ROGERS, GEOLOGY OF THE CORTLANDT SERIES 69 



siderably altered to a hydrated mica (damourite or margarite), so that 

 their double refraction is destroyed. They give the usual tests. The 

 magnetite is in somewhat smaller grains scattered among the spinels, and 

 it is usually not nearly so abundant. It seems to have crystallized simul- 

 taneously with the corundum (fig. 4). 



Williams mentions an analysis of the spinel "whose results were unfor- 

 tunately lost," but from the fact that the analyst remembered that the 

 MgO ran only about 9 per cent., he concluded that the mineral closely 

 approached the ferrous aluminate, hercynite. In the analyses (Nos. 14 

 and 16) of the emery, made by the writer and given above, the spinel was 

 chemically separated from the magnetite and ilmenite, and the analysis 

 of the mineral as recast gave the following results : 



(14) (16) 



A1 2 3 64.86 65.19 



FeO 21.78 20.78 



MgO 13.36 14.03 



From the analyses of pleonaste given in Dana's Mineralogy, there 

 appears to be no reason for doubting that the spinel in question is of this 

 variety, and not a hercynite. 



Pure Emery 



Pure emery, which has a peculiar reddish black tint, is rare. It has 

 the fracture and other characteristics of the other variety, but no corun- 

 dum can be distinguished. In thin section, however, it appears that 

 corundum is the chief constituent. It occurs in small square grains which 

 contain reddish brown inclusions in great abundance. These may com- 

 pletely fill the center of the crystal, or they may form a ring, as is often 

 observed in leucite. They are probably ilmenite. The corundum is 

 badly altered in this rock, as shown by the high percentage (2.65) of 

 water in the subjoined analysis. The magnetite constitutes about one 

 third of the rock. Spinel rarely occurs ; biotite has been noticed in small 

 amount. 



Feldspathic Emery 



Feldspathic emery resembles the spinel variety when well developed ; 

 and a surprising amount of feldspar may be present without being no- 

 ticeable in the hand specimen. Streaks of almost pure magnetite may 

 occur in this ore, which was the one formerly mined for iron. In thin 

 section, the plagioclase is seen to make up from one third to one half of 

 the rock. It is usually basic, though not always. The corundum is scat- 



