PETROGRAPHY 



333 



gives a fair illustration. Occasionally a little garnet is seen, but it 

 is a very light colored or quite colorless one, in contrast with the 

 deeper pink variety of the anorthosite derivatives. It shows also no 

 tendency to characterize the contacts. between the dark silicate and 

 the feldspar, as is the case with the anorthosites. It contains quartz 

 inclusions and is thus doubtless a result of metamorphism and of 

 late formation in the history of the rock. A few minute zircons are 

 also seen, and a few grains of magnetite. The gneisses do not show 

 wide spread evidence of dynamic disturbances. AVhile one or two 

 exposures, exhibit the operations of crushing forces, the others from 

 the main ridges are quite without such effects. These latter are 

 far more widely shown by the anorthosites in which they seldom 

 fail. Prof. Smyth after examination of the writer's slides of gneisses 

 states that dynamic effects are much more widely shown by the 

 gneisses on the western side of the mountain. 



Along the lake front just above the Cheever dock, and near the 

 great gabbro mass, are several exposures of brecciated gneiss. This 

 seems to have been a micaceous variety originally, but the dark 

 silicate is now altered beyond recognition. Even in the outcrop 

 the rock is visibly crushed into small angular fragments, now 

 recemented, and in the slides this comes out in a still more marked 

 way. Strained fragments of quartz, microperthite and chlorite fill 

 the field. The crush was apparently caused by the intrusion of the 

 neighboring gabbro. A less common variety of gneiss contains, 

 instead of biotite, dirty green hornblende, but all the other features 

 are essentially like the micaceous ones. 



In the walls of the non-bessemer ore bodies the gneisses appear to 

 become somewhat richer in dark silicates as the ore is approached. 

 Near the ore the slides show chiefly plagioclase, with abundant 

 green hornblende, green augite, scattered magnetite and titanite. 



The lean ore is chiefly mixed with pyroxene, black in the hand 

 specimens, but of a beautiful, emerald green in the sections. 



Much black hornblende is also met. Diamond drill cores near 

 Mt Bob, Mineville, have shown as the wall rock of the Bessemer 

 lores a very feldspathic gneiss, with scarcely a trace of a dark silicate. 

 The lean ore is mixed with quartz and feldspar. There is some 

 reason for thinking that this contrast of wall rock may often show 

 itself. Gneisses are also met, practically like those cited as most 



