132 C. R. Van Hise — Iron Ores of Michigan. 



base of the flow, forming thick layers of magnetite. The 

 residual magma has crystallized as the interstitial minerals, 

 and thus we have the titaniferous magnetites of the gabbros. 

 A recent petrographical study by Dr. W. S. Bayley has shown 

 that a part of the magnetite is secondary. As these ores are 

 associated with abundant secondary quartz it is not impossible 

 that much of the magnetite has been brought to its present 

 position after the solidification of the rock. 



General. — It is evident that the ores of the Upper Marquette 

 series, like those of the Lower, were concentrated during and 

 subsequent to the folding and erosion which affected both. 

 Thus while the ore bearing formation of the Lower Marquette 

 series is far older than that of the Upper Marquette, and had 

 undergone great changes before the latter was deposited, the 

 local concentration of the iron into workable deposits occurred 

 simultaneously as a consequence of the same great causes. 

 This statement also applies, so far as present knowledge goes, 

 to the two iron-bearing series of the Menominee district. The 

 Penokee series is equivalent to the Upper Marquette and 

 Upper Menominee. The equivalency of the Animikie and 

 the Penokee series and their simultaneous tilting into mono- 

 clines on opposite sides of the Lake Superior synclinal has been 

 shown in another place.* It is also shown that this tilting, and 

 consequently the concentration of the ores, occurred subse- 

 quently to Keweenawan time. It was suggested that the basic 

 intrusives of the Penokee series upon which the ore-deposits 

 rest are of Keweenawan age. Is it not probable that many of 

 the basic intrusives of like character in the Marquette and 

 Menominee districts are products of the same period? 



It appears then probable that the local concentration of the 

 iron ores into workable bodies did not generally begin before 

 Upper Huronian time, while in certain districts it did not 

 begin until much later. That the process has yet ended we 

 have no evidence. The ores of the Lake Superior region oc- 

 curring in two geological series are remarkably alike. This 

 likeness is plainly due to their similar genesis. 



This paper must be considered as an account of the progress 

 of an investigation rather than a complete exposition. While 

 it is possible that a further study will make necessary a modi- 

 fication of certain minor points, it is believed that the principle 

 of secondary concentration along great channels of downward 

 percolating waters, and particularly at places where these 

 waters are converged by tilted impervious formations is nearly if 

 not quite of universal applicability to the workable ore-deposits 

 of the Lake Superior region. 



United States Geological Survey. 

 Lake Superior Division, Madison, Wis., December, 189!. 



* Tenth Ann. Rept. U. S. Geol. Survey for 1888-89, pp. 402-408, 458-460. 



