408 THE VERMILION IRON-BEARING DISTRICT. 



relations to other rocks have varied greatly, although in the Final Report 

 (Vol. IV) it is regarded as of Keweenawan age, in general agreement 

 with the results obtained by others who have worked upon this problem. 

 Winchell" makes it the igneous base of the Keweenawan. 



As the result of work done during the field season of 1900 fairly 

 conclusive proof has been obtained of the fact that the gabbro is in reality 

 intrusive in the volcanic Keweenawan, and consequently younger at least 

 than a portion of the Keweenawan. This intrusive relation was observed 

 just west of the west end of Brul^ Lake. Brul^ Lake lies within a syncline 

 of Keweenawan lavas, bounded on the south and west by the Duluth gabbro 

 and on the north by the "red rock" of the Minnesota survey. From this great 

 gabbro mass at the west end of the lake an eastward-projecting tongue of 

 gabbro was traced into the volcanics. This tongue near the gabbro pos- 

 sessed the normal characters of the main gabbro mass, but to the east 

 it narrowed rapidly and its lithologic characters changed until where nar- 

 rowest, just before it disappeared in a topographic depression, it had graded 

 into a porphyritic rock of comparatively fine grain, with a selvage which 

 was very nearly basaltic. The lavas were upon both sides of this tongue. 

 The actual contact between the gabbro and the lava was not found, but 

 they were separated at one place by an interval of only about 1 foot, 

 and this was the place where the tongue showed its finest grain. The 

 connection of this tongue with the gabbro and the variation in grain in 

 the tongue seem to be very good evidence of the intrusive character of the 



RELATIONS OF THE LOGAN SILLS TO ADJACENT FORMATIONS. 



Relations to the Upper Huronian (^Animikie). — Up to the time of the 

 publication in 1893 of Lawson's'' paper on the laccolitic sills of the 

 northwest coast of Lake Superior, all of the earlier writers on the Lake 

 Superior region, with the exception of Irving" and Ingall'* had held the 

 sheets of basic rock which they observed intercalated between the Huronian 

 slates to be of volcanic origin — that is, surface flows interbedded with the 



«Geol. and Nat. Hiat. Survey of Minnesota, Final Kept., Vol. IV, 1899, p. 298. 



6 The laccolitic sills of the northwest coast of Lake Superior, by A. 0. Lawson: Geol. and Nat. 

 Hist. Survey of Minnesota, Bull. No. 8, pt. 2, 1893, pp. 24—18 See this for references to writers giving 

 details of sills which can not be given in present paper. 



oMou. U. S. Geol. Survey Vol. V, 1883, p. 379. 



t^Geol. and Nat. Hist. Survey Canada, Ann. Kept., 1888, H, p. 25. 



