries along not only its cover- 

 ing of trees and shrubbery, but also a "moraine 

 profonde" of rocks, stones, tree-trunks, peat 

 and mud, denuding the hills, filling valleys, and 

 feeding the ocean. (R. W. Coppinger, 1881.) 

 Areas on the Palklands, called " stone rivers," 

 may have the same origin. (W. Thomson.) 



Soil-cap movements and land-slips sometimes dam up valleys and make 

 lakes. But loading with waters is only one of the methods of producing 

 such movements. 



Amount of absorbed water ivitliin the earth. — The amount of absorbed 

 water in the earth has been increasing from the time of the earth's consoli- 

 dation. The thickening of the supercrust, by the addition of sedimentary 

 strata, has been attended by a continued addition to the amount. Ejected 

 igneous rocks take in water on cooling. Other sources of augmentation are 

 the making of hydrous iron oxides through oxidation, of clays through the 

 decomposition of feldspar, and of gypsum and other hydrous minerals. 



If the thickness of the supercrust over the continental portion of the 

 globe average 10 miles, and the average volume of moisture in the forma- 

 tions, both metamorphic and unaltered, be 2-5 per cent, the whole amount 

 of water absorbed and confined would be -^-^ of 10 miles, or about 1300 

 feet in depth, for the area of the continents. The deposits over the oceanic 

 basins have relatively little thickness. Whatever reasonable allowance be 

 made for them, the whole loss to the ocean waters, in depth, from this 

 source, will not exceed 800 feet. The confined water of the rocks, while a 

 feeble agent of change at the ordinary temperature, is one of immense 

 importance when much heat is present. 



II. THE OCEAN AS A MECHANICAL AGENT. 



The working agencies of the ocean of a mechanical kind are, as has been 

 stated, those of (1) the tidal wave ; (2) the wind-made waves and currents ; 

 and (3) earthquake waves. Besides these agencies, the sun's heat, by vary- 

 ing the temperature and density of the water, affects the ocean's movements. 



In mechanical work, the waters of the ocean have an advantage over 



fresh waters in being of greater specific gravity by -Jg- to ■^-^. They have 



also the important quality of depositing sediment more rapidly, because 



less viscous, owing to the saline condition of the waters. A fine sediment, 



Dana's manual — 1 4 



