PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 639 



Cordierite gneiss. Amphibolyte.} 



100, and perpendicular to the optic plane. This mineral might be mistaken for 

 hypersthene, owing to its good cleavage and parallel extinction; but it forms a stiff 

 jelly quickly with hydrochloric acid. One section. 



Age. Modified Keewatin, an appendage of the gabbro. 



Remark. Magnetite is plentiful in some parts of this rock. It is quite probable 

 that, on extended examination, very much of the olivine which accompanies the 

 "olivinitic iron ore," would be found to be fayalite. N. H. w. 



No. 1042. CORDIERITE GNEISS (with hyperst/iene ). 



From the lake through which the river flows; sec. 11, T. 64-7. 

 Ref. Annual Report, xv, pages 361, 395. Compare No. 1039. 



Meg. Gneissic, biotitic, quartzose. 



Mic. Through a background of granular cordierite and quartz, the latter being 

 secondary to the former, and both together occupying visibly more than half the 

 total area, are biotite in large flakes which inclose cordierite and magnetite grains in a 

 poikilitic manner, hi/pet-stlit'iic which in the same manner surrounds cordierite and 

 magnetite, and occasionally a grain of twinned plagiwlaxe which is fresh and glassy. 

 This feldspar, like all the other minerals, is of secondary origin. As to the relative 

 dates of generation of the hypersthene and the biotite, they generally do not inter- 

 fere, but on close examination it is easy to see that a few detached hypersthene 

 grains of small size are surrounded by the biotite, which, therefore, excepting the 

 quartz, was the latest of the secondary generations. One section. 



Age. Modified condition of the Keewatiu, due to the gabbro revolution. 



No. 1043. AMPHIBOLYTE. 



From the ridge which separates the above lake in the river from Kekequabic lake. 

 Rcf. Annual Report, xv, pages 362, 395. 



Meg. Dark greenish, finely granular. 



Mi>-. The rock consists almost wholly of a faintly pleochloric green or colorless 

 hornblende, or pargasite, whose maximum extinction angle is 18 to 21, and whose 

 acute bisectrix is n s , with positive elongation. Sections perpendicular to n f show 

 little longitudinal cleavage, but a distant transverse cleavage. Hence, the mineral, 

 while having a faint prismatic cleavage, has also an imperfect basal cleavage (001). 

 Sections that show the highest coloration (010), also have very evident coarse longi- 

 tudinal cleavage, which therefore must be parallel to 100. The pleochroism is so 

 faint that in sections of normal thinness (.003 millimeter or less), it is hardly 

 apparent. 



There is also a notable amount of biatHc and of con/ierite, an occasional grain of 

 magnetite, and also a little feldspar. Two sections. 



Age. Probably Upper Keewatiu. N. H. w. 



