164 THE PEINCIPLES INVOLVED IN EOCK-WEATHEEINa 



begins anew without a moment's intercession, and continues 

 until the entire mass disappears, — becomes itself converted 

 into loose sand drifted by the wind and an agent for destruc- 

 tion. Professor W. P- Blake was the first, I believe, to call pub- 

 lic attention to this phe- 

 nomenon, having observed 

 it while in the Pass of 

 San Bernardino (Cali- 

 fornia) in 1853. G. K. 

 Gilbert has also published 

 some interesting facts as 

 noted by himself while 

 geologist of the Wheeler 

 Expedition west of the 

 100th meridian, in 1878.^ 

 In acting on the hard 

 rocks, the sand cuts so 

 _ _ slowly as at times to pro- 



'^r^^^'^^'^J-.^^^^^^rft^ cluce only grooved or fan- 

 PiG. 16.— Eock undermined by wind-blown tastically carved surfaces, 

 sand. often with a very high 



polish. The geologists of the 40th Parallel Survey in 1878 de- 

 scribed like interesting phenomena as observed on the western 

 faces of conglomerate boulders exposed to the sand blast of the 

 desert regions of Nevada. The surface of the otherwise light- 

 colored rock was found to have assumed a dark lead-gray hue and 

 a polish equal to that of glass, while the sand had drilled irregular 

 holes and grooves, often three-fourths of an inch deep and not more 

 than an eighth of an inch in diameter, through pebbles and matrix 

 alike. Professors W. M. Davis,- G. II. Stone,^ and J. B. Wood- 

 ward^ have described pebbles occurring in the glacial deposits 



^ It should be noted that the ' ' sand-blast carving ' ' described by Gilbert in 

 this report is not due wholly to the action of wind-blown sand. The rock 

 is fine calcareous shale. Through the solvent action of meteoric water the 

 calcareous cement is removed, the fine, argillaceous interstitial material 

 mechanically eroded, while the more resisting granules of quartz sand stand 

 in relief, giving rise to elevated points and ridges. 



sProc. Boston Soe. of Natural History, Vol. XXVI, 1893, p. 166. 



3 Am. Jour. Science, Vol. XXXI, 1886, p. 133. 



* Ibid., Jan., 1804, p. 63. 



