PLATE LXXXI. 



AKELEY LAKE PLATE OF THE MESABI IRON RANGE, 1899. U. S. GRANT. 



It will be seen from the report of Dr. Grant on this plate that there are different 

 opinions as to the age and nature of the iron-bearing rocks of this area, and that 

 this difference extends to the muscovadyte phase of the gabbro. 



The writer's views of the iron-bearing rocks of this area are expressed on the 

 "map of the Mesabi Iron range," inserted at the close of volume iv. The Keewatin 

 iron belt, which is mentioned near the centre of sec. 11, T. 59-14, in connection with 

 the Partridge River plate (vol. iv. p. 385) as distinct from the Animikie, seems to 

 continue, in connection with the gabbro in which it becomes involved, interruptedly 

 across the Dunka River plate area, where its chief known outcrops are east of the 

 Dunka river and north of Birch lake, across the Gabbro Lake plate, where it is known 

 at but one point (at Muskrat lake), and to Disappointment lake, on the Snowbank 

 Lake plate, where it shows unmistakably its alliance both with the jaspilyte and the 

 greenstone of the Keewatin, as it is located outside of the gabbro and conformably 

 in the strata of the muscovadyte stage of the greenstone. Thence northeastward it 

 is again encroached on and lost in the gabbro, appearing on the Fraser Lake plate 

 at but three places, so far as known. But on the Akeley Lake plate it is represented 

 by a long, narrow series extending nearly across the plate and apparently running, 

 with the greenstone, beneath the Animikie. 



As in Michigan, these two ore horizons, where they exist in close proximity, 

 have sometimes been confounded in one. In Michigan they were for many years 

 all referred to the horizon of the Vermilion ores, and that was originally done for 

 Minnesota by the officers of the United States Geological survey. It required con- 

 siderable research and frequent presentation of the varied facts to convince some 

 geologists that the Animikie ores were in a different horizon from those of the Ver- 

 milion range; but owing to the simplicity of the grand structure of northeastern 

 Minnesota, on which the distinction was firmly based in the early years of the Min- 

 nesota survey, the two ore horizons have been recognized by all geologists. 



Now the pendulum has swung into the opposite end of the arc, and in its initial 

 movement in that direction the Minnesota survey admits the responsibility for the 

 error. These ores, through the gabbro area, and at Disappointment lake, were, at 

 first, before any adequate study had been given to their structural relations, and 

 before their petrology had been examined into, mapped with the Animikie, thus 

 making a continuous belt of Animikie ore from Gunflint lake to the Mississippi river. 

 Some ores were thus hypothetically transferred from the lower horizon to the upper; 



