152 THE CEYSTAL FALLS IRON-BEARING DISTRICT. 



admit of successful mining. Such ore bodies probably owe their presence 

 in great part to processes active subsequent to tlie formation of the schists. 

 (See p. 134.) 



According to the quantity and association of the minerals described 

 above as occurring in the schists, the following rocks may result from 

 the complete metamorphisra of the basic volcanics : Amphibolites, chlorite- 

 schists, epidote-schists, mica-schists, mica-gneisses, and possibly siliceous 

 hematite and magnetite ore. The complete metamorphism of dense basic 

 lava flows into crystalline schists has been described by Williams^ for the 

 Menominee and Marquette districts, and also by Van Hise and Bay ley ^ for 

 the Marquette district. Williams^ has also described the production of 

 schists from the igneous elastics in the Menominee district and similar 

 products have been described from the Marquette district both by Williams* 

 and by Bayley.^ 



The above-described schists cover a considerable area, with only iso- 

 lated exposures of rocks associated with them in which volcanic characters 

 are recognizable. They are confidently believed to represent extremely 

 metamorphosed volcanics of the same general original character as those 

 constituting the Hemlock formation and belonging to the same relative 

 period of extrusion. 



The same conclusions have been reached by Smyth for similar schists 

 along the Fence River to the southeast of those described. It is noticeable 

 that the most intense metamorphism of the A'olcanics has taken place in the 

 northern and northeastern part of the Crystal Falls district, that part in 

 which the crystalline schists have been produced, though the explanation 

 for this can not be offered. 



KORMAL, SEDIMEK^TARIES OF THE HEMLOCK FORMATION. 



The normal sedimentaries are in small quantity. It has been seen 

 (pp. 64, 78) that the Mansfield slate is overlain by a conglomerate in which 

 volcanic material predominates, but which contains partly rounded frag- 

 ments of chert and slate and round quartz grains derived from the under- 

 lying sedimentaries. But for the intermingling of this normal clastic debris 



I Bull. U. S. Geol. Survey No. 62, cit. < Op. cit., p. 158. 



= Mou. U. S. Geol. Survey, Vol. XXVIII, cit., pp. 152-159. "Op. cit.. pp. 160-169. 



= 0p. cit., p. 133. 



