INFLUENCE OF TOPOGRAPHY. 1219 



For instance, in the Gogebic and Menominee districts of Michigan, 

 where the drainage is across the range, the great ore deposits reach the sur- 

 face on the slopes or crests, and comparatively little ore has been discovered 

 below the strong cross drainage lines where the waters are probably ascend- 

 ing. All of the great iron-ore deposits reach the surface, but many of them 

 have a pitch which carries them below the rocks. An ore deposit which at 

 the surface starts on a slope or crest may extend below a subordinate cross 

 valley. In such cases the water had head sufficient to carry it down and 

 make it effective below the valley; but, so far as I know, it has yet to be 

 proved that any great deposit contiuues in force across the valley of an 

 important stream." 



The topography in the Mesabi range of Minnesota exerts a somewhat 

 di fferent influence. The productive portion of this range extends from a point 

 west of Hibbing to Embarrass Lake. Here the Giants range to the north is 

 strongly emphasized, and there are good drainage lines to the south. West 

 of Hibbing the Giants range is much less marked ; indeed, for much of the 

 distance to the Mississippi River is almost imperceptible. 6 While ore depos- 

 its have been discovered in this part of the range, they are not nearly so 

 numerous and extensive as in the area to the east, and the ores which have 

 been found are sandy (indicating an imperfect removal of the silica). 

 There is no known stratigraphic reason why the ore deposits should not 

 have developed as extensively between Hibbing and the Mississippi River 

 as to the east. The only suggestion which has occurred to me in reference 

 to the matter is that because of the dying out of the Giants range and the 

 lack of sufficiently marked elevation and good slopes to produce a vigorous 

 circulation, the groundwater circulation has not been sufficiently vigorous 

 to concentrate the iron oxide and to remove the silica to the same extent as 

 farther east. 



An excellent illustration of the third corollary is furnished by the 

 San Juan district of Colorado. In that district the richer parts of the lodes 

 are generally below the middle slopes of the mountains. Nearly all of 

 the great lodes have been opened on veins which outcropped in the so-called 

 "basins" at altitudes of 3,000 to 3,500 meters. Above these basins are the 



" Van Hise, C. R., The iron-ore deposits of the Lake Superior region: Twenty-first Ann. Rept. U. S. 

 Geol. Survey, pt. 3, 1901, pp. 329-330. 



& Van Hise, cit., Section on the Mesabi district, by O. K. Leith: pp. 352-353. 



