ATAIOCLASTIC ROCKS 541 



the loose rock debris is streaming toward the ultimate repository, 

 the sea, or to the rock-rimmed desert basins of the earth, from 

 which in many cases there is no escape. This great waste stream, 

 as it has been aptly termed, covers much of the surface of the land, 

 and may consist wholly of del)ris, or of debris and water in varying 

 proportions. As the amount of water increases the fluidity of the 

 mass rises, and the rate of its motion is increased, while the angle 

 at which graded conditions prevail is decreased. Thus where the 

 material is pure waste the angle will commonly be more than 20 

 degrees, and steeper in proportion as the material is coarser. Fur- 

 thermore, the material of the talus heap is constantly undergoing 

 disintegration, and, as it becomes finer, the potential angle of slope 

 is lowered. The actual lowering will be accomplished by a process 

 of "creep," which is often greatly aided by frost action and by tem- 

 porary saturation with water, which may become instrumental in 

 suddenly lowering the angle through landslides. 



The totality of the deposits formed by subaerial agencies on 

 the plains in front of the mountains is classed as piedmont waste 

 deposits. They may be formed wholly by "dry creep" from the 

 mountains, or, as is most generally the case, by combined creep 

 and the wash of mountain streams. Alluvial fans or dry deltas 

 thus form the connecting link between the atmoclastic and the 

 hydroclastic types of deposits, for, though strictly belonging to the 

 fluviatile division of the latter, they partake of many of the charac- 

 teristics of the former, and are indeed partly produced by subaerial 

 creep. (Walther-52: Penck-37 :i>5 ; Barrell-2.) 



Characteristics and Occurrence of Modern Atmoclastic 



Formations. 



Extensive recent formations of atmoclastic origin are found only 

 in the comparatively arid regions of the world, and as these are the 

 last which have come under the detailed observation of the geolo- 

 gist, though they cover about a fifth of the land surface of the earth, 

 their significance with reference to the former history of the earth 

 has been largely overlooked. It is only within recent years that the 

 desert regions of the world have been studied in detail and their 

 phenomena applied toward the interpretation of the genesis of older 

 formations of the earth's crust. So long as the destructive powers 

 of the atmosphere and streams were regarded as their chief at- 

 tributes, while the sea was held to be the chief receptacle of the 

 product of this destructive activity, it was natural that the marine 



