PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 825 



Argillyte and graywacke.] 



No. 2023. GRAYWACKE. 



Same place as No. 2016. 



Ref. Annual Report, xxiv, page 14. 



Meg. Overlies No. 2022; one of the series of clastic material. 



Mic. This is also slightly greenish, much altered, sheared and calcified. It 

 contains a few old feldspar forms, but in general the whole rock is darkened between 

 the nicols by chloride alteration. One section. 



Age. Upper Keewatin. N. H. w. 



No. 2024. GRAYWACKE. 



Same place as No. 2016. 



Ref. Annual Report, xxiv, page 14. 



Meg. Overlies No. 2023; twenty or more feet thick. 



Mic. This rock is like No. 2020, a rather even-grained graywacke, but there 

 Avere evidently pebbles of feldspar that have disappeared as feldspar by the formation 

 of the same granulitic fine secondary substance (micro-granulitic) seen in numerous 

 cases in the Ogishke conglomerate. One section. 



Aye. Upper Keewatin. N. H. w. 



No. 2025. ARGILLYTE AND GRAYWACKE. 



Across the little bay in sec. 20, T. 62-15 (i. e., on the east side of the bay), Vermilion lake. Sinuous alter- 

 nations of black slate with graywacke, a little southwest from the cliff near the extremity of the point. 

 Ref. Annual Report, xxiv, page 15. 



Meg. Black slate, rigid, and graywacke in contact. 



Mic. The graywacke consists, as usual, of a rather uniform grain of feldspar 

 and quartz, the former much altered, lying in a finer debris of the same, which debris 

 is now changed, with much of the larger feldspar grains, to an interlocking fine 

 plexus. There are a few of the larger grains of this feldspathic material embraced 

 within the slaty portion of the slide near the line of separation. The slaty portion 

 is quite different when viewed in natural light. While it is apparent that there is a 

 small amount of the same feldspathic material running through this dark mass, even 

 including a few scales of sericite, the slate consists essentially of a single other 

 substance, which it is difficult to determine. It has a high refractive index, and 

 being very fine and granular, it gives a darkness to the microscopic field which, with 

 low power, is almost that of an isotropic substance. It may be a mixture of epidote, 

 chlorite and hornblende with leucoxene. One section. 



Age. Upper Keewatin. 



Remark. This "black slate" was supposed, when collected, to be allied to the 

 jaspilyte, and to be a large fragment from the Lower Keewatin involved with the 

 graywackes of the Upper Keewatin, but its composition is rather like the graywackes, 

 and it is hence a portion of the Upper Keewatin, considerably broken and folded. 

 It has not a single element of the jaspilyte. N. H. w. 



