194 G. H. Wiltiams—Norites of the “ Cortlandt Series.” 
parative freedom from inclusions. These are not altogether 
wanting, but consist almost wholly of small plates of biotite or 
erains of magnetite, presenting, both in their number and irreg- 
ular arrangement, a contrast to the characteristic red plates of 
the hypersthene. Both of these slides contain considerable 
biotite in addition to the augite. This mineral, as well as the 
feldspar, exhibits in a beautiful manner the effects of pressure. 
5. Pyroxenite. 
Very interesting and comparatively coarse-grained rocks 
composed wholly of hypersthene and diallage or augite may be 
regarded as a modification of the type last described, due to the 
. disappearance of the feldspar. They are quite abundant in 
the eastern part of the township near the emery mines and are 
represented by Nos. 127, 129, 181 and 132 in the Johns Hop- 
kins University collection. Such rocks also occur on the 
northern half of Montrose Point, but here they carry more or 
less brown hornblende and are intimately related to the horn- 
blendites which are the prevailing rock in‘ that locality and 
which will be described later. 
Diallage-hypersthene aggregates have recently been found by 
the writer, associated with the hypersthene-gabbros of Balti- 
more and Harford Counties, Md., which are quite analogous to 
those occurring in the Cortlandt Series.* 
The Iron ore and Emery in the Cortlandt Norite. 
The deposits of magnetic iron ore and emery mentioned in 
Professor Dana’s papert as occurring in the massive rocks of 
the Cortlandt Series, although of but little practical import- 
ance, possess considerable scientific interest. The veins, which 
appear to be segregations in the norite, have been opened north 
and northeast of Cruger’s Station only for iron ore, while quite 
similar deposits in the southeastern part of the township, are 
still worked to some extent for emery.t The ore is heavy and 
black, but does not turn out as rich as would be expected from 
its appearance. A microscopic examination of it shows that it 
is largely composed of a dark green mineral in which octahe- 
dral crystals of magnetite are imbedded. Professor Dana con- 
sidered this mineral to be chlorite, but a more careful examina- 
tion shows it to be the iron-magnesian spinel, pleonaste. The 
action of chlorite upon polarized light, as is well known, is ex- 
tremely feeble and so it was not unnatural to mistake for it 
* Bulletin of the U.S. Geol. Surv., No. 28, p. 55. 
+ This Journal, September, 1880, xx, p. 199-200. 
t The emery mill at Peekskill owes its origin to the discovery of these deposits, 
although at present it procures its raw material almost exclusively from Asia 
Minor. f 
