PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 601 



Dioryte. Chlorite schist. Quartz-porphyry.] 







No. 918. DIORYTE (with quartz). 



From a boulder near the railroad, Breitung mine, Soudan. 

 Ref. Annual Report, xv, pages 275, 336, 389. 



A coarse-grained granitic rock, composed largely of hornblende, but with 

 some white to red feldspar and quartz. Some of the hornblendes have compact 

 borders and cores which are granular. 



Mic. The section shows much green hornblende and smaller amounts of feldspar 

 and quartz. The feldspar is generally much clouded and kaolinized, but in places 

 still retains traces of albite twinning. Epidote, spliene, magnetite and hematite are 

 also present. 



One section. 



Age. Boulder, probably from the Archean. u. s. G. 



No. 919. CHLORITE SCHIST. fJaspUitic. ) 



Railroad cut south of the Stone mine, Soudan. 



Ref. Annual Report, xv, pages 224, 389; Bulletin vi, pages 44, 52, 231, 421. 



Meg. A fine-grained, grayish-green, schistose rock. It contains considerable 

 white, fine quartz. 



Mic. The rock is composed essentially of small grains of quartz, similar to the 

 quartz of the jaspilytes, and flakes of chlorite. These are sometimes intimately 

 mingled together, but quite commonly each is massed in areas largely free from the 

 other. A few magnetite crystals are present. 



Three sections. 



Age. Keewatin. 



Remarks. This is one of the rocks intermediate in composition between 

 jaspilyte and green schist. Compare Nos. 885, 894, 895, 897. u. s. G. 



No. 920. QUARTZ-PORPHYRY (?) 



S. E. % sec. 6, T. 62-15 W.; east side of bay on south shore of Pine island, Vermilion lake. 

 Ref. Annual Report, xv, pages 303, 389. 



Meg. A gray schistose rock of fine grain and holding many small hornblendes. 



Mic. The groundmass of the section is of fine grain and is composed of quartz, 

 feldspar, chlorite, muscovite, pale green hornblende and epidote. In this groundmass 

 are larger grains of feldspar and quartz. These are without crystal outlines, interlock 

 irregularly with the smaller grains of the groundmass, and in many cases have been 

 fractured and their parts more or less separated. There are also some larger grains 

 of brownish-green hornblende, which often have attached to them growths of pale 

 green hornblende. 



One section. 



Age. Keewatin. 



