INTERESTING LOCALITIES OF ISHPEMING FORMATION. 433 



moi'e proiniiieut here. At the Barron mhie the basal recomposed rock so 

 closely resembles the original jasper that it is difficult, if not impossible, 

 to exactly locate the place at which the Negaunee formation ends and 

 the Goodrich quartzite begins. The two have been mashed into apparent 

 conformity, and at the contact plane is tlie ore deposit which welds them 

 together; but upon the east side of the ore deposits it is plain that we 

 have the Negaunee jasper, and upon the west side the Goodrich quartzite. 

 At this mine and at the west end of the Jackson mine are the two places 

 in the main area of the district where it is most difficult to discriminate 

 between the two formations. A short distance from the Barron, at the old 

 Humboldt mine, north of Mount Humboldt, there is no difficulty in making 

 the discrimination l^etween the formations, as the mashing was less severe. 

 The Humboldt conglomerate grades up into a coarse graywacke or into 

 a feldspathic quartzite. At one or two of the open pits the finer-grained 

 detritus has been mashed into a peifect, finely laminated mica-schist. 



In thin section the mashed conglomerates closely resemble the mashed 

 jasper of the Negaunee formation, but the small, distinctly fragmental grains 

 of quartz of larger size than the granules of jasper, and flakes of sericite, 

 mark the difference. The finer-grained conglomerate has become a fei-- 

 ruginous sericite-schist. The large clastic grains of quartz are broken, 

 elongated, and often wholly granulated. Around these areas the mashed 

 hematite and lustrous sericite wrap in the usual manner in such rocks. As 

 these conglomerates pass up into those in which no coarse detritus was 

 present, we have the sericite-schists. In one case the rock consists almost 

 wholly of flakes of sericite having a minutely crenulated, parallel arrange- 

 ment of a majoritv of the blades. There are present a few larger blades of 

 muscovite Avhich are arranged transverse to the others, or at a large angle 

 to them. Tliese are doubtless original clastic flakes. The quartzose mica- 

 schists differ from the pure micaceous rock only in that between the leaflets 

 of sericite are very numerous small particles of quartz, either single or 

 clustered, about which the sericite jjasses, like the meshes of a stretched net. 

 The quartzite close to Humboldt has largely recrystallized, and approaches 

 a quartz-schist. Many of the original fragmental grains have been granu- 

 lated, and the new quai'tz which has developed sometimes approximates in 



MON XXA'III lis 



