866 THE GEOLOGY OP MINNESOTA. 



[Graywacke. Diabase. 



Meg. Gray, siliceous, hardly porphyritic, a near approach to graywacke, but 

 apparently, in the field, a variation of the quartz-porphyry No. 2229. 



Mic. The rock is largely composed of very fine but irregular matrix of epidote, 

 feldspar, quartz, calcite, biotite, Muscovite, and a few cubes of pyrite. In the midst of 

 this matrix are a few larger, broken plagioclase feldspars, much altered by biotite 

 and epidote growths, and occasionally a large quartz. In the midst of the slide are 

 areas crowded with a very fine, granulated structure consisting of epidote and 

 apparently minute secondary feldspars. These are, in the main, a fine debris more 

 or less regenerated, but some of them are also due to old feldspars that have been 

 lost by micro-granulitization. One section. 



Age. Lower Keewatin. 



Remark. This rock is believed to have been originally composed of debris of 

 the same elements as the quartz-porphyry. Ordinarily it might be classed as an 

 altered quartz-porphyry, but its essential characters appear in the field to grade into 

 those of the quartz-porphyry a remark which is equally true of the microscopic 

 characters. N. H. w. 



No. 2234. DIABASE. (Uralitized. ) 



Fifty two paces south of the section corner of section 9. Apparently a dike cutting the graywackes and 

 slates which strike northwest and southeast. 

 Ref. Annual Report, xxiv, page 69. 



Meg. This is a dark, heavy rock, and on the weathered suiiace reveals an 

 original ophitic structure. 



Mic. This rock was originally an oZm'we-bearing diabase, but that mineral is 

 altered to a dark, opaque, ferruginous substance which is unidentifiable, while the 

 augite is now in the form of a green pleochroic hornblende. It contains also pyrite. 

 One section. 



Age. Dike cutting the Keewatin. N. H. w. 



No. 2236. DIABASE. 



From a dike 650 paces west of the corner post, northwest corner of sec. 8, T. 63-9, running north and south, 

 two feet wide, passing through quartz-porphyry No. 2229. 

 Ref, Annual Report, xxiv, page 70. 



Meg. Very fine grained, with specks of pyrite. 



Mic. The ferro-magnesian mineral is chiefly hornblende in small crystals that 

 interlock with the fine microlitic feldspars. There is a suggestion of an original 

 porphyritic structure, due to the appearance of areas that have become micro- 

 granulitic, which once were apparently of feldspar phenocrysts. The minerals now 

 occupying these areas are granular epidote, minute feldspar grains, or remnants of 

 the old crystal, a little hornblende and less of chlorite. In the slide is one large 

 porphyritic area occupied almost wholly by many hornblendes that have independent 

 orientations. It has the shape of a feldspar phenocryst. 



