ROGERS, GEOLOGY OF THE CORTLANDT SERIES 71 



green), giving the mineral a curious washed-out appearance. The analysis 

 shows it to contain 11.5 per cent, of FeO, so that it is bronzite. It may 

 become very abundant, so that the rock is almost a bronzitite, which was 

 never noticed in the typical norite proper; and magnetite, moreover, is 

 almost lacking. The writer has three slides containing the contact of this 

 rock with spinel emery. In every case, the feldspar is continuous over the 

 line; in two the spinel and magnetite are sharply segregated from the 

 norite, while in the other the minerals mingle for the space of about 1 

 mm. The feldspar of the emery is somewhat serictized, and its amount 

 is surprising, considering the black heavy aspect of the ore. In one of 

 the slides, a line of spinels diverged from the main mass and ran out over 

 the norite, crossing the bronzites indiscriminately. 



Sillimanite Schist 



Sillimanite schist is usually light gray in the specimen and of a fine,, 

 somewhat fibrous texture. The blades of sillimanite, however, can gen- 

 erally be distinguished, and they 



sometimes reach one half an inch in Cord,.,-*. ^^ lfe*^ Sl, ! ,mBni * r 



length, or rarely an inch or more. 

 This rock is extremely tough and so 

 hard that it will turn a drill at times. 

 The "ore" from the Dalton property 

 and from the latest cutting of the 

 McCoy mine is largely this rock, al- 

 though it is a poor abrasive, powder- 

 ing when ground. 60 Under the mi- 

 croscope in typical cases, it is seen 

 to be made up chiefly of sillimanite. 

 This is largely fibrolite, with the 

 blades scattered more or less abun- Fig. 5. Sillimanite Schist. SI. 304 

 dantly through it. These latter are 



blotched with a fine brown dust in places, and they also carry abundant 

 magnetite inclusions. Cordierite is almost always present in varying 

 amount ; its biaxial figure and the strongly pleochroic yellow halos which 

 surround its inclusions serve to distinguish it from quartz. Allanite, in 

 large brownish prismatic crystals, is often present. It shows parallel 

 extinction and good pleochroism from yellowish brown to greenish brown, 

 but the color, pleochroism and birefringence often vary even in one 



/ 



80 Mr. John H. Buckbee rather aptly characterized this rock to the writer as being 

 from a practical standpoint, "like gristle, — neither bone nor meat." 



