344 THE GEOLOGY OP MINNESOTA. 



[Jaspilyte. 



very slightly pleochroic mineral, which gives as an interference color greenish gray 

 of the fourth (or higher) order as indicated by the color bands on the thin side of 

 the section. The extinction is wavy. It gives an interference figure which is 

 apparently uniaxial, although the optic axis is a little out of the field of the microscope. 

 This interference figure is positive. The species of this mineral is not known. 



One section. 



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



Remark. Later investigation has shown that a debris from a quartz-porphyry 

 will, when compacted and sheared, take the characters of this rock. A quartz grain 

 showing embayments of the matrix may have carried that character with it from the 

 original magma into the debris of the clastic, or, if the embayments have open wide 

 mouths, as in this case, the fine debris of a clastic derived from quartz-porphyry, 

 entering such re-entrant angles, could hardly be distinguished from the original 

 matrix. N. H. w. 



No. 385. JASPILYTE.* (Dark, slaty. ) 



N. E. y N. E. J4 sec. 22, T. 62-15 W.; Southeasterly shore of Vermilion lake. (Compare Nos. 1958 and 1959, 

 Annual Report, xxii, page 16.) 



Ref. Annual Report, ix, page 97. 



Dark siliceous slate, in thin, rigid laminatious. Mainly in regular, thin 

 sheets, but in some places confused, the slates running west to southwest, and nearly 

 perpendicular, but sloping toward the south. This jaspilyte streak rises on to the hill, 

 and can be traced for a quarter of a mile. 



Mic. The section is apparently composed of silica only, in fine grains, yet with 

 a few scattering rhombs of siderite. It is evidently from one of the white laminae of 

 the rock. 



Another section, made so as to traverse the structure, shows alternations of layers 

 of quartz in fine grains and of magnetite. Siderite rhombs are frequent, as shown by 

 their rusty coloration, but calcite is also distributed with the siderite. The latter is 

 apparently converted, in some instances, to hematite which is sparsely distributed 

 amongst the magnetite. Piercing the quartzes, and interlocked amongst them, 

 are numerous needle-shaped, highly polarizing fibres whose elongation is positive 

 (i. e. they have the axis n f practically parallel to their greater dimension), but on 

 careful measurement their extinction is found to depart from parallelism about 15. 

 Their pleochroism is hardly apparent. They are supposed therefore to be some form 

 of non-aluminous amphibole, probably actinolite or f/runerite. Two sections. 



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



"This term was proposed by M. E. Wadsworth in 1880 in a discossion of the iron ores of the Marquette region. Bulletin 

 3/ujtpum Cuiiipnrntivi' Xnoliiiij/, Cambridge, Geological series, vol. i, p. 75. It is current in the Minnesota reports and has a definite 

 significance and application in the discussion of the Archean iron ores of the Vermilion range. Wadsworth's idea that this rock 

 had an eruptive origin, an idea which he included in his definition of the term, is the only objection we can see to the use of the 

 designation as a petrographical term. The term jasper is also applicable, but has not the definiteness which jaspilyte implies. 



