312 THE PENOKEE IRON BEARING SERIES. 



fitting grains. Wliether any of it is fragraental is difficnlt to say. A few grains are 

 found which are larger than the remainder of the mineral, and these are clear and 

 appear to be clastic. Biotite in very small flakes, with the greatest diameters ordi- 

 narily in a common direction, compose fully one-half of the section. Mingled with 

 this biotite, and giving the rock its color, are large quantities of the black or very 

 brown material before mentioned. This is not magnetite, but appears to be a very 

 dark colored ferrite, or such ferrite mingled with jiyinte or carbonaceous material, or 

 both. In each section there chances to l)e but one or two garnets. Incluiled in each 

 of the garnets are all of the remaining constituents. In the main the garnets are 

 not crystal outlined, but in places they are, and here biotite blades often abut sharply 

 against tlu^ garnets and abruptly terminate. These rocks are ])lainly intermediate 

 between tln^ Iron-bearing and Upper-slate members. 



8. Biotite-slate, from east side of fault and near base of formation. Specimen 

 9568 (.slides 4497 and 4498), 1700 N., 1000 W., Sec. 14, T. 44 N., R. 3 W., Wisconsin. 



This rock is black, very tine grained, massive, breaks with hackly sul)conchoidal 

 fracture, contains many grains of colorless and tlesh-colored feldspar, which upon the 

 broken surface of the rock show well developed cleavage planes. 



The thin sections are composed of comparatively coarse fragmental particles of 

 feldspar set in a much more abundant matrix, consisting largely of biotite, ((uartz, antl 

 feldspar. The larger fragments are mostly more than 1 mm. in diameter. They vary 

 in magnitude from this to those so line as to be lost in the nmtrix, their inimbers 

 increasing as their magnitudes dimini.sh. These sections differ from any of the other 

 biotiteschists and biotite-slates in the remarkable freshness of the larger feld.si)ars. 

 They are in their interiors generally clouded but slightly, and the iilagioclase gives 

 shai-p twinning bands. Tlie greater part of the feldspar is, however, orthoclase. The 

 smaller feldspars have all largely and some of them wholly altered to biotite and 

 quartz, and even the larger feldspars are often affected more or less dcei)ly upon their 

 exteriors by this biotitic decomposition, and sometimes complex areas of biotite are 

 found quite a distance in the feldspars. Every stage of this change is seen from per- 

 fectly fresh unaffi'cted feldspar to that in which but a trace of feldspathic material 

 remains in a line aggregate of biotite and quartz. The amount of fragmentid (piartz 

 is very small, not more than from a tenth to a twentieth as jilentilul as the felds])ar. 

 The biotite is in small dark brown folia intiinately mingled with finely crystalline 

 quartz, and all the particles of these two minerals have without doubt formed by 

 the decomjiosition of the feldspar. The absence in many cases of large fragmental 

 grains of(iuartz in the more altered mica-schists and mica-slates is explained by this 

 section. Here are abundant fresh larger fragmental feldspars and but few small frag- 

 mental quartzes. It is plain that this rock was once nminly a. felds])athic sediment, the 

 metasomatic alterations of which have formed a dark colored mica-slate. Doubtless 

 the greater mass of the fragmental material was tiner grained than the unaltered 



