750 PRINCIPLES OF STRATIGRAPHY 



STATIC METAMORPHISM OR DIAGENISM. 



Of the cliagenetic processes affecting rocks the following may be 

 especially considered : (I) lithification, (II) recrystallization, (III) 

 dolomitization, (IV) replacement of limestone by silica, etc., (V) 

 desalinification, (VI) formation of concretions, (VII) hydration 

 and dehydration. 



I. Lithification or Induration. 



The lithification of a rock is not restricted to diagenetic proc- 

 esses, but may be greatly aided if not altogether caused by the 

 other processes of metamorphism, especially the dynamic ones. Nor 

 is lithification a natural result of aging, for time has little or no 

 influence as a primary factor, though it may become important 

 when lithification is primarily due to some other factors. As ex- 

 amples may be cited the still unconsolidated early Palaeozoic sands 

 and clays of the undisturbed plains of Russia, and the much meta- 

 morphosed Eocenic rocks of the Alps, or of the Coast Range of 

 California. In the last two cases cited, the alteration of the rocks 

 is of course due to the dynamic disturbances which have affected 

 them, but consolidation by purely diagenetic processes of recent 

 sediments is not unknown. Thus the consolidation of the coral 

 sand of Bermuda furnishes a good example of a lithified rock of 

 modern origin, while the Nagelfluh of Salzburg, Austria, and other 

 districts illustrates the solidification of a clastic deposit of Pleisto- 

 cenic origin. (See Chapter XIV, p. 6oi.) 



Lithification takes place with varying rapidity in rocks of dif- 

 ferent origin. Thus pyrogenjc rocks, lavas or intruded masses 

 solidify comparatively rapidly through cooling. This results either 

 in congelation into an amorphous mass or in crystallization — wholly 

 or in part. Water solidifies with extreme rapidity by crystalliza- 

 tion when subjected to the proper reduction of temperature. Snow 

 crystals (atmogenic rocks) solidify somewhat more slowly from 

 granular neve into glacier ice, a typical process of diagenetic meta- 

 morphism, though involving to a certain extent recrystallization. 

 The same method of solidification probably affects most hydrogenic 

 rocks. Organic rocks are solidified at the time of their formation, 

 except organic oozes and granular organic rocks (pulverites, granu- 

 lites, etc.), which may be combined into masses much as clastic 

 rocks are. The lithification of clastic rocks is due to pressure- 

 cohesion, to cementation, or to recrystallization. 



