Areas of Granitoid and Gneissoid Rocks. 



The rocks which on the evidence of relative position would he regarded 

 as the oldest rocks accessible to observation in the Northwest are granitoid, 

 as every one understands ; but I have not found, as yet, any general grani- 

 toid nucleus of the continent, occupying the surface uninterruptedly, in any 

 direction, for more than a hundred miles. Even the granitoid areas are not 

 occupied chiefly by rocks conforming to the standard defiuitiou of granite — 

 a non-bedded and non-foliated mixture of quartz, feldspar, and mica, or of 

 quartz, feldspar, and hornblende. Limited areas approaching, or perhaps 

 attaining, this condition are found ; but the principal expanses of crystalline 

 rock are gneissic— consisting of quartz, feldspar, and a dark element, with 

 the quartz in many cases deficient in amount, but also very extensively 

 disseminated in porphyritic development. The feldspathic element is pre- 

 dominantly orthoclase, but generally one or more triclinic feldspars is also 

 present. The dark or ferro-maguesian element is generally biotite or horn- 

 blende, or both together. Sometimes muscovite appears with one or both of 

 these, and occasionally it excludes them. In rare cases the dark element is 

 augite* and not unfrequently individuals of hornblende are found with 

 augitic nuclei. Over considerable areas the hornblende has undergone 

 uralitization, and even chloritization. A large part of the hornbleude, 

 however, is black, lustrous, and fresh. The orthoclase is often found in por- 

 phyritic development, but generally it occurs in the ordinary granular state. 

 In the chloritic portions the feldspar is chiefly of late generation, and forms 

 a more or less perfect grouudmass, with a greenish stain in the vicinity of 

 the amorphous, chloritized hornblende. In mineralogical composition the 

 areas strictly granitoid are uudistinguishable from those properly gneissoid. 

 In structure the distinctions of successive generations are less obvious, and 

 the chloritization of the hornblende has made less progress. 



Within the limits of northeastern Minnesota four distinct areas of grani- 

 toid and gneissoid rocks have been surveyed. The accompanying diagram 

 shows their relative positions. 



These are the Basswood Area, the White Iron Area, the Saganaga Area, 

 and the Vermilion Area, so named from large lakes lying upon their borders. 

 Only the White Iron and Vermilion Areas have been followed along all 

 their borders. They are the only ones embraced wholly in Minnesota. 



The White Iron Area is elongated from Snowbank and Disappointment 

 lakes south westward to Birch lake and beyond. It is overlain along its 

 southeastern border by the great gabbro formation ; and this, at one place, 

 laps quite across the Area, dividing its surface exposure into two areas. 



* M. Alf. Lacroix has very recently made a study of pyroxenic gneiss from various parts of 

 Europe. "Contributions <> Vttwde des gn< - d,pyroxtoru rides roches a werneritt ," Paris, 1889, pp. 1-280. 



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