PR/NCIPLES OF ROCK WEATHERING 85 I 



show the same thinjj; on a scale so gigantic as to be at first scarce 

 comprehensible.' An item of no insignificant imj:)ortance to be 

 considered here, is the possibility, indeed probability, of an 

 incidental chemical decomposition taking place during this 

 abrasive action. 



Daubree has shown = that when feldspathic fragments were 

 submitted to artificial trituration in a revolving cylinder contain- 

 ing water, a decomposition was effected whereby the alkalies 

 were liberated in very appreciable amounts. He found further 

 that the principle product of mutual attrition of feldspathic frag- 

 ments was an impalpable mud (limon) of such tenuity as to remain 

 for many davs in suspension, and which on desiccation became 

 so hard as to be broken only with the aid of a hammer, resem- 

 bling in many respects the argillites of the Coal Measures, but 

 differing in that it carried a high percentage of alkalies. Granitic 

 rocks thus treated yielded angular fragments of quartz and very 

 m.inute shreds of mica, while the feldspars ultimately quite dis- 

 appeared in the form of the impalpable mud above mentioned. It 

 was noted that after a certain degree of fineness had been reached 

 further rounding of the particles 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. It is to a similar 

 action on the part of sea water that Shalers would attribute the 

 lasting qualities of the sand grains upon our sea beaches. Indeed 

 the conditions of Daubree's experiments as a whole were not so 

 different from those existing in nature that we need hesitate 

 to conclude that similar action, both chemical and physical, may 

 be going on wherever abrasion takes place in the presence of 

 continual moisture, as in the bed of a river or glacier. 



The hammering action of the waves upon the seacoast exert 



■ Captain C. E. Button has estimated (Tertiary History of the Grand Canyon 

 of Colorado) that from over an area of 13,000 to 15,000 square miles drained by the 

 Colorado River an average thickness of 10,000 feet of strata have been removed. 



^Geologic Experimcntale, p. 268. 



3 Bull. Geol. Soc. of America, Vol. V, p. 208. 



