76 ANNALS NEW YORK ACADEMY OF SCIENCES 



section to be a biotite norite carrying abundant spinel and some corun- 

 dum. Adjoining it is a poor ore, which is a sillimanite schist with abun- 

 dant cordierite. Then comes a brown norite, which is followed by a 

 good spinel emery ; and hornblende pyroxenite separates this from the 

 diorite. This is, however, an ideal cross-section, found only in one 

 place; in other places, the succession is less regular and often repeats 

 itself. In a 40-foot cut to the northeast, the structure is exactly the 

 same, except that the spinel norite is replaced by spinel diorite, as in the 

 large pit, and that the brown norite is lacking. 



In these three pits of the McCoy mine, then, the ore occurs in a 

 schistose, spinel-bearing micaceous rock, either diorite or norite, and is 

 associated with sillimanite schist and hornblende pyroxenite. 



The Chase mine, which is the last to be described, appears to be similar 

 in structure to these; but garnets in great abundance are found there. 

 The ore occurs in a ledge, flanked on one side by a corundum-bearing 

 sillimanite schist, which at times becomes very coarse; and on the other 

 by a very biotitic rock, similar to that in the large pit of the McCoy mine, 

 except that it carries corundum in irregular masses instead of spinel. 

 In irregular association with the ore occurs a feldspathic rock contain- 

 ing abundant garnets. From a rotten zone in this rock, well-formed 

 trapezohedrons one and a half inches in diameter can be readily ex- 

 tracted. This rock in thin section is seen to be chiefly a basic altered 

 feldspar, with a little biotite, spinel and corundum and the large garnets. 

 It may occur in the ore in irregular masses, but is chiefly separate. In 

 the ore itself, garnet occurs abundantly, both as crystals and as rounded 

 flow-like masses, often four or five inches across. The ore is somewhat 

 peculiar aside from this, being more than half corundum in fairly fresh 

 white crystals ; for the rest, it is made up of spinel, magnetite, allanite 

 and garnet. This is a high grade ore, but it is injured by the abundance 

 of brittle garnet and allanite. Around the cutting for a few hundred 

 feet, a very garnetiferous rock extends, whose dark mineral is probably 

 an altered hornblende, constituting a diorite. The true country rock is, 

 however, a p3Toxenite, which varies an indicated on the map. 



The foregoing description of the emery deposits may be epitomized as 

 follows : 



1. The ore usually occurs in a region in which mica schist inclusions 

 are abundant and often within a hundred feet or so of such an inclusion ; 

 and the largest mines (McCojr, Dalton, etc.) are within 1000 feet of the 

 border of the Cortlandt Series. 



2. The ore is always in sharply defined veins, pockets or lenses, but its 

 constituents often occur disseminated through the rocks immediately 

 adjacent. 



