REVIEWS 171 



western portion of the district, and the intervening iron-bearing Agawa formation 

 where present in the eastern portion of the district, there occurs a thick series of slates 

 of varying character, to which the name " Knife Lake slates " has been given. These 

 slates have been very closely folded, and have been more metamorphosed where 

 intruded by granites of Giant's Range, Snowbank Lake, and Cacaquabic Lake, and 

 by the Duluth gabbro. These igneous rocks occupy a considerable area, and their 

 intrusive relation to the Lower Huronian are unquestionable. The Lower Huronian 

 sediments now stand nearly vertical. 



The Upper Huronian or Animikie series is found in the extreme eastern portion of 

 the district, where it is continuous with the Animikie of the Mesabi district to the west 

 and Thunder Bay to the east. At the bottom of the series occurs an iron-bearing 

 formation known as the Gunflint formation. Above this occurs a great slate-gray- 

 wacke formation, to which the name "Rove slate" has been given. The Gunflint 

 formation is correlated with the Biwabik formation of the Mesabi district. It has a 

 very limited development in the Vermilion district, and its most interesting phases are 

 especially well developed in the vicinity of Akeley Lake. In general the formation 

 has a monociinal dip to the south-southeast at a low angle. It has been extremely 

 metamorphosed by the Duluth gabbro, and where most metamorphosed the rocks are 

 composed of coarsely crystalline bands of quartz, of varying width, alternating with 

 coarsely crystalline bands of magnetite ore reported to vary from one inch up to ten 

 or twelve feet in thickness, and of bands of dark-green, brown, or black rocks that 

 consist of combinations of quartz, augite, hypersthene, hornblende, olivine, and mag- 

 netite as the principal minerals, but associated occasionally with some ferruginous 

 carbonate, actinolite and griinerite. 



The Duluth gabbro and the Logan sills, referred to the Keweenawan, occur m 

 the eastern portion of the district. 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 intrusive in the Keweenawan volcanics. A number 

 of facts are enumerated to show that the gabbro and the Logan sills are of essentially 

 the same petrographic character, although they 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 compo- 

 sition 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 but small size with some associated minerals. These masses grade into the 

 surrounding gabbro, and were formed as the result of processes of segregation. 



Cutting the Duluth gabbro are acid dikes and dikes of basalt and diorite. 



The entire district has been much folded and metamorphosed, resulting in a 

 marked north of east and south of west trend of the Archcean and Lower Huronian 

 formations, marked principally by schistosity. 



Comment on the Vermilion and Mesabi Reports. — Detailed work in these districts 

 has developed a number of points bearing on the general stratigraphy and correlation 

 of the rocks of the Lake Superior and Lake Huron districts. 



In the Vermilion district the rocks now called Lower Huronian had previously 

 been referred to the Upper Huronian by the U. S. Geological Survey, and the sedi- 

 mentary Soudan iron formation, now mapped as Archaean, had previously been called 

 Lower Huronian, and separated from the greenstones and granites supposed alone to 



