HISTORY OF NEPHELINE BASALTS. 



285 



crystals are larger and more abundant, and there is more magnetite. 

 There is an entire absence of felspar in the American rock. 1 





Nepheline-Basalt. 



Nepheline-basalts consist typically of nepheline, augite, magnetic 

 iron, and olivine. Some varieties contain triclinic-felspar and a 

 little leucite, the latter being more often found. Biotite and Hum- 

 boldtilite are sometimes met with. Usually the structure is granular, 

 with a certain amount of glassy base; the porphyritic condition is 

 comparatively rare. Good types of the rock occur at Pflasterkaute in 

 the Thiiringerwald, and Kohlback, near Bayreuth, in the Western 

 Erzgebirge, near Ador Between Joachimsthal and Flatten a rock 

 occurs with more augite than nepheline. Nepheline-basalts are found 

 in the Mittelgebirge, and at Kaltennordheim in the Khb'n, where the 

 nepheline is recognised by its six-sided section, and is associated with 

 triclinic felspar. In the Swabian Alps the nepheline-basalt is altered, 

 chiefly by the decomposition of the nepheline. The nepheline-dolerite 

 of Katzenbuckel, near Eberbach, in the Odenwald, contains large 

 crystals of nepheline, which are sometimes altered. They lie in a 

 ground mass, rich in green augite microliths, small crystals of nephe- 

 line and nosean, magnetic iron, and a little glass. A rock of this 

 character is found at Oberbergen in the Kaiserstuhl. It contains 

 green or brown augite, nosean, nepheline often penetrated by micro- 

 liths of green augite, with sanidine and garnet penetrated by apatite. 

 Some lavas of the Eifel are characterised by a predominance of 

 nepheline, which is associated with Humboldtilite. Such a rock is 

 seen at Hannebacher Ley, north of the Laacher See : in addition to 

 these minerals, it contains well-crystallised augite, magnetic iron, and 

 a very little leucite. At the Scharteberge, near Kirchweiler, the rock 

 is a compound of nepheline, Humboldtilite, and augite, with hauyine, 

 olivine, and magnetic iron. A similar rock forms the Mosenberg, and 

 occurs near Bertrich by the Moselle. 



1 For further details see Zirkel, Basaltgesteino ; Zirkel, Microg. Petrog. ; 

 Rosenbusch, Micro. Physiog. ; Clarence King, Report Fortieth Parallel, vol. i. 

 3 Timins, Q. J. G. S. vol. xxiii., p. 357. 



