HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 587 



13. The leucite rocks. 



Mineral composition. — The essential constituent is leucite and a basic 

 angite. A variety of accessories occur, including biotite, hornblende, 

 iron ores, apatite, olivine, plagioclase, nepheliue, nielilite, and more 

 rarely garnets, hauyn, sphene, chromite, and perowskite. Feldspar as 

 an essential fails entirely. 



Chemical composition. — The average chemical composition as given 

 by Blaas* is as follows: Silica, 48.9; alumina, 19.5; iron oxides, 9.2; 

 lime, 8.9 ; magnesia, 1.9 ; potash, 6.5 ; soda, 4.4 per cent. 



Structure. — The rocks of this group are, as a rule, fine-grained and 

 often slightly vesicular, presenting to the unaided eye little to distin- 

 guish them from the finer graiued varieties of ordinary basalt. 



Colors.-— The prevailing colors are some shades of gray, though some- 

 times yellowish or brownish. 



Classification and nomenclature. — The varietal distinctions are based 

 upon the presence or absence of the mineral olivine and upon struc- 

 tural grounds and various minor characteristics. We have the olivine 

 free 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 (specimens 36877 and 72846). 



The localities now represented are as follows : 



Leucitite : The following localities in the province of Rome, Italy : Capo di Bove, 

 36560; Aqua acetosa, via Laurentia, 36561; Bagnorea, 36562 ; Cava di Marino, 

 36673 ; Foutanadi Papa, Strada d'Albano, 36564 ; Sta Maria di Galera Bracciano, 

 36565; Mte. Salumone, Mte. Compatri, 36566; Villa Lancellotti, Frascati, 

 36567 ; Colle del Eremita, Mte. Compatri, 36568 ; Cima del Tuscolo, 36569 ; Colle 

 dei Cypressi, Mte. Compatri, 36570; Italy, 70232; near Conca, Roccamonfina, 

 Italy, 73020 ; Serra des Pocos di Caldos, province de la Sao Panlo, Brazil, 69985 ; 

 N. W. of Points of Rocks, Leucite Hills, Wyoming, 36377 and 72846. 



Leucite hasalt. — Laacher See, Prussia (with rubelan), 36571; Diefelderstein, Kungs- 

 kopf, Bausenberg, and Veitskopf, Laacher See, Prussia, 36571 to 36575, inclusive; 

 Pohleberg, near Annaherg in the Erz-Gebirge, Saxony, 36576. 



14. The nepheline rocks. 



Mineral composition. — These rocks consist essentially of nepheline 

 with a basaltic augite and accessory sanidin, plagioclase, mica, olivine, 

 leucite, minerals of the sodalite group, magnetite, apatite, perowskite, 

 and melanite. 



Chemical composition. — Below is given the composition of (I) a neph- 

 elinite from the Cape Verde Islands, and (II) a nepheline basalt from 

 the Vogelsberg, Prussia.t 



* Katechismus der Petrographie, p. 117. 

 t Roth's Gesteine Aualysen, 1884. 



