THE UPPER SLATE MEMBER. 305 



The only exposure of qnitrt^ifr and ronffhmcrate of any magnitude is in 

 Sec. 11, T. 45 N., R. 1 W., Wi.sfousin. At this place the rocks of the 

 Copper series lie immediately to the northward, there Ijeing between the 

 quartzites and greenstones an interval of but a few feet. The rocks of this 

 exposure, described in the tabulations, are feldspathic. They are notable in 

 showing nicely the alteration of fragmental feldspar to both muscovite and 

 biotite with a sinudtaneous develoi)ment of quartz, which is thus inclosed in 

 the felilspar and mingled with the other secondary products. (PI. xxxii, Fig. 

 1.) The rather unusual alteration of feldspar to actinolite is also here illus- 

 ti-ated. This actinolite occurs in the eidargehients of the quartz grains, but 

 not in their cores, thus showing that it has formed subsequently to the 

 deposition of the fragments of tlie rock. 



The claif-dates or phijllites are not numerous, being found in Ijut a few 

 localities in the eastern part of the Upper-slate member. It is probable 

 that these rocks make up a greater part of the upper l)elt than would be 

 inferred from the few exposures, for they are so soft that they would rarely 

 outcrop even if widespread. The known exposures of clay-slates are in 

 Sees. 11, 12, and 17, T. 47 N., R. 4(i W., Micliigau; Sec. 13, T. 47 N., R. 

 47 W., Micliigan; and near the line between Sec. 1, T. 45 N., R. 1 E., and 

 Sec. 0, T. 45 N., R. 2 E., Wisconsin. Among the clay-slates are placed 

 only rocks, which are excessively fine grained, the particles that compose 

 them being so minute as to make the determination of their mineral com- 

 ])Osition in part uncertain. A porti(,)n of each thin section is sufficiently 

 coarse grained to certainly show that it contains small fragmental par- 

 ticles of (piartz and feldspar. It can also be seen that it contains much 

 chlorite, kaolin or sericite, finely crystalline quartz, and iron oxides. As 

 the minerals thus known to be jjresi'ut are the same that ordinarily con- 

 stitute clays, and as they are those which compose the graywackes and 

 grayvvacke-slates with which the day-slates are closely associated, there is 

 little doubt but that the greater nuuiber of them are thus chiefly consti- 

 tuted. One of the slates contains a suiall quantity of biotite. The iron 

 oxide in the slates along the line of Sec. 1, T. 45 N., R. 1 E., and Sec. 6, T. 

 45 N., R. 2 n, Wisconsin, is so abundant as to l)e one of the (-hief constit- 

 uents of the slate. This iron o.xide is very largely magnetite in minute 



MON XIX 'M 



