INTERESTING LOCALITIES OF MIGHIGAMME FOKMATION. 455 



of Clarksbui-g formation, in sec. 1, T. 47 N., R. 29 W., and in sees. 5, 

 6, and 7, T. 47 N., R. 28 W., there is a complex ridge along which are 

 very numerous exposures of the slates, novaculites, and graywackes of the 

 Michigamme formation. The different varieties of tlieso rocks are particu- 

 larly well seen adjacent to and north of Clarksburg. The rocks of this 

 area differ in a number of particulars from those north of Champion. The 

 coarse-grained graywackes are only rarely found, but in their place there 

 is present a considerable quantity of fine-grained, banded novaculite. The 

 slates near Clarksburg are not so carbonaceous as nortli of Cham))ion, and, 

 finally, they are somewhat more crystalline, a strongly developed slaty 

 cleavage being prevalent and many of the rocks being biotite-.slates. Tins 

 slaty cleavage varies from parallelism to the l^edding to rigdit angles to it. 

 The cleavage is in general approximately east and west, and its dip is 

 usually nearly vertical. 



In thin section, the semicrystalline appearance observed macroscop- 

 ically is borne out. The mica-slates, which are rare north of Cliampion, 

 are the usual rocks north of Clarksburg. With the biotite there is abun- 

 dant chlorite. In some slides one is predominant, in others the other. In a 

 number of slides the original bedding is recognized bv alternating layers of 

 different degrees of coarseness, and across the bedding the cleavage cuts, as 

 indicated by the parallel folia of mica. The novaculites have a fine-grained 

 background, consisting of small, closely-fitting gi-ains of quartz. Between 

 these quartz grains more or less of sericite, biotite, and chlorite have devel- 

 oped, and also occasionally griinerite. The pure nonfragmental rocks of 

 the iron formation are only rarel}^ found in this area, but many of tlie rocks 

 are half fragmental — that is, mingled with the clastic material are much 

 siderite and its alteration products. The alteration of the siderite usually 

 results in the production of needles of griinerite and crystals of magnetite, 

 although the other oxides of iron occur. Commonly in the sti-ongly griin- 

 eritic rocks chlorite, rather than biotite, is found. 



In the broad belt of the Michigamme formation running from Clarks- 

 bm'g to Ishpeming the area is low and swampy, with only occasional out- 

 crops. The most important cluster of these extends north of the Clarks- 

 burg formation, through sees. 9, 10, 11, 12, T. 47 N., R. 28 AV., and sec. 18, 



