286 



PRINCIPLES OF STRATIGRAPHY 



Sise of Grain. In recent years attempts have been made to es- 

 tablish a more precise mode of ■ designation of the various types 

 based on size of grain. The following is the standard adopted by 

 the New York City aqueduct commission and may well be adopted 

 as a general standard with the emendations given beyond, 



{■"iG. 39. Diagram showing the interrelations of the exogenetic rocks. ABC 

 represent the crust of the earth or Hthosphere. From within this 

 shell, hut still external so far as the crust is concerned, the volcanic 

 explosions produce the pyroclastic rocks on the surface of the 

 earth. Within the crust itself are formed the autoclastic rocks. 

 From without the shell the attack by atmosphere, hydrosphere and 

 biosphere produce, respectively, atmoclastic (and anemoclastic), 

 hydroclastic and bioclastic rocks. The final result of the destruc- 

 tive activities of the various agents is the return of the material 

 either to the air as vapor or to the water in solution, or more 

 rarely to assimilation by the biosphere (salt, etc.). Fusion may 

 transfer some of the material to the realm of the pyrosphere. The 

 cycle of change is thus complete and redeposition will be as endo- 

 genetic rocks. 



7, Coarse gravel above 5. mm. ; 2, fine gravel 5 to i mm. ; ?, 

 coarse sand i to 0.5 mm. ; 4, medium sand 0.50 to 0.25 mm. ; 5, fine 

 sand 0.25 to o.io mm.; 6, superfine sand o.io to 0.05 mm.; 7, rock 

 flour (silt, Merrill) 0.05 to o.oi mm. ; 8, superfine flour (fine silt, 

 Merrill) o.oio to 0.005 ^^i''- ! P> ^'^Y size 0.005 to o.oooi mm. 



