PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 329 



Chlorite schist.] 



may be separated into distinct types on their differing optic properties. In this sense 

 the entire rock is essentially a serpentine, retaining still a sort of porphyroidal 

 microscopic structure, such as mentioned by Dr. Wadsworth. 



In the section examined there are five minerals, not including magnetite. 



1. An isotropic, pale green or yellowish mineral, which occupies areas of 

 considerable size, entirely different and distinct from the other areas. This mineral 

 is very finely fibrous, or structureless, and cannot be distinguished from that 

 frequently seen in the centres of decayed olivines in the Keweenawan eruptives. 



2. Calcite (or dolomite] is present, not only in microscopic veins, but more or 

 less disseminated in the mass of the rock. 



3. A highly bi-refractive mineral, which occasionally is in distinct fibres, but 

 for the most part is in flakes or scales, and which occurs scatteringly in the 

 porphyroidal areas of the original olivines referred to by Dr. Wadsworth. They do 

 not probably consist of antigorite, owing to their high double refraction. They have 

 positive elongation and parallel extinction, and with a thickness of .03 millimetres, 

 their highest colors are in the second order, viz.: red, yellow and green. They 

 appear, therefore, to be talc, of which there is a microscopic vein visible on one end 

 of the hand specimen. 



4. The most of the space of the supposed old olivines is occupied by a much 

 less bi-refractive mineral. Its highest colors are a faint yellow, and most of it does 

 not rise above the white of the first order. It is also in fine scales and is probably 

 antigorite. The network which occupies these areas renders it impossible to get a 

 uniform darkness. 



5. There is also a noticeable amount of a fibrous or fibro-lamellar mineral, 

 whose double refraction is still lower, viz., in the grays below the white of the first 

 order, which is referable to pennine. 



The rock is therefore a serpentine with a considerable amount of steatite. 



Two sections. 



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



No. 350. CHLORITE SCHIST (?) 



"Chloritic (?) slate f rom Pipestone rapids, just above the pipestone rock. The slate stands nearly vertical, 

 but dips to the south." S. W. % sec. 22, T. 64-11 W. 



Ref. Annual Report, ix, page 91; Annual Report, x, page 95. For description of this locality see, also, 

 Annual Report, xv, pages 104, 105. 



Meg. A very fine-grained, soft, fissile, greenish-gray schist. It has minute 

 laminae', green in color and probably composed largely of chlorite; also gray or flesh 

 colored laminae perhaps composed of feldspathic material. 



No section. 



Age. Archean (Keewatin). u. s. a. 



