ENDOGENETIC ROCKS 



273 



a simple matter in all but the pyrogenic rocks. In refined work 

 the chemical composition of the latter is by some subjected to a 

 recalculation into the minerals which satisfy the composition (Cross, 

 Iddings, Pirsson, etc.-4), though this at best seems an artificial 

 procedure. 



Texture and structure have been used with rather indefinite 

 limitations. In general, texture is the fine structure or microstruc- 

 ture, concerned especially with minute arrangement of the compo- 



FiG. 38. Interrelations of the Endogenetic Rocks. 



"The material of the earth may be in three states before solidification, 

 i. e., that of fusion, of solution in water, and that of vapor. From these states 

 by direct deposition (endogenetically) we get the three types of fundamental 

 rocks — the Igneous, or Pyrogenic; the Aqueous, or Hydrogenic ; and the 

 Atmospheric, or Atmogenic (snow and snow-ice). By the physiological ac- 

 tivities of organisms we have formed the Organic, or Biogenic, rocks, the 

 material of which is derived either from the atmosphere (chiefly through 

 plants — Phytogcnic, though also in a minor degree through animals, as indi- 

 cated by the heavy dotted line) ; or from the water (chiefly through animals — • 

 • — Zoogcnic, though also in a minor degree through plants). The lighter 

 arrows show to which states the rocks finally return, the dotted lines indicating 

 the less likely change. Biogenic rocks are either vaporized or dissolved, hence 

 the return arrows rise from the center. Pyro-atmogenic rocks, half way 

 between Pyro- and Atmogenic are illustrated by sublimates, and Pyro-hydro- 

 genic rocks, half way between Pyro- and Hydrogenic, by pegmatites." 

 (Grabau-Q-.^jJ".) 



