PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 591 



Greenstone. Serieitic schist.] 



No. 889. ' ' GREENSTONE '' ( with pebbtes of jaspilyte) . 



Point in the S. E. ^ sec. 20, T. 61-15 W., Vermilion lake. 



Eef. Annual Report, xv, pages 231, 388; Bulletin vi, pages 233, 421. 



Meg. A fine-grained, roughly-schistose, green rock, having many small, irreg- 

 ular areas of jaspilyte-like white quartz. On some of the cleavage surfaces is a 

 brownish, micaceous mineral. These areas of white color give the rock a peculiar 

 appearance, suggesting a sheared and decayed greenstone amygdaloid, or perhaps a 

 greenstone conglomerate with white pebbles. 



Mic. The section shows a very fine-grained rock composed mainly of chlorite 

 and minute grains of quartz, and perhaps also of feldspar. A few tmiscovite scales are 

 also present. The white areas so noticeable in the hand sample are composed of 

 finely crystallized quartz similar to the quartz of the jaspilytes. In one of these 

 white areas was a large grain of quartz surrounded by finer grains of the same mineral, 

 its margin interlocking with the fine grains. These areas of quartz are a little 

 coarser grained than the main mass of the rock. 



One section. 



Age. Upper Keewatin. 



Remark. Compare No. 897. u. s. G. 



No. 890. SERICITIC SCHIST. (Brown.) 



Stone mine, Soudan. 



Ref. Annual Report, xv, pages 235, 253, 388. 



Meg. A fine-grained, roughly-schistose, brown rock. The specimen has an 

 indistinct mottled appearance and a rough texture. 



Mic. One noticeable feature of the section, when examined with a low power 

 or with the naked eye, is the presence of small, often not very distinct, areas which 

 are darker than the main mass of the rock. Under the microscope these darker areas 

 are not so distinct, and they are seen to be of essentially the same composition as 

 the rest of the rock, except for the abundance of grains of iron ore, which is com- 

 monly accompanied by chlorite. The main mass of the rock is very fine grained and 

 is composed of quartz, or quartz &nd feldspar, Muscovite and iron ore. These are all 

 common, and chlorite is less common. The iron ore seems to be both Umonite and 

 lii'inatite. The rock may be a very fine breccia. One section. 



Age. Keewatin. u. s. G. 



Remark. This rock is plainly a finely fragmental one, and the quartz is finely 

 divided like that of the jaspilyte. The irregular dissemination of the iron ores sug- 

 gests that originally the rock was of the nature of a volcanic tuff. Indeed, there is 

 is an indistinct patchy distribution of the other minerals concordant with that 

 hypothesis. N. H. w. 



