DIAGENESIS : RECRYSTALLIZATION 755 



feldspar upon worn grains of that mineral, the old and new mineral 

 being in optical continuity. ( \'an Hise-32.) The sandstone con- 

 tains both orthoclases and plagioclases, and both are enlarged by 

 deposition of new material in o])tical continuity with the old. Horn- 

 blende has also been found to be secondarily enlarged in old volcanic 

 tuffs. 



Oiiartzitcs and Novaculites. 



When quartz sandstones are so completely cemented by second- 

 ary silica, whether deposited independently or in optical continuity 

 with the original quartz grains, that the rock will break across the 

 original grains rather than between them, the rock is called a 

 quartsite. If the original grain of the quartz rock was a luta- 

 ceous one, the result oi this excessive induration is a novaculite. 



Lithiftcation of Clastics Largely a Supramarinc Process. 



Since lithification of clastics by cementation and recrystallization 

 requires the active circulation of ground water, it is apparent that 

 it is chiefly effective after the deposits in question have been lifted 

 above sea-level, if they originally were marine. This is not entirely 

 true for processes of recrystallization, which may go on even be- 

 neath sea-level. 



II. Recrystallization, 



Recrystallization of the mineral constituents may affect all rocks, 

 and occur under static, dynamic or contactic conditions. As a proc- 

 ess of diagenism it often produces marked results, though these 

 are never carried to the extremes which are attained when it acts 

 as a process of symphrattism. When it takes place in unconsoli- 

 dated material it may become a method of lithification, but it is 

 more commonly found in rocks already consolidated by one or the 

 other method. As a method of change from a less stable to a more 

 stable form of mineral it is of the greatest importance. Thus the 

 original less stable forms of CaCO.,, aragonite, ktypeit, found in 

 marine oolites and organic deposits, are changed to the more stable 

 form calcite. (See Chapter IX.) In the case of organic remains so 

 altered, the finer structural features are commonly lost. 



Recrystallization is especially efifective in the more soluble rocks, 



