756 PRINCIPLES OF STRATIGRAPHY 



such as limestones, gypsum and salt, though the secondary enlarge- 

 ment of quartz crystals in reality also belongs here. Gypsum, anhy- 

 drite, rock salt and granular snow are other substances easily sub- 

 ject to recrystallization. In this process the smaller particles are 

 commonly dissolved and their material added to the larger ones. 

 In the zone of katamorphism, solution and redeposition are going 

 on throughout the limestone with the result that the entire mass is 

 gradually recrystalHzed. This may affect both loose aggregates of 

 calcite grains which thereby become consolidated, and it may affect 

 indurated limestones which are then gradually altered toward the 

 condition of marble. True marble is probably formed only under 

 the influence of dynamic forces, but many recrystallizations come 

 close to approaching this state. 



It is often assumed that recrystallization has affected most of 

 the older Paljeozoic limestones, because of their lack of organic 

 remains, which, it is argued, are destroyed by recrystallization. It 

 may be questioned whether organic remains are ever destroyed by 

 ordinary recrystallization, though there is no doubt of this when 

 recrystallization under dynamic influences goes on. In the case of 

 many of the older Palseozoic limestones, however, the absence of 

 organic remains is a primary character. Many of these limestones 

 were deposited as lime muds and silts derived from the erosion of 

 still older limestones, and without the direct participation in their 

 formation of lime-secreting organisms. 



Rock salt deposits on recrystallization tend to become coarser, 

 as in the case of the Polish deposits. The same is true for gypsum, 

 which sometimes crystallizes out into masses of large dimensions. 

 The largest found up to date in Utah measured in some cases 150 

 cm. in greatest dimension. When deeply buried, gypsum loses its 

 water under the influence of pressure and recrystallizes into anhy- 

 drite. This brings with it a decrease of volume of 38%. 



An important point for consideration lies in the fact that recrys- 

 tallization is favored by pressure. The greater the pressure, the 

 more likely is the deformation to be accomplished by recrystal- 

 lization. 



Pressure Phenomena Due to Recrystallization. 



In rocks of homogeneous character and fine grain, recrystalliza- 

 tion may have a deformative effect on the original structure lines 

 and not infrequently upon the enclosing strata. This is especially 

 well seen in the salt deposits of undisturbed regions, such as the 

 Zechstein salt of north Germany and the Salina salt of New York. 



