DIAGENESIS; LITHIFICATION 751 



The methods of Hthification, not confined to diagenetic processes, 

 however, may thus be tabulated : 



1. Congelation — in amorphous bodies. 



2. Crystallization — chiefly in pyrogenic rocks. 



3. Recrystallization — chiefly in atmogenic and hydrogenic 



rocks. 



4. Welding or pressure cohesion — chiefly in clastic rocks. 



5. Cementation — chiefly in clastic and organic rocks. 



Igneous rocks solidify by congelation into an amorphous glass 

 (obsidian, etc.) or by crystallization. Atmogenic snow crystals con- 

 solidate into firn and glacier ice by a process of recrystallization, 

 when the smaller crystals are destroyed to the gain of the larger 

 ones. This is also true of granular hydrogenic rocks such as rock 

 salt and gypsum. Hydrogenic rocks are also solidified through 

 cementation by precipitated material of the same kind. Biogenic 

 rocks are usually consolidated by the precipitation of calcium car- 

 bonate under the influence of decaying organic matter and the for- 

 mation of ammonium carbonate. 



Sphfcrites, granulites and pulverites, of whatever origin, are gen- 

 erally consolidated by the same agents which consolidate the clastic 

 strata. 



LiTHiFicATiON OR INDURATION OF Clastic Rocks. Under this 

 heading will also be included the granular or pulverulent endoge- 

 netic substances. The two chief methods are : ( i ) Pressure-cohe- 

 sion, or welding, and (2) cementation. Recrystallization, especially 

 through secondary enlargement, also consolidates loose material, but 

 is more common in rocks already lithified. Though distinct proc- 

 esses, they seldom if ever occur wholly alone, both welding and 

 cementation generally taking place at the same time, though in 

 unequal amount. Recrystallization may accompany these processes. 



I. Welding. (Van Hise-34 : 505-597, d/O-d//.) This is a proc- 

 ess of mechanical consolidation caused either by the pressure of 

 superincumbent rocks or by tectonic movements. This pressure re- 

 sults in bringing closely together the particles of which the rocks 

 are composed. If water is present, this is squeezed out, while the 

 mineral particles are mechanically readjusted with reference to one 

 another. The particles will cohere, because they are brought so 

 close together by the pressure that they are within the limit of 

 molecular attraction of one another. This takes place especially in 

 the zone of anamorphism, where the pressures in all directions are 

 greater than the crushing strength of the rocks, and hence sufficient 

 to bring the particles within the sphere of molecular attraction. 



