Plate XXXIIl. — The Devklopment of ISIica-Slates. 



Fig. 1. Hiiititic ami iruisidvitic grii.v\v:irkr, Si«^(.'iiiicii H.")!!, sliilc .'idlll'. From the NW. i of Sec. 11, T. 

 ii N., U. 3 W., WiseoMsiu. In llolurin(^(l lislit, X <)l). The sectinu is one of the lea.st 

 altered feldspathie frayineutal rocks. Large rounded areas of feldspar make n]i most 

 of the section, althou.uli .sniallcr particles of fragmental feUlspar and ([iiartz arc contained. 

 The incipient alteration of the feldspars to the micas is ol)surved, and the .smooth ont- 

 lines of tlu' fraismental grains have already been lost by alteration. This is the original 

 ])hasc ol' roik. the alterations of which carried to Ihe extreme! ha\e jiroduccd the crystalline 

 mica-schists. (.See p. 316. ) 



Fig. 2. Biolite-slatc. Siieeimeu 126, Wright. From the NK. j of Sec. it. T. 44 Js'.,-K. 3 W.. Wisconsin. 

 In polarized light, X 25. The section is fr<nn a rock which was originally like Fig. 1. 

 The alterations olithe feldspar to the micas and cjnartx lia\ e been carried to a greater degree. 

 The regular outlines of the original areas are wholly lost, so that while they have a general 

 roundish or oval form their exteriors are minutely angular. The micas are much more 

 abnnilaut at and near the ('Xteriors of the grains of fclds])ars than in the interiors, although 

 the alterations have extended (piite to the centers, (See \t. 310.) 



Fig. 3. Kiotite-slatc. Spcciiricii IL'S, Wright. From the NW. i of Sec. !t, T. 44 N., R. 3 W., Wis- 

 <-onsiii. Ill polarized light, X 25. Thi' original condition of this rock was precisely like 

 that of Figs. I and 2. The series of alteratirtTis there shown liavi-. h()wever, gone much 

 larthcr. Thclragmenlal character of tin; feldspais is iinite lost. They have been pene- 

 trated by sccoudaiy mnseovite and biotite until they arc most irregular in form, only 

 ;i single grain a]ipr<i\imately I'claiuing its (t\;tl outline. As a further result of this process 

 a considerable ])ioportion of interstitial crystalline (piarlz and mica, as compared with the 

 two previous ligures, is found, which has conu' directly from the decomposition of the 

 feldsiiar, (See p. 310.) 



Fig. 4. Hiotite-slate. In polaiized li.ght, X 25. This ligure is taken froLU part oC the same section 

 as the last, ill which the alterations .just described have gone still farther. The crystal- 

 line i-haracter of the dev*doj)ing rock is better shown, 'riicrc I'cuiain. however, areas of 

 fehls|var which act as a unit, although they ari' iiit in every direction liy the alteration 

 jiroducts, ipiartz and mica. (See p. 310) 



514 



