54 



GEOLOGY. 



than otherwise a fraction of an inch in diameter. Strictly speaking 

 these results are the effect of the electricity of the atmosphere rather 

 than of the atmosphere itself, but they are best mentioned in this 

 connection. 



Allusion has already been made to the chemical changes in the 

 atmosphere occasioned by electric discharges. 



SUMMARY. 



Weathering. — The result of all atmospheric processes, whether 

 phj^sical or chemical, by which surface rock is disrupted, decomposed 



Fig 35. — Details of a weathered rock surface, due partly to wind work and partly to 

 solution. The particular phase of weathering illustrated by this figure is known 

 as " honeycomb " weathering. (Gilbert^ U. S. Geol. Surv.) 



or m any way loosened, is weathering. This convenient term also 

 includes similar results effected by ground w^ater, plants, etc. The 

 tendency of weathering is to produce a mantle of residuary earth over 



