208 T. B. Broohs— Youngest Huronian Rocks. 



3. Not only does this granitoid formation thin out and disap- 

 pear in its eastward prolongation, but the same is true of the 

 whole Huronian series, the belt of which becomes narrow as 

 followed east, and finally disappears in the neighborhood of 

 Gogebic, where the Lauren tian is seen very near the Copper 



4. The fact that the granite mass does not cross either the Cop- 

 per or Huronian series, or, so far as observed, give off dikes in 

 either, renders it improbable tbat it came into its present posi- 

 tion as an eruptive mass subsequent to the formation of both 

 series of rocks. 



5. The various ores of iron, which are so generally and abun- 

 dantly diffused in the Lower and Middle Huronian, are entirely 

 absent so far as observed from the upper three or four members as 

 developed in the Marquette and Menominee regions, and also in 

 the Penokie series if the following hypothesis is true ; but they 

 occur in all forms, although, it is believed, not abundantly, in 

 the uppermost exposed member on Black Eiver.f If we sup- 

 pose this iron to have been mostly precipitated as a carbonate, 

 then we might expect it would' be more generally diffused 

 through the rocks of certain epochs than those materials derived 

 from the erosion of adjacent coasts. 



There is evidently but one hypothesis which will reconcile 

 these facts, which is : that the granitoid formation in question 

 is of the Huronian period, and probably the youngest member; 

 which series are here non- conformably overlaid by the Copper- 

 bearing rocks. I conceive that this view is supported by the ob- 

 servations in the Menominee region above recorded, and suppose 

 this Penokie granitoid formation may be the equivalent of gra- 

 nitic bed XX of the Huronian series as developed in that re- 

 gion. On this hypothesis, it is possible that the valley dividing 

 the Penokie Eange proper from the granitoid belt may be un- 

 derlaid by a soft slate, the equivalent of the micaceous schist, 

 bed XIX. ^ 



I would anticipate the objection which many will make to 

 attaching much weight to lithological evidences in determining 

 the age of formations 100 miles apart, by repeating that the 

 staurolitic mica schist formation (XIX) maintains its min- 

 eralogical character for over one half this distance. I fail to 

 understand why conditions favorable to the formation of exten- 

 sive areas of particular rocks may not have existed occasionally 

 in Archaean Time, since they were so prevalent in the fol- 

 owing ages. This idea of equivalency is further supported by 

 facts given in my "Eevised Descriptive Catalogue of the Mich- 

 igan State Suite of Huronian Rocks," in preparation. 

 * Pumpelly and Brooks, this Jour., voL iii. 1872. 

 t The beat point for observing the Huronian between Lake Gogebic and Mon- 



