QUARTZITE. 865 



The question as to where the process of cementation from sand to 

 sandstone takes place is discussed under the next heading, "Quartzite," 

 since the problem is precisely the same with both, and it can be better 

 understood after the facts relating to the cementation of quartzites have 

 been considered. 



QUARTZITE. 



One of the most important rocks produced by the cementation process 

 is quartzite. (PL IX.) The term "quartzite" is here restricted to quartzose 

 sand rocks which have been so firmly indurated by the cementing processes 

 that when broken the fractures pass through the original grains and not 

 around them. The process of cementation continues, until the enlarged 

 sand grains or the independent quartz deposited interfere and interlock so 

 as to nearly or quite fill the interspaces. (PL IX, B.) In the production 

 of indurated sandstone and quartzite the quartz may be deposited either as 

 enlargements of old grains or as independent interstitial material. In either 

 case the induration may be so nearly perfect that a vitreous quartzite is 

 produced. If composed of very fine grains of sand, so that the indurated 

 rock may serve as a whetstone, the rock has been called novaculite. Novac- 

 ulite is therefore no more than an even- and fine-grained quartzite. In 

 general, there is a strong tendency for the quartz to deposit upon the old 

 grains rather than as independent mineral particles. The reason for the 

 deposition of the quartz from the solutions and the optical orientation of 

 this quartz with the orig'inal sand grains has been discussed on pages 75-76, 

 121-123. It may here be said in summary that the explanation lies in 

 the power of a mineral to abstract from solutions material like itself, for 

 large particles to grow rather than new ones to develop, and to the fact 

 that silica is one of the most abundant of the materials which are transported 

 by ground waters. Many of the cemented quartzites show little or no 

 evidence of strain, which proves that mechanical action has not been 

 potent in the induration. Neither is there any evidence that a high degree 

 of heat is requisite to dissolve the silica or to deposit it as quartz upon or 

 between the old grains. The sands from which quartzites develop contain 

 various impurities. During the transformation of the sands to quartzite 

 these impurities are rearranged to a greater or less extern by metasomatic 

 changes and new minerals develop, as described in the cases of the minerals 

 in the feldspathic sands and the grits. (See p. 870 et seq.) As with sand- 



MON T XLVII — 0-1 55 



