PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 337 



Schist. Graywacke.] 



No. 370. HORNBLENDE SCHIST. (Siliceous.) 



North side of Burntside lake. 



Sef. Annual Report, ix, pages 94, 95. 



Meg. Gray, firm, apparently siliceous, like a quartzyte, with an angular, sharp 

 fracture. 



Nic. The rock consists very largely of quartz, but holds also plagiodase and 

 linn/blende. There are three feldspars. One is closely striated and rather fresh. 

 One is clear and glassy, and the third is non-striated, but not glassy. The first has 

 an extinction angle that sometimes reaches 18. The second appears to be the same 

 that has sometimes been identified in the sheared Archean as andesine-oligoclase, a 

 secondary feldspar generated by the dynamic deformation to which the rock has 

 been subjected, and the third appears to be of orthoclase. The quartz is broken and 

 largely of secondary generation also, but probably pre-existed in the rock in large 

 percentage in a free state. The hornblende constitutes a loose frame-work surrounding, 

 in a lenticular manner, the quartz and feldspar, and producing by its prevailing 

 direction a structure produced probably by pressure accompanied by some shearing. 

 Muscovite is seen in the older feldspar, and some apatite is embraced in the secondary 

 feldspar. There is also a little epidote. 



One section. 



Age. Archean (Coutchiching). 



Remark. This rock was probably at first a gray wacke, but has been recrystallized 

 by the heat and pressure that accompanied the granitic intrusions. N. H. w. 



No. 371. GRAYWACKE. ( ' Metamorphic. ) 



At the mouth of the river entering Burntside lake at the portage to Mud lake; N. E. ^ sec. 36, T. 63-14 W. 

 JRef. Annual Report, ix, page 95. 



Meg. Similar to No. 370, but somewhat more gneissic. 



Mir. In place of hornblende this rock has an isotropic chlorite, varying to a 

 mint, and more cjiidoft-. Otherwise the thin section does not differ from No. 370 in 

 any noteworthy manner. 



One section. 



AIJC. Archean (Lower Keewatin). 



Ih-HKirk. The foregoing (Nos. 359-370) are all conformable when they show any 

 stratification at all, which is always the case except where there is a full transition 

 from No. 363 or No. 364, or even from No. 360 to No. 368. In that case, when No. 368 

 is fairly set in, the parallel structure, always dipping at a high angle to the south (or 

 a little east) becomes more and more indistinct, or is lost, and in its place a jointage 

 running in different directions, hardly ever parallel with the schists, is substituted. 

 Yet even then, in some weathered situations, a natural parting of the rock brings 



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