158 THE PENOKEE IKON BEARING SERIES. 



ill diameter. The quartz fragnieut.s arc both simple and complex, and have often 

 received secondary enlargements. The most notable thing abont these rocks is the 

 fact that the crystals of magnetite, and also numerous minute needles of actinolite, 

 besides being contained in the matrix, are included abundantly in the enlargements 

 of the fragiiieiital grains of quartz. These minerals have developed in situ. These 

 very singular rocks come from a belt lying above the chert, which at Penoke« gap 

 forms all of the upper part of the limestone member, and the fragmental slates of the 

 member now under consideration. (PL xvii, Figs. 1 and 2.) 



S. Biotitic and chloritic slates, from low horizons. S])ecimeiis O.lfii; (slide ."14.3), 

 0563 (slide 3097), from IISO X., 1700 W.; '.t5(;4 (slides ol-14, 3145), 9505 (slide .3098), 

 from 15.30 N., IKiO W., Sec. 14, T. 44 N., R. 3 W., Wisconsin. 



These are tine grained, tliinly laminated rocks, breaking at times with a sub- 

 conchoidal fracture across the planes of lamination. In color they range from dark 

 brown or black, tlu'ough gray, to various shades of green. 



The thin sections present a ground mass which is always chiefly quartzose, the 

 grains being of small but uiiitbriu size and g:enerally of roundish or oval shapes. In 

 detail, however, the outlines of tiie grains are angular, and the iirojcctions of the 

 different particles commonly interlock. In many of these minute particles the out- 

 lines of the original cores are perceptible, the enlargements being narrow and pro- 

 ducing the angular projections referred to. Tlie line of division between the enlarge- 

 ment and the original core is generally eiiiiihasized by secondary dcvelo|iineiits of 

 {larticles of chlorite and biotite. ^lany more (juartz particles, liowever, do not show 

 any traces of the original cores, and it is not certain that a portion of them may not 

 have crystallized in situ. Small fragments of feldsi)ar make up a jiart of the ground- 

 mass, .scattered through which are particles and clusters of particles of chhuite and 

 biotite. The single flakes of tliese two minerals are larger than the iiarticles of the 

 groundmass itself, bill are still \ery minute. In some slides the biotite almo.st 

 excludes the chlorite, while in others the reverse of this is true. A considerable 

 portion of these two iiiincrals seems jilainly to be due to the decomposition of the 

 feldspar fragments. Accessory constituents are sericite, kaolin, tourmaline, and 

 magnetite. 



9. Quartzite, from the upjici most i)arl of the shite, and immediately beneath the 

 Iron-bearing memlier. Specimens n.")(i(> (slide 3192), 9.501 (slide 3193). Fnnn 1075 N., 

 1280 AV., Sec. 14, T. 41 N., I{. ;; W.. Wisconsin. 



A fine grained, quite gi;iyisli i|iiartzite, having a conchoidal fracture. 



Quartz in latlu'r siiiiill liiil iiiiifoini sized particles composes threefourths of the 

 sections:" The jiarticles of this mineral, although they very often closely fit and inter- 

 lock, are i)lainly in the main of a fragmental nature. I'.iotite in 3192 and chloiite in 

 3193 occur sparingly bclwcen I lie quartz jiarticles. Nuiiieroiis fragments of feldspar, 

 flakes of sericite, aii<l a lew ]);iiti(lcs of limonite are seen. The sections are closely 

 jiUied to those of 8. 



