446 THE MAEQUETTE lliOls BEAiaNU DISTRICT. 



the southern part of the belt, several miles east of Clarksburg. It can not 

 be said that these divisions are in any why stratigraphical, unless it be true 

 of the feiTuginous phases, which appear to occupy a somewhat persistent 

 horizon; but these latter rocks are not so well defined that they can be 

 mapped as a separate formation. 



The slates and grayivackes differ from each other chiefly in coarseness 

 of grain, the two often being interlaminated in the same exposure or ridge. 

 There are all gradations, from the aphanitic, black shales or slates to a 

 graywacke so coarse as to approach a quartzite, or, in one case, a conglom- 

 erate. The rocks vary in color from gray to black. Where they are fine- 

 gi-ained they usually have a well-developed slaty cleavage, and are often 

 carbonaceous, feiTUginous, and pyritiferous. In some places the amount of 

 carbon is so gi-eat as to give a black streak. When broken apart parallel 

 to the cleavage the graphite is frequently in a lustrous form, due to move- 

 ments parallel to the parting. An examination of two specimens (16671 

 and 16678) of the most carbonaceous rocks, by H. M. Stokes, in the chem- 

 ical laboratory of the United States Geological Survey, showed that they 

 contain respectively 18.92 and 15.69 per cent of carbon, but no hydrocar- 

 bons. A portion of the carbon appears to be in the form of anthracitic, 

 coaly substance, but much of it has been transformed to graphite. The 

 pyrite is in detached crystals, and in laminte parallel to the parting. 

 The least altered of these rocks could properly be called shales or grits. 

 In places where they are more altered the shales pass into mica-slates, and 

 bv more extreme alteration into the mica-schists. 



The feri-uginous slates and graywackes contain much iron. In the 

 least-altered phases the iron compound is largely siderite, and thus the rock 

 is a sideritic slate. Rarely the siderite becomes the predominating constit- 

 uent, and in this case the rock is similar to the sideritic slates of the Negaunee 

 formation. As a consequence of weathering and metasomatic changes, 

 ferruginous slates, ferruginous cherts, and griinerite-magnetite-schists have 

 developed from these sideritic slates. In the few localities where the 

 ferruginous material was very abundant small ore bodies also have fornjed. 

 Such are known at three places north of Champion and at one south of 

 Spun-. Pits also occui* in the south halves of sees. 29 and 35, T. 48 N., 



