28 PHYSIOGRAPHY 



surface of many a sharp mountain peak is covered with cracked and 

 broken rock, so insecure that a touch or a step will loosen many 

 pieces and start them down the mountain (Fig. 18). Great piles 

 of such debris (called talus) bury the bases of some of the western 

 mountains to the depth of many hundreds of feet (Fig. 19). The 

 pieces of talus range from tiny bits to masses tons in weight. 



This process of rock-breaking is a phase of weathering. The 

 debris loosened in this way moves from higher to lower levels under 

 the influence of gravity, if it moves at all. The general effect of 

 the process is to make high places lower, and to build up low lands 

 about the bases of steep slopes. 



The breaking of rock through changes of temperature is not the 

 work of the atmosphere; but the atmosphere has much influence 

 n the changes of temperature on which the process depends. 



' SUMMARY 



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

 of the work which is controlled by it is to lower (degrade) the sur- 

 face of 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 this work is weathering, or the preparation of 

 material for removal by other and more powerful agents of erosion. 

 As we shall see, however, the atmosphere is not the only agent con- 

 cerned in weathering (see p. 44). 



REFERENCES 



1. UDDEN, Erosion, Transportation, and Sedimentation Performed by the 

 Atmosphere: Jour, of Geol., Vol. II, pp. 318-331, and Pop. Sci. Mo., Sept., 1896. 



2. GEIKIE, Earth Sculpture, Chapter XII: Putnam. 



3. HITCHCOCK, Controlling Sand Dunes in the United States: Nat. Geog. 

 Mag., Vol. XV, pp. 43-47, 1904. 



4. SALISBURY, Physiography, Advanced Course, Chapter II: Henry Holt 

 &Co. 



5. Standard text-books on Geology. 



