578 BEPORT OF NATIONAL MUSEUM, 1890. 



rarely occur tridymite, nielanite, zircon, and olivine. The rock under- 

 goes ready decomposition, and calcite, chlorite, linionite, and various 

 minerals of the zeolite group occur as secondary products. 



Chemical composition. — The average of six analyses given by Zirkel* 

 is as follows: Silica, 58.02 per cent.; alumina, 20.03; iron oxides, 6.18; 

 manganese oxide, 0.58; lime, 1.89; magnesia, 0.80; potash, 6.18; soda, 

 6.35 ; water, 1.88 ; specific gravity, 2.58. 



Structure.— rThe phonolites present but little variety in structure, be- 

 ing usually porphyritic, seldom evenly granular. The porphyritic struc- 

 ture is due to the development of large crystals of sanidiu, nepheline, 

 leucite. or hauyn, and more rarely hornblende, augite, orsphene, in the 

 fine-grained and compact grouudraass, which is usually micro-crystalline, 

 never glassy or amorphous. 



Colors. — The prevailing colors are dark gray or greenish. 



Classification and nomenclature. — Three varieties are recognized by 

 Professor Kosenbusch, the distinction being founded upon the varia- 

 tion in proportional amounts of the three minerals sanidin, nepheline, 

 or leucite. We thus have, 1st. Nepheline-phonolite, consisting essen- 

 tially of nepheline and sanidiu, and which may therefore be regarded 

 as the volcanic equivalent of the nepheline syenite. 2d. Leucite phono- 

 lite, consisting essentially of leucite and sanidin, and 3d. Leucitophyr, 

 which consists essentially of both nepheline and leucite in connection 

 with sanidin, and nearly always melanite. 



The following localities and varieties are represented: 



Nepkeline-phonolite: Black Buttes, Black Hills, Dakota, 39096 and 70608 ; Rio Janeiro, 

 Brazil, 69974 ; Serra dos Pocos de Caldas, Prov. do S. Paulo, Brazil 69968; Serra 

 de Tingua, Brazil, 69965 and 70237 ; Una de Fernando Poroula, Prov. de Peruain- 

 buco, Brazil, 69970 and 69971 ; Eifel, Rhenish Prussia, 36344; Ober Scheffhausen 

 Kaiserstuhl, Switzerland, 36346; Schlofsberg, Bohemia, 36347 ; Kleiue Priessen, 

 Bohemia, 36350 ; Kletschner Berg, Bohemia, (Nosean-sanidin phonolite), 36348 ; 

 Mileschauer, Mittelgebirge, Bohemia, (Noseau sanidin-phonolite), 36349; Hohen- 

 Mahlberg, Nassau, Germany, 36345 ; Iiohen Krahen, Baden, 38349 and 35371 : 

 Gemersbold, Baden, 38350. 



Leucite- Phunolite: Civata Castellana, Viterbo, Rocca Monfina, and Baguoria Cimin 

 Mountain!,, Italy, 36541, 365-10, 36562, and 38790. 



Leucitophyr: Englenkopf, Eifel, Prussia, 38337 ; Burg Olbruck Eifel, Prussia, 35753 

 aud 36351; Dachsbusch, Laacher See, Prussia, 36352; Rieden and Perlerkopf, 

 Laacher See, Prussia, 36353 and 36354. 



6. The porphyrites. 



Mineral and chemical composition. — The essential constituents of the 

 porphyrites are the same as of the diorites, from which they differ mainly 

 in structure. 



Structure. — The porphyrites, as a rule, show a felsitic or glassy ground- 

 mass, as do the quartz porphyries, in which are imbedded quite per- 



* Lehrbtich der petrographie, II, p. 193. 



