72 ANNALS NEW YORK ACADEMY OF SCIENCES 



crystal. Corundum, in any of the three varieties, may be present, and 

 garnet (almandite) has been observed. Quartz was found in one instance. 

 Magnetite and ilmenite are usually abundant. 



In the rock just described, the important minerals are of a peculiar 

 type, but the relation of this abnormal schist to true igneous rocks is indi- 

 cated by the fact that it often contains hornblende, biotite, hypersthene 

 and even feldspar, chiefly orthoclase. In fact, a pure sillimanite schist 

 is more rare than one containing a little biotite or hornblende, and in two 

 cases a hornblende pyroxenite was found which carried merely subordi- 

 nate amounts of the sillimanite and its associates. 



Other rocks are found associated with the ore, such as quartz gabbro, 

 etc., which have not been before considered, but their peculiarities ' are 

 indicated by the names given. The statement that shearing is often 

 apparent at the mines may be recalled in this connection ; in several cases 

 the wiiter mistook for mica schist rocks which appeared under the 

 microscope to be diorite and norite, and the schistosity was so striking 

 that he was led to record the dip and strike. Faulting has also often 

 been seen, and the ore itself is always at least cracked. 



TYPES OF OCCURRENCE 



Gradational Type 



In the area of biotite norite which lies northwest of Crugers, a number 

 of small emery pits have been sunk. The ore seems to be widely devel- 

 oped in this district, but the veins are all small and rapidly pinch out. 

 The ore is of the spinel emery type; it is sometimes in sharp veins, but 

 more often it grades into the quartz-emery schist. 



The small pit which offers the best opportunity for study lies 250 feet 

 due south of the point at which the mica schist inclusion outcrops on the 

 post-road. In this pit, an unmistakable quartz emery schist with streaks 

 of coarse red garnet lies to the north, i. e., nearest the schist inclusion; 

 it is adjoined on the south by the remnants of the ore, about two feet in 

 width. This ends abruptly as far as could be ascertained in the field; 

 but specimens of the adjoining rock taken one, two and four feet from 

 the contact were respectively spinel norite with garnet, spinel norite and 

 norite. This rock was of the ordinary variety described on page 30, with 

 the spinels merely superposed upon it. This gradation appears to be 

 the typical association of the emery in this district, although the cuttings 

 are not confined to this distance from the large schist inclusion. The 

 ore is important chiefly from a theoretical standpoint; only a very lim- 

 ited amount has been taken out. 



