74 ANNALS NEW YORK ACADEMY OF SCIENCES 



tions of the various rocks are, however, much more complicated than in 

 the previous types, and three varieties may be distinguished. 



In one of the Dalton pits, a simple succession from the brown norite 

 to ore to sillimanite schist was observed. The former is typical; the 

 latter is coarser than usual, carries some garnet and sometimes shows 

 crumpling and contortion. The ore varies in width from two to ten 

 feet and is of the pure variety. 



At the Buckbee mine, the underground work is inaccessible, having 

 all caved; there are two open cuts, however, which seem to indicate the 

 structure. These are both lenticular in ground plan, curving into the 

 hill and out again. The ore is an excellent one, being often nearly half 

 corundum in large gray-brown crystals, and practically fresh and un- 

 altered. It may at times carry garnet and occasionally pyrite in half 

 inch crystals. In both of these pits, the wall rock is the brown norite, 

 which in this case, however, resembles ordinary norite, in thin section, 

 more closely. It is altered to chlorite in patches. Such of the ore as 

 has not been taken out occurs in this rock in irregular veins, which often 

 "break" into pockets. The poorest grades of ore resemble the streaky 

 quartzose emery, but this in thin section resolves itself into a quartz 

 sillimanite schist, often garnetiferous. There is a broad irregular band 

 of coarse pyroxenite running horizontally through the cliff of brown 

 norite, but it seems to be a local segregation, having nothing to do with 

 the ore. A vogesite dike appears in both of the pits. The succession 

 here may or may not be as in the gradational type; it would seem rather 

 that the sillimanite schist encloses the ore and is itself surrounded by 

 the norite. 



In the two other large pits on the Dalton property, a more complicated 

 relation exists. The wall rock is a quartz gabbro ; this becomes a much 

 chloritized pyroxenite of the usual type, which passes into brown norite. 

 The latter is at first chiefly hypersthene, but it becomes feldspathic near 

 the ore, which is of the pure variety. Sillimanite schist is plentiful; in 

 one case, it occurs in irregular fine-grained masses on the other side of 

 the ore, while in the second case, it seems to occur mingled with the ore. 

 Essentially the same succession is to be observed in the latest cut of the 

 McCoy mine. This is about 30 feet wide by 45 feet deep, and 35 feet 

 high and gives the complete cross section. At a short distance away, 

 the whole ledge resembles a sandstone with vertical dip, owing to the 

 regular zones in which the types occur (Plate V, fig. 1). Starting from 

 the west, the succession is biotite norite (typical); sillimanite schist, 

 "ore" (sillimanite schist with spinel, very fissile, or "slate/' as the 

 miners call it), then the typical sillimanite schist again; this succession 



