CHAPTER VL 



THE KEWEENAW AN. 



INTRODUCTION. 



The only rocks of Keweenawan age iu the Vermilion district are 

 gabbros which form a part of the Duluth gabbro mass of northeastern 

 Minnesota, certain great basic sills to which the name Logan sills has been 

 given, and some few basic and acid dikes which cut all of the rocks of the 

 district, including the aforementioned gabbros and Logan sills. As a 

 result of the studies reported in this monograph, it has been determined 

 that stratigraphically the Duluth gabbro and the Logan sills belong together, 

 although they sliow slight differences in lithologic character. These 

 differences are due essentially to variations in the conditions of consoli- 

 dation. Since these two rocks belong together, they will be described 

 imder the same section in the following pages. A second section will be 

 devoted to a brief mention of the basic and acid dikes, which are the 

 youngest rocks of the Vermilion district, excluding always the Pleistocene 

 glacial-drift deposits. 



SECTION I.— DULUTH GABBRO AND LOGAN SILLs! 



References to the great gabbro mass of northeastern Minnesota are 

 common in the geologic literature of the Lake Superior region. The name 

 Duluth is given to this gabbro since it is so well developed near the city of 

 that name. This rock is conspicuously developed on the north shore of 

 Lake Superior, where it forms a prominent part of the Keweenawan series 

 of northeastern Minnesota, underlying several hundreds of square miles. It 

 is also well known upon the south shore in the Keweenawan district of 

 Wisconsin. 



North of the Duluth gabbro, and extending all around tlie north shore 

 of Lake Superior as far as the Slate Islands of the northeast shore of the 

 lake, it has been found by the various geologists who have worked in tliis 



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