GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 9I 



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

 its constituents often occur disseminated through the rocks immedi- 

 ately adjacent. 



3 The ore is immediately associated with abnormal rocks, con- 

 taining sillimanite, cordierite, garnet, quartz, or allanite, which are 

 found nowhere else in the area, except around certain schist inclu- 

 sions near Crugers ; or more rarely it adjoins rocks which are normal 

 except for the spinel scattered through them. There is often a great 

 abundance of biotite around the ore, which is also characteristic of 

 these inclusions. 



4 These rocks often exhibit evidences of shearing, faulting or 

 cracking, which is rare in other parts of the district, except around 

 schist inclusions." 



The theory advanced to account for the occurrence is that the 

 emery is formed by the absorption of the Manhattan schist in the 

 basic magma of the Cortlandt series. The streakedness of the ore 

 resembling the structure of a schist, and its occurrence near the con- 

 tacts of the igneous rock and around schist inclusions, the presence 

 of sillimanite, garnet and biotite, and the gradation from norite. 

 through emery into schist, are the chief arguments for the theory 

 advanced. 



Considerable amounts of material of this type are available from 

 the district. It is not a high-grade product, however, and con- 

 sidering the fact that the market is chiefly supplied by other types 

 of abrasives and that a higher grade emery comes to the market 

 from foreign sources there is no immediate prospect of more exten- 

 sive development, although it is likely that small production will 

 continue for an indefinite time. This is an interesting economic 

 resource, although it is not one of large consequence. Its interest 

 centers largely in the rarity of its mineralogy and in its origin. 



Ball mill pebbles. An interesting special use of the very hard 

 and tough material forming the emery deposits has been tried out 

 by Edres Herbert. He conceived the notion of using this material 

 in ball mills for grinding purposes as substitute for flint pebbles. 

 The foreign supply of flint was cut off by the war and substitutes 

 were in demand. Preliminary tests have shown unusually encourag- 

 ing results and there seems to be promise of a limited use of this 

 kind for this material. Its weight and toughness are two points in 

 its favor. 



Graphite. The mineral graphite is not at all unusual in the 

 Grenville rocks. It is found in considerable abundance in certain of 



