222 A TREATISE ON METAMOEPHISM. 



as, for instance, carbonic acid, thus also producing silicates. The heat and 

 volume relations of these reactions are discussed under "Quartz." 



The reactions of dehydration, crystallization, and lessening of volume, 

 as seen on pages 167-170, are particularly characteristic of the zone of 

 anamorphism, and it is in this zone that the change from opal to quartz 

 probably most extensively occurs. As evidence of this is the frequent 

 occurrence of opal in the zone of katamorphism, and the general absence 

 of opal in the rocks which have been metamorphosed in the lower zone. 



CHERT, CHALCEDONY, ETC. 



Standing between opal and quartz are numerous varieties of partly 

 crystallized or very finely crystallized silica, of which chert and chalced- 

 ony may be taken as the more important kinds. With these substances 

 are frequently small but variable amounts of opal containing combined 

 water. The specific gravities of chert and chalcedony are intermediate 

 between those of opal and quartz, i. e., between 2.15 and 2.65. Their most 

 frequent occurrence is as veins, nodules, belts, and members in carbonate 

 formations. Ordinarily they are derived from organic forms, such as 

 radiolaria, diatoms, and sponges, which lived under conditions similar to 

 those under which the limestone-building animals lived. (See p. 817.) 



Chert and chalcedony are derived from opal. The material here 

 included varies from that which is close to opal, having- only a few minute 

 crystallized spots, through material which shows more and more evidence of 

 crystallization, to material which contains comparatively little amorphous 

 silica, and thence into fully crystallized silica or quartz. The transition 

 varieties may have the peculiar spotty appearance in polarized light char- 

 acteristic of ordinary chert or the peculiar radial fibrous polarization of 

 chalcedony or any combination of the two. 



The alterations of chert and chalcedony are into quartz, or by combi- 

 nation with bases producing silicates, the same as opal. The chemistry and 

 physics of the change are the same as for opal except that the decrease in 

 volume is less, and therefore they need not be repeated. 



