26 OUTLINE OF MONOGEAPH. 



some ferruginous carbonate, actinolite, and griinerite. The I'ounded granules are 

 sometimes preserved in these rocks, showing their derivation from the least 

 metamorphosed forms previously mentioned, although the granules consist of 

 minerals different from those in the least metamorphosed forms. The Rove slate 

 conformably overlies the Gunflint formation. The rocks of this series show nothing 

 of especial interest. They have been metamorphosed slightlj'' as a result of the 

 contact action of the adjacent Duluth gabbro mass and the intrusive Logan sills. 



Chapter VI. This chapter treats of the Keweenawan series. The only rocks 

 of this age in the Vermilion district are gabbros forming 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 the rocks 

 of the district, including the aforementioned Duluth gabbi'o and the Logan sills. 

 The studies of the writer and his associates have been confined chiefl}'' to the 

 northern edge of the Duluth gabbro, which appears in the Vermilion district. 

 Several reconnaissance trips have also been made into the area underlain by the 

 gabbro. As a result of these studies, the Duluth gabbro is found to vary in 

 texture from a coarse-grained granular rock to a relativelj^ fine-grained rock. It 

 also has in places a gneissic structure. Under the microscope the texture is seen to 

 varjr from granular to ophitic. The Logan sills are great masses of doleritic rocks 

 that occur for the most part as sills interbanded with the Upper Huronian sediments, 

 and at times cut in dike form across them. Petrographicallj^ these dolerites range 

 from coarse-grained rocks, found in the centers of the sills, with an imperfect 

 granular texture and very similar to the gabbro, through normal ophitic dolerites, 

 to intersertal textured basalts on the selvages of the sills. The gabbro is found to 

 metamorphose all of the sediments already enumerated, and is thus shown to be 

 one of the youngest rocks of the district. It is also found to be intrusi\'e in the 

 Keweenawan volcanics. A number of facts are enumerated to show that the gabbro 

 and the Logan sills are of essentiall}' the same petrographic character, although 

 the}'' exhibit minor differences that are readily explicable when one considers the 

 relative amounts of the two rocks. After a consideration of these facts and of 

 the stratigraphic relationship of the rocks the conclusion is reached that the gabbro 

 and the sills are of essentially the same composition and age, having been derived 

 from the same parent mass of magma. In certain localities in the Duluth gabbro 

 there are found masses of titaniferous magnetite of varying size, with some associated 

 minerals. These masses grade into the surrounding gabbro, and were formed as the 

 result of processes of segregation. No published description has yet been given, so 

 far as the writer knows, of any large continuous masses of titaniferous magnetite in 

 these gabbros, and he knows of none from personal observation. If, however, large 

 masses do exist their content of titanium would prevent them from being of value at 

 the present time, when, according to the modern iron-smelting practice, titaniferous • 

 ores can not be smelted economically. In a short section mention is made of the 

 acid dikes that are younger than the Duluth gabbro, and of certain basalt and dolerite 



