114 THE PENOKEE IRON-BEARINO SERIES. 



treal river are even more basic than tliis rock (PI. xv, Fig. 2), containing 

 little or no quartz They have a well developed, coarse foliation, and were 

 regarded in the field as coarse gneisses rather than syenite-schists. The speci- 

 mens vary from mottled red and black to black, dei)ending upon the quantity 

 of the iron-bearing silicates present and the color of the feldspar. When 

 examined in thin section, they are found to be very much coarser grained 

 than woidd liiivc been suspected from the hand specimens. They contain 

 as a l)ackgroiuid large — upon an average, 2 mm. in diameter — closely fit- 

 ting and interlocked individuals of alkaline feldspar comprising the three 

 species, orthoclase, microcline, and plagioclase. 8o far as seen, the schist- 

 ose structure of the rock does not in any measure affect these feldspars. 

 In the t'-euters of some of the larger orthoclases, in irregular areas, the mi- 

 crocline twinning is developed. This peculiar appearance indicates either 

 that one of them may alter into the other or that in their growth one has 

 enveloped the other. In most of the syenites the abundant mineral, aside 

 from the (pi;irtz, is hornblende accompanied with ii considerable quantity 

 of biotite and chlorite. The hornblende is in well defined blades and 

 crystals and tlie Idotite in its usual broad leaflets. Sometimes transverse 

 sections of liorul)leude show almost perfect ci'ystals which exhibit the 

 prismatic faces or these condjined with the orthopinacoids. If these min- 

 erals are original crystallizations, they must have formed liefore the feld- 

 spars. If they are secondary, they must liave developed within the feld- 

 spars. The strongly marked schistose structure characteristic of the rock 

 js wholly due to the arrangement of the hornblende and biotite minerals, 

 the larger blades usually having their greater lengths in a common 

 direction. In places tliey are massed togetlier in large areas so as to 

 wholly exclude tlie feldspar. At times a single blade of biotite or liorn- 

 bl«nde cuts through several individuals of feldspai-. The hornblende of 

 these rocks is somewhat abnormal in its optical character. It has strong 

 relief and gives very brilliant interference colors. The angles c : C run as 

 high as 21". Its })leochroism is: c, bluish-green; b, yellowish-green; a, 

 pale 1 1< )ney yellow ; b ^ O H- Differing from these hornblende-syenites is a 

 mica-pyroxene-syenite, the pyroxene in this case replacing for the most 

 part the hornl)lende. This rock is like the other syenites in all other 



