1894.] Petrography. 103 
reexamined by Pirsson,' who finds it to be a phonolite with phenocrysts 
of anorthoclase and pyroxene, in a groundmass of the usual compo- 
nents of phonolite. The anorthoclase has the composition : 
BIO, ALO, Fe,9, CaO Na,O rer HO Total Sp. Gr. 
66.44 19.12 tr 7.91 10 67 =99.70 2.585 
The nepheline is all in the groundmass where it appears as idiomor- 
phic crystals. The density of the rock is 2.582 and its composition : 
SiO, TiO, Al,O, Fe,0, Ae MnO CaO BaO MgO Na,O K,O H,O CI SO, Total Cl 
61.08 18 18.71 1.91 tr 1.58 .05 .08 8.68 4.63 2.21 12 tr=99.86—.03=99.83 
A second occurrence of phonolite within the same region is in a dyke 
just south of Deadwood. It consists of phenocrysts of reddish feld- 
spars and black hornblendes that approach barkevikite in properties. 
The rock from the Black Hills sold by the dealers as tinguaite is a 
dense aggregate of pyroxene phenocrysts in a matrix of feldspar and 
aegirine, with an occasional patch of nepheline. 
| The Origin of Norwegian Iron Ores.— The iron and other 
i ores of many of the Norwegian localities are connected genetically 
with granites and gabbroitic eruptives. The iron ores in veins 
| are supposed by Vogt?to be due to contact action between granite 
and the surrounding rocks. Those connected with the gabbros are 
basic accumulations, whose origin is ascribed to differentiation of the 
| basic magma. In consequence of this differentiation, which is governed 
largely by Soret's principle and the differences in density of the various 
| differentiated products, the gabbro splits into labrador-rock and various 
iron-olivine and iron-pyroxene compounds, and in these latter are 
accumulations of magnetite and ilmenite large enough to constitute ore 
bodies. Each of the iron-pyroxene rocks is described by the author 
and the iron ores associated with them are characterized. The titanium 
of the iron is thought to have originated mainly in the olivine and 
other basic components of the normal gabbro. 
The Tonalites of the Rieseferner.—The tonalites of the 
Rieseferner in the Tyrol are again the subject of careful petrographi- 
eal study. The normal tonalite (hornblende-mica-quartz-diorite) 
* Amer. Jour. Sci., XLVII, 1894, p. 341. 
5Geol. Fóren Stock. Fórh. 13 and 14. 
5Becke: Min. u. Petrog. Mitth., XIII, p. 379. 
