44 



Minnesota Academy of Science 



eluded, not only that on the south side of lake Superior there were 

 two different ore horizons, hut that at Marquette they were both to 

 he seen easily in the mines that were being exploited. The first 

 formal announcement of this important generalization* which ever 

 was published was made in the sixteenth annual report, 1887, of 

 the Minnesota survey,- pp. 43-47. It was amplified in the seven- 

 teenth, 1888, pp. 43-45, and, when the priority of our discovery was 

 questioned by some of the Wisconsin geologists, it was defended in 

 the twenty-first annual report, 1892, pp. 87-99. It was very soon 

 recognized as an important fundamental datum in all research work 

 in the geology of the Lake Superior region. It is the starting point 

 in all intelligent exploration, for it is very evident that unless the 

 exploring geologist knows how the strata run, and what their posi- 

 tion with respect to the ore which he is to search for, he is very 

 liable to expend a large sum of money by working in the dark. 



NORTHERN MINNESOTA. 



Fig. 1. — Location of the Iron ranges of Minnesota. 



*The initial dissent from the idea of the "Huronian" age of all 

 the ores of the Lake Superior region was published in the thirteenth 

 report of the Minnesota survey, pp. 24-37, 1884. 



