EFFECTS OF ELECTRICITY 29 



slopes creep slowly downward. (4) When rain falls on dry sand or 

 dust the cohesion is at once increased, and shifting by the wind is 

 temporarily stopped. 



Effects of electricity. Another dynamic effect conditioned by 

 the atmosphere is that produced by lightning. In the aggregate, 

 this result is unimportant; yet instances are known where large 

 bodies of rock have been fractured by a stroke of lightning, and 

 masses many tons in weight have sometimes been moved appreciable 

 distances. Incipient fusion in very limited spots is also known to 

 have been induced by lightning. Thus where it strikes sand it 

 may fuse the sand for a short distance, and, on cooling, the partially 

 fused material is consolidated, forming a little tube or irregular 

 rod (a fulgurite) of partially glassy matter. Fulgurites are usually 

 but a few inches long, and more commonly than otherwise a fraction 

 of an inch in diameter. 



SUMMARY 



On the whole, the tendency of the work of the atmosphere, and 

 of the work which is controlled by it, is to degrade the land, and to 

 loosen materials of the surface so that they may be moved readily 

 to lower levels by other agencies. The most important phase of 

 the degradational work of the atmosphere is weathering, or the prep- 

 aration of material for removal by other and more powerful agents 

 of degradation. . As we shall see, however, the atmosphere is hot 

 the only agent concerned in weathering. 



The wind has doubtless been an important agent in the trans- 

 portation of dust and sand, wherever and whenever there was dry 

 land, ever since an atmosphere has existed. If it has been as 

 effective as now through all the untold millions of years since there 

 have been land and atmosphere, the total amount of work which 

 it must have done is past calculation. Wind-deposited sand, now 

 cemented into solid rock, has' been identified, even in very ancient 

 formations. 



Laboratory work. The study of topographic and geologic maps, photographs, 

 etc., illustrating wind work should be taken up in connection with this chapter. 

 Plates XVI to XXII of Professional Paper 60 of the U. S. Geological Survey afford 

 good illustrations of wind work. See also Interpretation of Topographic Maps, 

 Kxcrrise III, a laboratory manual (Henry Holt & Co.) which may be used with 

 this text. 





