312 THE GEOLOGY OF MINNESOTA. 



[Limestone. 



Chlorite gives color to the rock, which is a greenish gray. In a section parallel 

 with the laminated structure, the chlorite scales overlap each other and give a 

 general isotropic appearance to the matrix between crossed nicols, relieved by the 

 sprinkling of angular grains of quartz and a few other minerals. 



Age. Archean (Keewatin). 



Remark. This rock has had a long history and no name can properly be assigned 

 to it in its present form. It is equally impossible to state what its original nature 

 was. In many respects it resembles some of the " porphyritic " fragmentals near 

 the base of the Upper Keewatin, and its intimate structural association with, and 

 petrographic alliance to, the graywacke (No. 310) in the midst of which it occurs, 

 point to its original fragmental nature. It would then be a porphyrel. N. H. w. 



No. 312. LIMESTONE. 



Matrix of a flint conglomerate or breccia. Occurs on both sides of Gunflint lake, most conspicuously on 

 the north side, west of No. 311, on the points of the peninsula. 



Kef. Annual Report, ix, page 82; Annual Report, x, page 87; Annual Report, xvi, page 69; Annual Report, 

 six, pages 126, 127; Bulletin vi, pages 121, 129, 130, 420. 



Meg. This rock is gray, finely crystalline, and covered with a coating of iron 

 rust, from which fact it has been supposed to consist essentially of siderite. The 

 angular pieces embraced by it are of gray flint. It has a fine color-banding due to 

 sedimentation, and occasionally fragments of rock other than flint are scattered in it, 

 some of which is nearly black, resembling the dark rock of No. 307, and some is quartz, 

 the latter being quite rare. Some of the dark rock is apparently a calcite amygdaloid. 



Mic. The shimmering iridescence of calcite, or dolomite, is the most pronounced 

 feature between crossed nicols, in convergent light, due to the high double refraction 

 which is capable of giving the different colors even for roughness of the ordinary 

 grinding and especially for the varying thicknesses in the individual grains of the 

 rock as left by the slicing. There are a few other substances in the slide for the 

 most part opaque, and too fine for determination. They are not pyrite. The section 

 does not cut any of the vesicular dark ingredient of this rock. 



In another section, made so as to cut one of the darker portions, the darker por- 

 tion is seen to be made up largely of fine angular quartz with a matrix of finer grains 

 of the same. There is also a considerable percentage of chlorite. Between the nicols 

 the darker portion is so much darkened as to indicate the presence of an isotropic 

 substance, perhaps of glass. The slide still does not show the vesicular portion men- 

 tioned above. 



In still another sections made by Marchand, there are occasional areas which, 

 sprinkled with a few minute opaque specks, are for the most part transparent, but 

 between crossed nicols are either wholly dark or are indistinguishable from a 

 devitrified glass. 



