Plate XXXII. — Graywackes from the Upper Slate Member. 



Fig. 1. MicaceouK giiiywncke. Siiocimcu 9118, slide 2906. Fioui the HE. i of Sec. 11, T. 45 N., R. 1 

 AV., Wisconsin. In i)olariziMl light, x -5. The section shows the general rounded character 

 of the quartz grains, some of which an- now minutely angular by enlargement. The com- 

 plex, minutely crystalline areas rciiresent single fragments of feldsjiar, which are now 

 largely altered to mica and (piartz and thus appear more like tiuely crystalline interstitial 

 material tliaii siMi|)le fragnicnlal grains. (See p. 326.) 



Fig. 2. Biotitic and nuiscovitic graywackc. Specimen 9-I44, slide 3092. From NW. i of Sec. 11, T. 



44 N., R. 3 W., Wisconsin. In polarized light, X t>0. The figure illustrates a more 

 ([uartzosc part of the scc^tion. It slinws nicely the, allcraiion of feldspar. In the center 

 of the tigurc was a simple fragment of feldspar, whicli lias largi^ly altered into mica and 

 quartz, as a result of whieli the rounded area is now a conqdetely cr.vstalline mass of mica, 

 quartz, and feldspar. Its true naturi' is, however, shuwu Vi.v its rounded appearance and 

 by the fact that the many detacheil, small, irregular areas of fBhls[iar efJtiuguish together. 

 (See- p. 316.) 



Fig. 3. Biotitic graywacke. Specimen 9.598, slide 3334. From SE. i of Sec. 1.5, T. 45 N., R. 1 W., 

 AVisconsin. In polarized light, a 60. The Kgure illustrates the .same. thing as Fig. 2. 

 The large roundish area in the center of the figure was once a single fragmental feldspar, 

 but it has almcist wholly decomposed, and thus forms an interlocking mass of mica and 

 quartz. In this case detached areas of ipiartz thus formed are seen to l)e a unit over a con- 

 siderable area. This figure and the previous one illustrate the crystalline character which 

 can result fnmi the alteration of fragmental feldspar. Many individuals of mica and c|uartz 

 have been produced from a single feldspar, and, as th(\y form simultaneously by tlie altera- 

 tion, tliey interlock as completely as if they were original crystallizations, and also with the 

 residual feldspar in case there is any. (See pp. 324-325.) 



Fig. 4. Biotitic and chloritic graywa(■k(^. Specimen 9109, slide 441!S. From the NE. J of See.l2, T. 



45 N., R. 1 W., Wisconsin. In ]>olarlzed light, X 35. A typical fine grained biotitic and 

 chloritic graywacke. The fragmental character of the quartz is plain, although many of 

 the smaller particles are (iuit(^ angular. The fraguumtal feldspar was originally as a)>uii- 

 dant as the quartz, ))ut the figure docs not well show this, as umst of the grains of this 

 mineral are estousively altered to biotitc and chlorite. (Sec p. 325.) 



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