CRYSTALLIZATION OF AMORPHOUS CHERT. 849 



carbonation of the silicates and that furnished by the solution of amorphous 

 and semicrystalline quartz. 



Rearrangement of chert. — -Chert, as deposited by organisms or chemically, 

 may be hydrous and amorphous, semicrystalline, or completely crystalline. 

 In proportion as it is amorphous or semicrystalline, it is likely to be easily 

 and extensively dissolved and rearranged. Usually the opaline, semi- 

 crystalline, and completely crystalline materials, are mingled in an intricate 

 manner. Very often a single thin section shows all varieties of material. 

 In many sections the opaline or amorphous silica constitutes a background 

 in which there are innumerable polarizing spots of chalcedony and quartz. 

 The process of rearrangement may go on until the material, whether of 

 organic or chemical origin, becomes wholly crystalline quartz. 



If crystallization be complete, there may be produced a finely 

 crystalline rock composed of closely fitting, minute particles of quartz, 

 many of them with crystal outlines, which average only a small fraction 

 of a millimeter in diameter. Such rocks may be called finely crystalline 

 quartz rocks. Representatives of them are the whetstones of Arkansas and 

 the quartz rock of the Penokee district of Wisconsin. In many cases such 

 rocks are stained by oxide of iron, and these are often called jasper. 



The crystallization may go further, so as to produce coarsely crystal- 

 line quartz rock. This process of crystallization of the amorphous 

 material so as to produce first, chert ; next, finely crystalline quartz ; and 

 next, coarsely crystalline quartz, occurs under the laws of physical 

 chemistry. In so far as the silica passes from a hydrous to an anhydrous 

 condition the volume is lessened. In so far as it passes from an amorphous 

 to a crystalline condition the volume is lessened. The change in volume 

 in the passage of one of the ordinary opals to quartz has been calculated 

 on page 221 to be 22.81 per cent. All these processes may occur at any 

 depth, precisely the same as the process of dolomitization. But they are 

 promoted by depth of burying and by high temperature, and therefore the 

 finely crystalline and coarsely crystalline quartz rocks derived from organic 

 silica or siliceous, sinter have usually been buried to a very considerable 

 depth. 



If in addition to depth of burying- there be profound orogenic move- 

 ment, the rock may be further recrystallized and mechanically sliced, and 



MON XLVII — 04 54 



