( H3 ) 



CHAPTER XL 



SUBAERIAL DENUDATION AND ITS RESULTS. 



Wasting Effects of the Atmosphere. 



THE gradual wasting of the surface of the superficial parts of the earth 

 is an important element in geological theory and history. The follow- 

 ing examples of the varied results of atmospheric influences in 

 modifying the surface of the works of nature and of man, form but a 

 small fraction of current information on the subject. 



The wasting effects of the atmosphere are those terrestrial processes 

 by which materials are provided for rivers and the sea to transport and 

 deposit in new situations. These processes are dependent on general 

 humidity, variations of moisture, precipitation of rain, and variations 

 of temperature. 



It is not, however, always possible to distinguish accurately the 

 effects of these several causes. Many natural agencies are often con- 

 cerned in one operation, and the general result is the sum or the 

 difference of their effects. The chemical action of the atmosphere is 

 evident in buildings, and on the surface of certain rocks. The same 

 process which slowly reconverts the mortar of walls into crystalline 

 carbonate of lime frequently causes the pulverisation and bursting of 

 the bricks, in consequence of the expansion of the small masses of 

 lime which they contain. 



The surface of bricks is often covered with a saline efflorescence', 

 which is generally nitrate of lime, but sometimes chloride of sodium. 

 The surface of the yellow limestone near Doncaster is sometimes 

 covered with a nitrous efflorescence, and so is the calcareo-magnesiau 

 mortar made from it. 



The Wind. The agency of the wind as a denuding power is easily 

 underestimated, though the amount of dust deposited from the atmos- 

 phere under ordinary circumstances demonstrates that much matter is 

 carried by the air from a higher to a lower level. The modern inven- 

 tion of the sand-blast, by means of which glass, granite, and. other 

 substances are easily etched, illustrates experimentally the ways in 

 which wind, blowing in prevalent directions^ abrades rocks. And when 

 we remark that the contours of the sandhills of Holland are exactly 

 the contours of mountain chains, it is quite possible that the outlines 

 of mountains are in the main to be attributed to the agency of 



