PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 585 



AmpMbolite. Conglomerate.] 



Meg. A fine-grained, gray, roughly schistose rock. The matrix, which is too 

 fine grained for the macroscopic determination of its constituents, contains small, 

 scattered, porphyritic hornblendes and more numerous, glistening, silvery, porphy- 

 ritic micas. The latter are especially noticeable on cleavage surfaces of the rock, 

 the cleavage of the mica being approximately parallel with the cleavage of the rock. 

 One of the hand specimens contains a fragment of dark-gray, flinty argillyte. 



Mic. A noticeable feature of the section is the presence of porphyritic, light- 

 brown hornblendes. These lie in a confused groundmass of fine grain and of a very 

 schistose character. The groundmass is composed essentially of quartz, probably 

 also feldspar, zoixifc, ml rile, and small fibres of a nearly colorless amphibole. There 

 is also considerable Ihnonite which seems to have originated from the alteration of 

 pyrite. The porphyritic mica crystals are not prominent in the thin sections, and 

 where they do appear they are seen to be much clouded and altered and in general 

 have little or no effect on polarized light. Two sections. 



Age. Dike cutting Keewatin rocks. u. s. G. 



No. 873A. AMPHIBOLYTE (?) 



Rounded mass of green rock in No. 873. 

 Ref. Annual Report, xv, pages 309, 387. 



Meg. A gray rock of medium grain, composed largely of hornblende and effer- 

 vescing freely with hydrochloric acid. 



Mic. The section shows numerous, crowded, brownish hornblendes in a sparse 

 cement. The hornblendes frequently show crystal outlines. The cement is fine 

 grained and is composed essentially of calcite and quartz, with some hornblende. 



One section. 



Age. Mass in dike which cuts Keewatin rocks. u. s. G. 



No. 874. CONGLOMERATE. (Matrix.) 



Stuntz island, Vermilion lake; sec. 21, T. 62-15 W. 

 Ref. Annual Report, xv, pages 314, 387. 



Meg. A light-gray, fine-grained, roughly schistose rock, containing numerous 

 quartzes; the rock in general resembles a schistose quartz-porphyry with numerous 

 quartz phenocrysts. 



Mic. The sections show a very fine-grained matrix which is composed essen- 

 tially of quartz, feldspar and some flakes of muscovite. A little calcite is also present. 

 In this matrix and interlocking with it are numerous fragments of various shapes 

 and sizes of quartz and feldspar. The feldspar is somewhat altered and clouded; it 

 appears to be both orthoclase and acid plagioclase. A considerable number of these 

 quartz and feldspar fragments seem to have been broken since they were included in 

 the rock. Four sections. 



