52 



ALBERT JOHANNSEN 



Class 4. 



TABLE IV 



Quarfeloids 



Mafites 



between — and — 

 95 100 



CLASS I, ORDER I 



(no) Silexite Miller. The term silexite was proposed by 



Miller^ for pure igneous quartz rocks. Such rocks, frequently 

 described under the names igneous quartz, quartz dikes, quartz 

 veins, etc., represent the end members of pegmatitic intrusions. 

 Greisen, an old Saxon miner's term, cannot be used for the family 

 name, since it should properly be restricted to tin-bearing leuco- 

 granites. Furthermore, according to some authors, it is an altered 

 rock, feldspar having been changed to quartz. Beresite Rose* 

 likewise cannot be used; it was shown by Helmhacker^ to be an 

 altered quartz-porphyry. 



Besides pure quartz there are a number of other rocks falling in 

 this family, namely such as are free from feldspars and mafites, and 



^William J. Miller, "Silexite, a New Rock Name," Science, XLIX (1919), 149; 

 also "Pegmatite, Silexite, and Aplite of Northern New York," Jour. Geol., XXVII 

 (1919), 28-54, in particular p. 30. 



* Gustav Rose, Mineralogisch-geognostische Reise nach dem Ural, dem Altai und 

 dem Kaspischen Meere (Berlin, 1837), I, 186. 



3 R. Hebnhacker, "Der Goldbergbau der Umgebung von Berezovsk am ostlichen 

 Abhange des Urals," Berg- und Hiittenmdnnische Zeihing, LI (1892), 83-84. 



