MANSFIELD AND LARSEN I NEPHELINE BASALT 



467 



1. Xepheline basalt (III) IV. 1 (2). 3. 2. (1) 2 Rossweinose. About 24 miles 

 east of Blackfoot, Idaho. 



2. Periodtite, IV.1.'.3.'2.'2, Crystal Falls, Mich. H. N. Stokes, analyst. 

 Described by J. Morgan Clement, Jour. Geol., 6 : 386. 1898. 



3. Gabbro, 'IV. 1'. '3. 2. '2, Etzdorf, Rosswein, Saxony. Sachsse and Becker, 

 X. J., 1893, II, p. 503. 



4. Nepheline basalt, 'IV.2.3.2'.'2, Schafberg Plateau, Saxony, J. Stock, Tsch. 

 Min. Pet. Mitt., 9: 466. 1888. 



5. Nepheline basalt, IV.'2.3.2.'2, Black Mt., Uvalde Co., Texas. W. F. Hille- 

 brand, analyst. Described by Whitman Cross, U. S. Geological Survey, Bull. 

 419, p. 43. 



Classification. The great abundance of augite, olivine, and 

 biotite, and the presence of nepheline as the only feldspathic 

 mineral, places the rock as a biotite-bearing nepheline basalt 

 rather high in olivine. 



In the Quantitative Classification the norm and the systematic 

 position of the rock are as follows: 



Norm and systematic positions of nepheline basalt from about 24 miles east of 



Blackfoot, Idaho 



Class: Sal = 34.91 = 0.554 (III) IV 



Fern 63.03 

 Order: PO = 55.96 = 8.74 1 (2) 



M 6.40 



Section: P = 25.35 = 0.826 3 



O 30.61 

 Rang: Mg + Fe + K2O + "NazO 568 = 

 CaO =116 



4.90 '2 



Subrang: MgO = 496 = 6.89 (1) 2 

 FeO 72 



(III) IV. 1 (2).3.'2.. (1) 2, Rossweinose. 



100.12 



Comparison of the norm and mode. The presence of consid- 

 erable phlogopite in the norm and the presence of feldspar in the 

 mode and its absence in the norm are the most apparent differ- 

 ences between the two. The following calculation of the mode 

 from the chemical composition and the known mineral composi- 



