CHAPTER XIII. 



ORIGINAL STRUCTURE AND LITHOGENESIS OF THE ATMOCLASTiC 

 AND ANEMOCLASTIC ROCKS 



A. ATMOCLASTIC ROCKS. 



Typical atmoclastic rocks owe their origin entirely to disintegra- 

 tion or disruption of older rocks, while the accumulation and ar- 

 rangement of this material are brought about by atmospheric agen- 

 cies under the influence of gravity. The most typical atmoclastic 

 rocks are those produced /'// situ by subaerial decay and not subse- 

 quently disturbed. Here belong the laterite and kaolinite, the for- 

 mation of which was described in Chapter II, pp. 37, 39. Taken by 

 itself, such rock may show^ a complete absence of stratification and a 

 gradation in texture from coarse blocks next to the unaltered rock to 

 fine soil and humus at the surface where disintegration and de- 

 composition afifect rocks of uniform grain such as granites or other 

 holocrystallines. A downward gradation into the older rock may be 

 pronounced which, if the upper layers are reworked by water or 

 other agencies, forms an apparent transition bed between widely dis- 

 tinct formations. The products of atmospheric disintegration may 

 become rearranged by flowing, sliding or gliding under the influ- 

 ence of gravity. The talus at the foot of a cliff is an example of 

 such rearrangement. The surface gradient of such an accumulation 

 is generally high, varying from 60 degrees or over in the coarsest 

 series to 23 degrees or even less in the fine-grained series. 



Stagnant atmoclastic material is the exception ; in general move- 

 ment to lower levels occurs. When more or less saturated with 

 water, the angle of slope is correspondingly lowered, and the talus 

 approaches more and more the character of an alluvial fan. The 

 atmoclastic sediments then grade into the hydroclastic or aqueous 

 type. If, on the other hand, the wind is a factor in rearrangement 

 and deposition of the material, this becomes an anemoclastic rock 

 and has to be treated as such. 



The waste of the land is on the march to the sea. Everywhere 



540 



