342 THE GEOLOGY OF MINNESOTA. 



[Quartz. Graywacke. 



Meg. A grayish-green, fine-grained, rather -soft rock, with quartz veins. 



Mic. Dr. Wadsworth's description is as follows: 



" The section is composed largely of debris that appears to be altered melaphyr, 

 with a few argillyte fragments, quartz, a little augife, and much secondary pi/rite. 

 The groundmass of the section has sprinkled through it numerous gray and yellowish 

 granular masses resembling titanite in the process of formation, but none are in 

 sufficiently advanced a stage to be determined crystallographically. The melaphyr 

 material is altered as in the preceding porodytes.. Both sections of this rock show 

 portions of a vein made up of irregular quartz grains containing liquid inclusions 

 with moving bubbles. Portions of the rock material are arranged in wavy parallel 

 bands, along the vein in one section, but these bands have no relation to the quartz 

 grains themselves but pass through them indiscriminately, without regard to the 

 boundaries of the grains." 



Two sections. 



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



No. 382. QUARTZ. (Vein.) 



Same place as No. 381. 



Ref. Annual Report, ix, page 96. 



Meg. Vein quartz, with feldspar, pyrite, chalcopyrite and rock material. 

 Mic. The section shows much clouded quartz in grains closely interlocking. 

 The cloudiness is due to minute grayish inclusions and to cavities filled with liquid. 

 One section. 

 Age. Vein in Archean (Keewatin) rocks. u. s. G. 



No. 383. GRAYWACKE. (Debris of qunii^-/i(>i-/>/i //>;/. ) 



Vermilion lake; N. W. % S. W. J4 sec. 13, T. 62-15 W. North bluff at the entrance to Armstrong bay. 

 Ref. Annual Report, ix, page 96. 



Meg. The rock is very siliceous, and contains rounded, free quartzes. It is of 

 a light color, and apparently destitute of structure such . as schistosity. Yet, in the 

 field, where a close jointage crosses the rock dividing it into sheets diagonal to the 

 usual structure, when a specimen is taken from between two planes, so as to have 

 one of these joints on each side, the specimen will persistently remain triangular, 

 although it be broken till reduced to too small a size for preservation. 



Mic. The quartzes are evident, but have not regularity of shape. They are 

 evidently fragments of the quartzes of a quartz-porphyry. They are embraced in a 

 matrix which has a confused appearance, but throughout which are the forms of 

 large feldspars, much changed by saussuritic growths. Some of these feldspars are so 

 filled with minute grains that have resulted from alteration that they are almost lost as 

 feldspars and seem to be parts of the matrix, and can only be detected as actual 



