THE UPPER SLATE MEMBER. 317 



At first siglit the sections appear like ordinary l)iotitesc,liists. They coutaiii 

 grouuflmasses of quartz, througli which arc plentifully and uniformly scattered 

 brown folia of biotite and less numerous flakes of muscovite. On closer examination 

 many of the larger grains of this quartz are plainly fragniental, and they have often 

 been enlarged. The biotite is to a large extent secondary to feldspar, many flakes of 

 this mineral being found mingled with a small titiantity of feldspathic material, which 

 throughout a considerable area acts as a unit. In a few places fragments of ortlio 

 clase and plagiodase are found, in which tlie mica-schist decomposition is but begun, 

 but in the greater part of the sections but little of the original feldspar rcnuiins. AVe 

 have now the explanation of the vaguely outlined spots seen in the hand si>ecimens. 

 They represent original fragmental feldspars, which once couii)osed a very large pro- 

 portion of the rock; but now in most cases they have so completely altered to biotite, 

 muscovite, and (juartz that only here and there in the thin section is an area well out- 

 lined. This rock was then originally like 2, but now ajjproaches very closely to a 

 typical mica-schist. In l! there is everywhere a plain distinction between the feld- 

 spathic and nonfeldspathic areas. The larger portion of the sections resembles tlie 

 finegrained matrix of U. Oxide of iron in small particles is an abundant inclusion 

 in the biotite, feldspar, and mucli of the (juartz. The quai-tz containing such ([uaiiti- 

 ties of this oxide of iron is taken to be secondary, the larger plainly fragmental 

 grains being free from this ferrite. 



19. Muscovitic and biotitic graywacke, from west side of fault at an ui)per mid- 

 dle horizon. Specimen 9540 (slide 331(i), Irom IGOO N., W., Sec. 10, T. 4Jr N., R. 3 

 W., Wisconsin. 



A light gray, fine grained rock, which is quite massive, but which breaks most 

 readily along its obscure schistose plane. 



The section is chiefly conqmsed of particles of (piartz and feldsjiar of greatly 

 varying sizes and quite deeply interlocking. At first sight the only indication of a 

 fragmental nature is the general rounded contours of the larger ])article.s of quartz 

 and feldspar, these grains being minutely and sharply angular upon their borders. 

 Upon (dose examination many of the grains of (juartz show indications of enlargement. 

 The lines between the supposed nuclei and their enlargements arc in no case so 

 distinct as in the ferruginous fragmental (|nart/.ites, but consist of small detatched 

 particles of chlorite, kaolin, air bubbles, and sometimes iron oxide. In a few cases 

 the (H)res of the (juartz grains are clearly made out. Oftentimes there arc consiilera 

 ble breaks in the lines, and tlie whole line in most grains might be taken to he ordi- 

 nary inclusions. However, the constancy of the oval or rounded forms of tlie inclu- 

 sions and their positions near thi-, exteriors of the large grains lead to the conclusion 

 that they mark the outlines of true fragmental cores whitth have become enlarged 

 until they interlock. The irregularity of the exterior of the grains of feldspar is due 

 to another cause, their partial alteration to chlorite, muscovite, and biotite, w ith con- 



