422 THE VERMILION IRON-BEARING DISTRICT. 



magnetite elsewhere widely disseminated in small quantities through the 

 gabbro was locally concentrated, while the magma was in a more or less 

 fluid state. We conclude that these titaniferous magnetite bodies belong to 

 those iron-ore deposits of igneous origin, Avhich are at present of little value. 

 All of the other ore deposits of this district, even when now magnetitic, as 

 in the case of the Gunflint ores, are nontitaniferous, or contain only traces 

 of titanium, and were originally of sedimentary origin. 



SECTIOISr II.— ACID DIKES YOUNGER THAIS^ THE DULUTH GABBRO. 



The great Keweenawan gabbro mass is cut through at various places 

 by small dikes of red granite. Several of these were seen beyond the 

 limits of the Vermilion district. One especially was noted upon the island 

 on the east side of Gross River, just opposite the bay into which the portage 

 from Snipe Lake enters. This dike is 3 feet in Avidth and trends north and 

 south. It is a fresh biotite-granite. Grant" reports a granite dike cutting- 

 through the gabbro on an island in Gobbemichigamma Lake in the NW. ^ 

 of the NE. J of sec. 6, T. 64 N., R. 5 W. These occun-ences are sufficient 

 to prove that there is a granite later than the gabbro. 



South of the Vermilion district there are large areas underlain by a 

 bright-red weathering acid rock, varying from syenite to granite, which sends 

 off shoots into the adjacent gabbro. No attempt has been made to trace 

 the connection between the granite dikes mentioned above and these larger 

 masses of "red rock" occurring in the midst of the Keweenawan. The 

 possibility of their close relationship is suggested, however. 



SECTIO]S^ III.— BASIC INTRUSIVES YOUNGER THAN THE DULUTH 



GABBRO. 



At numerous places basic dikes have been noted in the gabbro. These 

 were found as the result of the limited studies made upon the gabbro. 

 These studies were confined chiefly to the margin of the gabbro and to a 

 few excursions made within the Duluth gabbro mass. Unquestionably great 

 numbers of other dikes of similar character would have been found had the 

 gabbro been iTiore closely examined. A number of basic dikes were also 

 found in the Upper Huronian sediments and in the older rocks of the Ver- 

 milion district, both the eruptive and sedimentary ones. These dikes corre- 

 spond in every detail with those found in the gabbro. All of these very 



«Geol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. IV, 1899, p. 479. 



