36 



University of California. 



[Vol. 2. 



rich in alkalies. They have been recognized in the soda rich gran- 

 ites and syenites as well as in the eleolite syenites. Among the 

 younger eruptive rocks it is the phonolites and leucitophyres as 

 well as related rocks in which occur aegirine or the nearly related 

 acmite, or, expressed more truly, the monoclinic pyroxene rich in 

 the acmite molecule. vEgirine-augites are regular components of 

 many leucitophyres, phonolites, and eleolite syenites." From these 

 quotations it would appear that the aegirine-augite is not neces- 

 sarily or even generally present in the basaltic nepheline-bearing 

 rocks. 



Zirkel* gives the following account of the occurrence of 

 flegirine : " In the younger eruptive rocks aegirine or an aegirine- 

 like pyroxene, rich in soda, is known in many phonolites, certain 

 phonolite-like trachytes, sanidinites, some leucite and nepheline 

 rich rocks. A relatively high soda content of the magma always 

 appears to be necessary for the crystallization of a soda rich pyrox- 

 ene." Of the augite of the nepheline rocks he says: " The augite 

 in these nepheline basalts is in general wholly like that of the feld- 

 spar basalts." This is sufficient to show that, as far as our knowl- 

 edge goes, the absence of the aegirine augite does not seriously 

 affect the question of the original presence of nepheline. It would 

 appear that this augite is generally present in the more acid nephe- 

 line rocks, but more often absent from the basic varieties. 



The history of the teschenites has many features in common 

 with that of other groups. The name was first used by Hoheneggerf 

 in 1 86 1 for some eruptive masses in the Cretaceous of central Europe. 

 The group was more narrowly defined by Tschermak,^ and later 

 by Rhorbach.§ The latter divided the teschenites of Tscher- 

 mak into two groups on structural grounds, depending on whether 

 the ferro-magnesian silicates crystallized earlier or later than the 

 feldspar. Zirkel || accordingly refers the rocks with the latter 

 structure (ophitic), which are less widely spread and contain no 



* Lehrbuch der Petrographie, 2d edition, Vol. II, p. 293. 



fGeognost. Verhalten. der Nordkarpathen inSchleisen, etc., Gotha, 1861. 



t Sitzgsber. Wiener Akad., Bd. 53, 1866. 



I Min. u. petr. Mitth. VII., 1886. 



|| Lehrbuch der Petrographie, 2d edition, Vol. II, p. 678. 



