432 A TREATISE ON METAMORPHISM. 



WATER, ICE, AND WIND. 



Ill the broadest sense wind, water, and ice above the surface are 

 included under the agents of the belt of weathering. But this subject is so 

 large that it is ordinarily given a separate treatment under, the terms erosion, 

 land sculpture, or physiography. Therefore the work of these three agents 

 will not be here considered or even summarized. However, it may be well 

 to recall that these agents are not efficient in the mechanical disintegration 

 of the rocks except as they use rock material as their tools; that is, wind, 

 water, and ice are incapable of rapidly cutting the rocks. When the wind 

 bears sand it may do a considerable amount of cutting; when water bears 

 rock material varying in size from bowlders to silt it may accomplish a vast 

 amount of work. The same statement is true of ice. All three of these 

 agents, therefore, furnish their energy of motion to rock material as tools of 

 cutting, and thus accomplish mechanical destruction of the rocks. This 

 energy is indirectly derived from the sun and gravity. It should also be 

 said that there appears to be a limit beyond which water, wind, and ice are 

 incapable of finer subdivision of material. Thus Daubree found that in 

 water there was a limit beyond which mechanical trituration for the hard 

 minerals like quartz ceased, "owing to the buoyant action of the water, 

 which in the form of a thin film between adjacent particles acted as a cushion 

 and prevented actual contact to the extent necessary for mutual abrasion.'" 1 

 Shaler appeals to the same principle in explaining the endurance of sand 

 grains on the seashore. He says that the grains are held apart by films of 

 interstitial water (this would be water of adhesion), and are therefore pre- 

 vented from strongly impinging against one another.'' Judd says that the 

 small particles of the alluvial deposits of the Nile which have been trans- 

 ported great distances are angular, while the larger particles of the same 

 deposits are subangular or rounded. Judd attributes the rounding of the 

 larger grains to eolian rather than river work; but this does not lessen the 

 force of the argument as to there being a lower limit beyond which the 

 particles are not rounded by river action. 



« Merrill, cit. , p. 197. See Daubree, A., Geologie exp6rimentale, Paris, 1879, p. 268 et seq. 



»Shaler, N. S., Phenomena of beach and dune sands: Bull. Geol. Soc. America, vol. 5, 1894, pp. 

 208-209. 



''Judd, J. W., Report on a series of specimens of the deposits of the Nile Delta, obtained by the 

 recent boring operations: Proc. Royal Soc. London, vol. 39, 1885,- pp. 213-220. 



