PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 347 



Arkose.] 



Remark. This locality, and the rocks associated with this rock, are thus 

 described in the Ninth Annual Report: 



" A mile further west, and near the entrance to the bay that leads to the portage 

 going south from Vermilion lake to Squagemaw lakes, the rock of this number, which 

 is a gray, chloritic schist, is seen to have a nearly east and west slaty structure, 

 varying to a little south of west. This slaty structure is intersected diagonally by 

 alternations in the rock due to sedimentation, running nearly northwest and south- 

 east. The kinds of rock exhibited by this alternation are as follows, from Nos. 389 

 to 394, both inclusive, the former being on the northeast side and the latter on the 

 southwest side." Other rocks from this immediate vicinity are Nos. 1 to 7 (W.), 

 Annual Report, xv, pages 19, 20. 



Another section bears the No. 389, but as it differs considerably from the foregoing 

 it is supposed to have be'en misnumbered. It is probably from the Lower Keewatin 

 of the region. The following description is given, since the section shows well some 

 important characters: 



Mic. The section consists essentially of feldspar (plagioclase) and muscovite, 

 with siderife. The feldspar is of the clear variety which in small fragments is difficult 

 to distinguish from quartz. It is not crowded with kaolinic scales, but has dust-like 

 particles which cross it in lines, but hardly impair its translucency. Some of the 

 larger grains interlock in such a manner as to indicate secondary growth, and in one 

 case a large grain has partially enveloped another by forming about its projecting 

 corner. It is distinctly twinned on the albite plan. A section cut perpendicular 

 to gives an extinction angle of 73, which agrees well with Fouque's extinction for 

 andesine-oligoclase. Although this translucent feldspar prevails, or exists almost 

 akone, in one section, there is still another which is quite different in its inclusion of 

 what appear to be mica scales set in every conceivable angle with reference to the 

 structure of the grain, and of the scales themselves. These are also of some plagio- 

 clase, but as the single grain in this slide has no cleavage nor crystalline character 

 on which to measure extinction the species cannot be determined. The siderite is 

 distributed irregularly, both as single rhombs and as shapeless aggregates of many 

 grains. It has a uniaxial interference figure, slight absorption and rather strong 

 refraction index approaching that of epidote. Its double refraction is similar to 

 that of calcite. It has a clear glassy color, when unaltered and in simple crystals, 

 but much of it in compressed masses appears of an ashen-gray color. When it is 

 altered it becomes brown or reddish with the resulting oxide of iron. Its cleavages 

 are conspicuous. Except for the rustiness that sometimes permeates it, and its ashen 

 color, it can hardly be distinguished from calcite in ordinary light, but its high 

 refractive index, in convergent light, is also a good diagnostic. 



Age. Archean (Keewatin). N. H. w. 



