Platj-; XXXrv. — The Dkvklopmknt of ]\Ii('A Schists and Mica-Slates. 



Fig. 1. Muscovitic biotite-sibist. Specimen 2039, Wisconsin. From the NE. i of See. 6, T. 44 N., R. 

 2 W., Wisconsin. In iioliirized ligtt, X 60. The ])roeesses iif alteration described in the 

 previous plate has here gone so far as to leave but little trace of original fragniental 

 feldspar. Areas of feldspar, however, remain in the section, wliich, while cut by quartz 

 and mica, so as to give all of the minerals a most crystalline material, yet polarize as a unit. 

 (See pp. 320-321.) 



Fig. 2. Biotite-schist. Specimen 1.53, Wright. From the NE. i of Sec. 4, T?44 N., R. 3 W., Wis- 

 consin. In polarized light, X 60. The section is a typical biotite-schist from the 

 Upper slate member. The examination of the thin section shows no evidence of frag- 

 mental origin. It is composed almost wholly of mica and quartz, the two minerals inter- 

 locking in the most intricate fashion. Taken alone it could not be shown that this rock 

 and the preceding were derived from fragmental feldspathie sediments, but taken 

 in connection with the previous figures and the formation in which they are found, it is 

 certain that they have been produced by the alteration of such a rock. The figures of 

 ■the previous plate and 1 and 2 of this i>late constitute a graded series which illustrates 

 well the series of alterations by which there has lieen produced from a completely frag- 

 mental rock, composed chielly of fcldsiiar, a rock which is a typical crystalline schist. (See 

 p. 311.) 



Fig 3. Hiotlte-slate. Specinun 1.30, Wright. From SE. i of Sec. 10, T. 44 N., R. 3 W., Wisconsin. 

 In polarized light, X 60. The section is one of the exceedingly finely crystalline black 

 biotite-slates. That it has been derived from a fragmental felil.spiitliic rock like the mica- 

 schists is .shown by the vagui'ly outlined fragmental feldspars, the alteration of which here 

 produces the finely crystalline biotitc and quartz. (See p, 31.5.) 



Fig. 4. lUotitc-slate. Specimen 9113, slide 2903. From the SE. i of Sec. 12, T. 45 N., K. 1 W., Wis- 

 consin. In polarized light, X 60. The section is (me of the ty])ical finely crystalline 

 liiotite-.slates. It was doul)tless produced from a fragmental feldspar rock, as in the previ- 

 ous figure, although it shows no remaining fragmental feldspar. However, in other parts of 

 the thin section and in the hand specimen the vague outlines of rounded feldspars are seen, 

 (Seep. 326.) 

 516 



