THE NEPHELINE EOGKS 



97 



Classification and Nomenclature. — The varietal distinctions 

 are based upon the presence or absence of the mineral olivine 

 and npon structural grounds and various minor characteristics. 

 We have the olivine~f ree variety leucitite and the olivine-holding 

 variety leucite basalt. 



These rocks have also a very limited distribution, and, so far 

 as known, are found within the limits of the United States only 

 at the Leucite Hills, Wyoming, and the Highwood Mountains of 

 Montana. 



(2) THE NEPHELINE BOOKS 



Mineral Composition. — These rocks consist essentially of 

 nepheline with augite and accessory sanidin, plagioclase, mica, 

 olivine, leucite, minerals of the sodalite group, magnetite, apa- 

 tite, perowskite, and melanite. 



Chemical Composition. — Below is given the composition of 

 (I) a nephelinite from the Cape Verde Islands, and (II) a 

 nepheline basalt from the Vogelsberg, Prussia.^ 



Chemical Composition of Nepheline Rocks 



Constituents 



Silica (Si02) .... 

 Alumina (AlsOg) . . . 

 Iron sesquioxide (^6203) 

 Iron protoxide (FeO) 

 Magnesia (MgO) . . . 



Lime (CaO) 



Soda (NaaO) . . . . 

 Potash (K2O) , . . . 

 Water (H2O) . . . . 



Specific gravity. . . , 



II 



42.37 % 



8.88 

 11.26 



7.80 

 13.01 

 10.93 



4.61 



1.21 



0.34 



100.29 % 

 3.103 



Colors. — The prevailing colors are various shades of gray 

 to nearly black. 



Structure. — Structurally they are porphyritic, with a holo- 

 crystalline or in part amorphous base, usually fine grained and 

 compact, at times amygdaloidal. 



Classification and Nomenclature. — These rocks differ from 

 the basalts, which they otherwise greatly resemble, in that they 



^Both, Abhandl. der Konig. Prens. Akad. der V7iss. zu Berlin, 1884. 

 8 



