G. H. Williams—WNorites of the “Cortlandt Series.” 195 
ap isotropic mineral, like spinel. In the massive ore this min- 
eral forms an aggregate of interlocking grains, but in the 
adjoining rock it is disseminated in small octahedral crystals as 
in section No. 6a described above. It was isolated from a 
specimen of iron ore collected near Cruger’s Station and ana- 
lyzed by Mr. W. 8. Bayley.* 
In the eastern and southeastern parts of the norite region sim- 
ilar deposits of pleonaste occur in’great number, and frequently 
contain more or less corundum and fibrolite. 
The Westchester Co. pleonaste is remarkable for its small 
proportion of magnesia. The eight analyses of this mineral 
given in Dana’s System of Mineralogy (p. 148) have from 13 
to 26 per cent MeO. The specimen which comes nearest in 
composition to the Cortlandt mineral is one from Tunaberg in 
Sweden, in which Erdmann found Al,O, 62°95, FeO 23°46, 
MgO 13:03.+ Still even here the amount of magnesia is con- 
siderably greater than in the mineral analyzed by Mr. Bayley. 
In this respect the Cortlandt pleonaste approaches the pure iron 
spinel (FeO,Al,O,) which Zippe, in 1839, named Hercynite.t 
This was analyzed in 1845 by B. Quadrat, who found that it 
contained 
Al,O, 61:17, FeO 35°67, MgO 2°92.8 
This mineral occurs in black masses near the villages of 
Natschetin and Hoslau, nor far distant from the town of Rons- 
perg, at the eastern edge of the Bohemian Forest. (Lat. “Silvia 
Flercynia,” Pliny, whence Zippe’s name). Quadrat states that 
it is only found in loose blocks in the soil, but von Hochstetter 
says that it occurs in position, as ‘‘a member of the Archeean 
Series, between amphibolite and amphibole-schist.” .The 
Bohemian hercynite was first microscopically studied by H. 
Fischer, who discovered in the spinel aggregate what he con- 
sidered to be magnetite, quartzand hematite.q H. Kalkowsky, 
subsequently showed the supposed quartz to be corundum, and 
the hematite, iron-hydroxide.** Certain very thin, dark gray 
plates which are frequently interpolated in the hercynite are 
referred by Kalkowsky to ilmenite. A microscopic section of 
the Ronsperg hercynite in the possession of the writer, contains 
* The exact results of this analysis were unfortunately lost, but it is definitely 
remembered that the mineral was found to contain only alumina, ferrous iron and 
magnesia; and that the latter constituted slightly over 9 per cent of the whole. 
+ Ak. H. Stockh., 1848. 
¢ Verh. der Gesellsch. des Vaterland. Museums, 1839. 
§ Annalen der Ch. und Ph., lv, p. 357. 1845. 
|| Geog. Studien aus dem Boéhmerwald. Jahrbuch der k.k. geologischen Reichs- 
anstalt, 1856, p. 785. 
a Kritisch micromineralogische Studien, Freiburg, 1869, p. 18, II. Fortsetzung, 
1873, p. 88. 
** Zeitschrift der deutschen geologischen Gesellschaft, 1881, p. 536. 
