16S THE CRYSTAL FALLS IKON-BEAEING DISTRICT. 



The iron-l)carinn- rocks of the Upper Huronian comprise cherts, 

 siderite-shites, ferruginous cherts, iron ores, aud subordinate quantities of 

 ferruginous graywackes and clay slates. 



The least altered of these is a siderite-slate. This is a fine-grained gi'ay 

 rock composed almost entirely of siderite, iisually in rounded rhombohedral 

 crystals, with very little minutely cr)^stallized silica between them in 

 places. Wherever they have been exposed to the weather any length of 

 time, these rocks have a deep reddish-brown oxidation crust. Alteration 

 also follows along crevices, and thus the siderite is rapidly oxidized. The 

 main products derived from these siderites are like those of the more 

 important ore-producing parts of the Penokee and Marquette districts, 

 uamelv, hematite and limonite. Little magnetite has been found. These 

 siderites are interbanded with the black carbonaceous clay slates. In some 

 cases the dividing line is sharp. In others, as the siderite lessens in quan- 

 tity, fragmental material increases until only a few crystals of siderite are 

 found scattered through the elastics. Their association with the carbona- 

 ceous fragmentals would seem to indicate, as pointed out by Van Hise,^ that 

 the siderite owes its formation to the presence of organic material. 



The ferruginous cherts (the term is here used as defined by Van Hise) 

 are banded chert and hematite, with some magnetite, in which the iron 

 oxide is derived from a jjreviously existing siderite, and the cherty bands 

 are not of fragmental origin. This alteration from the siderite to hematite 

 may be easily followed from the fresh siderite through that which is slightly 

 discolored, to the reddish-brown earthy mass, and then to the crystalline 

 hematite. Such alteration processes have been illustrated and clearly 

 described a number of times by Van Hise, so that no further mention will 

 be made of them. 



The ferruginous graywackes ma}' be described as rocks which are 

 partly of fragmental and partly of chemical origin. For instance, the tran- 

 sition ma}' be traced from a rather micaceous magnetitic graywacke, in 

 which ordinar}- and false Ijedding may be seen, to a rather schistose rock, 

 in which magnetite is predominant, but in which is considerable fragmental 

 quartz aud secondar}' muscovite and chlorite. This rock represents an 

 original grit containing more or less siderite. Metamorphism has changed 



' Fifteenth Ann. Rept. U. S. Geol. Survey, cit., j). 601; Mon. U. S. Geol. Survey, Vol. XXVIII, eit., p. 447. 



