PRINCIPLES OF ROCK WEA THERING 7 I 5 



large sheet of plate glass, once a window in a lighthouse on Cape 

 Cod. During a severe storm of not above forty-eight hours' 

 duration this became on its exposed surface so ground from the 

 impact of grains of sand blown against it, as to be no longer 

 transparent and to necessitate its removal. 



Window panes in the dwelling houses of the vicinity are, it is 

 even stated, not infrequently drilled quite through by the same 

 means. 



Apply now this agency to a geological field in a dry region. 

 The wind sweeping across a country bare of verdure and parched 

 by drouth, catches up the loose particles of dust and sand and 

 drives them violently into the air in clouds, or sweeps them 

 along more quietly close to the surface where they are at first 

 scarcely noticeable. The impact of a single one of these moving 

 grains on any object with which it may come in contact, is far 

 too small to be appreciable, but the impact of millions acting 

 through days, weeks and years, produces results not merely 

 noticeable but strikingly conspicuous. We have here in fact a 

 natural sand blast, an illustration on a grand scale of a principle 

 in common use in glass cutting and to a small extent in stone 

 cutting also. Constantly filing away on every object with which 

 they come in contact the grains go sweeping on, undermining 

 cliffs, scouring down mountain passes, wearing away the loose 

 bowlders and smoothing out all inequalities. Naturally the 

 abrading action on exposed blocks of stone is most rapid near 

 the ground, as here the flying sand grains are thickest. First the 

 sharp angles and corners are worn away, and the masses gradu- 

 ally become pear shaped, standing on their smaller ends. Finally 

 the base becomes too small for support, the stone topples over, 

 and the process begins anew without a moment's intercession and 

 continues until the entire mass disappears — becomes itself con- 

 verted into loose sand drifted by the wind and an agent for 

 destruction. W. P. Blake was the first, I believe, to call public 

 attention to this phenomenon, having observed it while in 

 the pass of San Bernardino (California) in 1853. G. K. Gil- 

 bert has also published some interesting facts as noted by him- 



