GEOLOGICAL WORK OF THE ATMOSPHERE 109 



gate, this result is inconsequential; 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 often fuses 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 only a few inches in length, and more com- 

 monly 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 readily 

 moved to lower levels by other agencies. The most important 

 phase of the degradational work of the atmosphere is weathering,' 

 or the preparation of material for removal by other and more power- 

 ful agents of degradation. As we shall see, however, the atmos- 

 phere is not the only agent concerned in weathering (see p. 140). 



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, photo- 

 graphs, etc., illustrating wind work may well be taken up in connection with 

 this chapter. Numerous sheets of the topographic maps published by the 

 United States Geological Survey afford illustrations of dunes; so also do a 

 few of the folios. Plates XVI to XXII of Professional Paper 60 of the U. 

 S. Geological Survey afford good illustrations of wind work. A fuller list 

 of topographic maps available for this purpose is to be found on p. 78 of the 

 junior author's Physiography (Advanced Course). 



