Plate XIV. — From the Southern Complex. 



Fig. 1. Biotite-granite. Specimen 9639, slide 4228. From the NE. i of the NE. i Si'c. 20, T. 44 N., R. 

 3 W., Wisconsin. In jiolarized liglit, X 25. Tlie section sliow.s the strinifjly feldspathic 

 character of tlie rock and tlie abundance ofuonstriated feldspar, altli(iui;li striated feldspar, 

 both mierocline and ]ilaf>ioclase, are seen. The dark areas are, largely biotite. 



Fig. 2. Biotitic granitoid gneiss. .Specimen 9674, slide 3394, steps N., 160, steps W., of the south- 

 east corner of Sec. 23, T. 44 N., R. ii W., Wisconsin. Upon the Marengo river. In polarized 

 light, X 25. The section shows the alteration of feldspar into biotite and quartz. Large, 

 irregular, much altered areas of orthoclase and plagiodase are contained in a fine ground- 

 mass composed of quartz aud biotite. The aggregates of these minerals, besides tilling the 

 interspaces, cut into the lai'ger feldspar areas, so that there is a gradation IVom the fine 

 graineil background into the feldspar. In certain cases this alteration has extended (|uite 

 to the centers of the feldspar individuals. This figure suggests that tlie rock was once very 

 much more strongly feldspathic than at present — perhaps as strongly feldspatlii<- as the rock 

 in the previous figure — and that the alterations h.ave changed it into a strongly l)iotitic and 

 quartzose rock. Th(^ foliation which is now apparent in the rock may also be one of the 

 results of this alteration. 



Fig. 3. Horiiblemle-sehist. .Specimen 90.50, slide 2776. From near the northwest corner of .Sec. 35, T. 

 46 N., R. 2 E., Wisconsin. In polarizid light, X 25. The groundmass of the section is com- 

 posed of finely crystalline, closely interlocking quartz and hornblende. Within this ground- 

 mass are numerous large roundi.sh areas of feldspar, which are now on their outer parts 

 ajqiarently altering into quartz and hornblende. Tlie appearance of the feldspar suggests 

 a fragmental character, but however closely examined, nothing else in the section gives 

 additional light upon this question. If any of the hornblende schists of the Southern Com- 

 plex are clastic (bnt this is doubtful), this is probably one of them. 



Fig. 4. Hornbleude-gueiss. Specimen 9060, slide 2921. From the south part of the SW. i Sec. 33, T. 

 46 N., R. 2 E., Wisconsin. In ordinary light, x 25. The light background is composed of 

 finely crystalline quartz mingled with a good deal of feldspar, the latter commonly being in 

 larger areas than the quartz and often having loundish outlines. These facts are not 

 apparent in the figure. The slide is reproduced to show the peculiar character of the horu- 

 bleude individuals. They run from minute fibers up to tolerably large Idades whicli have 

 extremely ragged outlines. They cut the quartz and feldspar through and through. Their 

 appearance is such as to suggest that tliey are now in the process of growth, and in tliin 

 section the relations of the hornblende and quartz to the feldspar fnrtliei- suggest that from 

 the feldspar these minerals have developed. If all of the minerals now present are original, 

 tlie horniileiide must certainly have been the first to crystallize, so thoroughly does it ])en"- 

 trate the others; but this is very iiiueli less probable than that it has ileveloiied as tl e last 

 mineral of the rock and subsequent to its consolidation. 



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