164 THE CKYSTAL FALLS IRON-BEARING DISTRICT. 



retjAtions to INTRUSIVES. 



The Upper Iluronian, as well as the Lower Huronian, has been pene- 

 trated by intrusive rocks. The difference in the character of the intrusives 

 of the two series is, however, interesting-. As has been seen (p. 77), the 

 Lower Huronian is cut by vast masses of basic rocks and by rare dikes of 

 acid rocks. Li the Upper Huronian of the southern pai't of the district the 

 acid rocks are more abundant but still subordinate to the basic intrusives. 

 North of Crystal Falls is a great east and west basic dike. Similar rocks are 

 known in a few small bosses near Cr3^stal Falls. Moreover, on the Michi- 

 o-amnie River, in sees. 31 and 32, T. 43 N., R. 31 W., and in a few places 

 to the southeast of this area, the Upper Huronian is cut by the southern 

 continuation of the basic masses whose principal occurrence is in the Lower 

 Huronian area. Finally basic and ultrabasic intrusives pierce the Upper 

 Huronian sediments in sees. 15, 28, and 29, T. 42 N., R. 31 W , and in a few 

 other places. The acid rocks occur in isolated knobs near Crystal Falls, 

 in sec. 28, T. 43 N., R. 32 W.-, and in sec. 4, T. 42 N., R. 32 W. They 

 increase in quantity toward the southeast, and in the vicinity of Lake 

 Tobin, in sees. 21 and 28, 42 T. N., R. 32 W., they form a series of small 

 hills rising- boldly out of the sand plains; and finally they occur in large 

 quantity in sees. 19, 20, and 29, T. 42 N., R. 31 W., between the Paint and 

 the Michig-amme rivers. 



CORRELATION. 



The Upper Hiu'onian sedimentary rocks were first studied in the field 

 by Brooks, and the coiiclusion reached by him that they are late Huronian.' 



Rominger correlates these same rocks very correctly with the schists 

 exposed around Lake Michigamme, although he has the erroneous idea that 

 they follow down the Michigamme River, instead of making a wide curve 

 to the west, as subsequent study of the area has shown.^ He considers the 

 rocks as forming- the middle portion of his Arenaceous Slate group.^ 



Recent work in the district has shown the Upper Huronian rocks to be 



' Geology of the Menominee region, by T. B. Broolis : Geol. of Wisconsin, V<51. Ill, 1880, p. 44. 



-"The mica-schists seoiu to contimio southward along the coiarse of the Michigamme River, as 

 we find in its lower course, 5 or G miles north from its entrance into the Brule River, and from there 

 down to the mouth, mica-schist to be the prevailing surface rock. Along the lower course of the 

 adjoining Paint River the mica-schists likewise are the only rocks seen in the exposures." (Geol. of 

 Michigan, Vol. V, 1895, p. 81.) 



'Op. eit., p. 79. 



