320 THE PENOKEE IRON BEARING SERIES. 



tioii: Silica, 50-73; alumina, 22'VS; irou sesijiiioxidi', O-ll ; iron protoxide, 5-08; man 

 fjiinous oxide, O-OO; calcium oxide, 0-53; maguesium oxide, -!-94; jjotassa, 3-4:8; soda, 

 1-41; water, 3-liS = l()(l-33. 



The section of this rock is much like 3322 in U). The chief diflerences are, 

 that much of the large fragmental feldspar shows the distinct twinning bands of 

 plagioclase, ami tliat none of these large feldspars have altered to such a degree as a 

 part of them have in the secticni referred to. 



27. Chloritic biotite-schists; the uppermost liorizon shown at this point, the rock 

 being mingled with the eruptives of the Keweenaw series. Specimens !ir)S(; (slide 

 3329), 11)80 N., 540 W.; 9590 (slide 4425), 1850 N., (i4(> W.; 9591 (slide 4420), 1940 

 N., 585 W,, Sec. 5, T. 44 N., R. 2 W., Wisconsin. 



The rocks are light to dark gray, break with a couehoidal fracture, and are 

 quite massive, although showing traces of lamination. 



In the thin sections fragmental quartz and feldspar are the most i)leutiful con- 

 stituents. Mingled with these minerals are much biotite, chlorite, ami muscovite, 

 the biotite being far the most abundant. Many of the larger of the quartz grains 

 have taken a. second growth, as a result of which their exteriors are minutely angular. 

 Much of the feldspar is quite fresh, although iu many places it has largely altered to 

 nuiscovite, <-hlorite, and biotite. Less freipieutly the feldspar is replaced by sat- 

 urating ([uartz. While much of the mica and chlorite are secondary to fehlsjiar, this 

 statement could not with certainty be made of all of it. Quite plentiful black par 

 tides of ferrite are scattered through the sections. Iu the interstices is finely 

 crystalline! (luartz. 



28. Muscovitic biotite-schist. The ui)permost horizon shown here — higher than 

 the preceding. Specimen 2030, Wis., NW. ^ of NE. ^^ Sec. 0, T. 44 N., R. 2 W., Wis- 

 consin. 



The section has a line grained grouudmass of quartz and feldspar, in which are 

 abundantly found muscovite and biotite. Many of the larger quartz grains include 

 numerous folia of mica. The feldspar areas are reticulating and include quartz and 

 both the micas. The nuiscovite and biotite, besides being found in the small folia 

 thus included iu tlie (juartz aiul feldspar, are present iu uumerous large blades. This 

 section, taken by itself, would be considered a conqiletely crystalline mica schist in 

 which the interlocking and mutual inclusions of the different minerals are of the 

 most intricate sort. But, like the other mica-schists of the Up])er slate, it is believed 

 to be an (U'dinary clastic rock in which the series of metasomatic changes have been 

 great. The large, areas of (piartz including folia of mica rci)resent clastic individuals 

 of feldspar. As these, feldspars have altered to mica the excess of silica has sepa 

 rated out as (pxartz, aiul the areas, as stated above, which were once orthoclase or 

 plagioclase — in most cases the former — are now complex, juost intricately interlocking 

 areas of quartz and mica. Erequently the alteration of a single fclds])ar has resulted 



