708 THE GEOLOGY OF MINNESOTA. 



[Conglomerate. Chert. 



adjacent and parallel, they do not extinguish as if twinned. There are more numer- 

 ous isotropic areas similar to those me ntioned in the marble (No. 1371), and these are 

 sometimes specked with fine grains of calcite and with feldspar microliths. There is 

 a considerable amount of dust of a dark leucoxene or of magnetite and red hemalih- 

 scattered through the slide, and the differences in the manner of distribution of these 

 materials have much to do with the pebbly aspect of the slide, for they are wanting 

 in places and abundant in others, or very fine in some and coarse in others. The 

 isotropic areas are occupied mainly by a faintly-green substance, apparently chloritic, 

 and this extends also through the body of the rock as a bond of coloration. This 

 substance is also unevenly disseminated, adding to the pebbly aspect of the section. 

 These differences of aspect accompany other slight differences of composition, viz.: 







in some of the pebble-like spots the calcite is in fine round pellicles, and having high 

 colors, appears like globular pyroxene. Such areas are markedly contrasted with the 

 surrounding rock in which the calcite is in distinctly crystalline forms. Again, the 

 feldspar spicules are more abundant or are nearly absent in some angular or sub- 

 rounded areas. One section. 



Age. Archean (probably Upper Keewatin). 



Remark. The differences mentioned can be explained apparently only by 

 assuming original differences in the composition of the rock, however uniform and 

 simple the mineralogical composition at present. The isotropic areas, which in 

 bright light also show very fine, globular, indistinct spots, which faintly transmit 

 light, are therefore to be considered as bits of glassy volcanic debris now devitrified. 

 This rock illustrates well the effusive or agglomeratic composition of the great bulk 

 of the greenstones of the Keewatin in the region south and east from Ogishke Muncie 

 lake. N. H. w. 



No. 1373. CHERT (?) 



Fragment in No. 1372, near the marble. 

 Ref. Annual Report, xvi, pages 96, 123. 



Meg. Chert of a light greenish-gray color, connected with rock No. 1372. 



Mic. The most of the slide is exceedingly fine grained and its individual com- 

 ponents cannot be made out, presenting an overlapping interference and an aggre- 

 gate gray color in common light. Between crossed nicols the field is nearly dark, 

 but sprinkled with small angular spots in which some colored light gets through. 

 It is probable that the rock consists largely of chloritized hornblende and calcite, for 

 there are quite a number of idiomorphic crystals of calcite, porphyritically scattered 

 throughout the slide. One section. 



Age. Keewatin. 



Remark. Owing to its composition this rock is perhaps not properly called 

 chert. N . H. w. 



