Plate XV. — From the Southern Complex. 



Fig. 1. Hornblende-granite. Specimen 12873, slide 5504. From the NK. i of the SW. i Sec. 23, T. 47 

 N., R. 47 W., Michigan. South of Aurora mine. In polarized light, X 25. The strongly 

 feldspathic character of the granite of the Southern Complex, shown by PI. xiv, Fig. 1, is 

 again brought out, although more quartz is present than in the previous figure. A larger 

 proportion of the feldspar is microcliue. The pegmatitic structure of quartz and feldspnr is 

 nicely shown iu one place. The dark areas are mostly hornblende. In one individual twin- 

 ning is seen. 



Fig. 2. Hornblendo-biotite-syeuite. Specimen 7615, slide 2070. From near the northeast corner of 

 Sec. 27, T. 47 N., K. 47 W., Michigan. In ordinary light, X 25. A coarsely crystalline 

 feldspathic background is apparent. The individuals of feldspar are large and tit in much 

 the same manner as does the feldspar in the massive syenites and granites from this vicinity ; 

 but iu this rock it is cut through and through with hornblende and biotite, which are 

 arranged approximately with their longer axes in a common direction and thus give the 

 rock its foliation. The relations suggest the secondary development of the hornblende and 

 biotite within the feldspar by dynamic mctamorphism. 



Fig. 3. Biotite-gneiss. Specimen 7529, slide 1963. From a short distance south of the north quar- 

 ter post of Sec. 18, T. 47 N., R. 45 W., Michigan. In polarized light, X 25. The section is 

 composed of small particles of nearly uniform size, consisting of quartz and feldspar min- 

 gled with biotite. The roundish and yet closely fitting character of the quartz and felds- 

 par is nicely shown. There are also seen a few larger roundish grains of (juartz. This is 

 one of the rocks of the Southern Complex which has a strong fragmental appearance, and 

 yet there is no certain evidence that it is clastic. The parti(*les now perfectly fit one another, 

 therefore they could not thus have been mechanically deposited. None of them show evi- 

 dence of enlargement, so the crystalline appearance can not be .accounted for iu this way. 



Fig. 4. Hornblende-gneiss. Specimen 94.58, slide 3077. From near the south quarter post of Sec. 

 16, T. 47 N., R. 45 W., Michigan. In ordinary light, x 25. The background of the section 

 is composed of small, perfectly fitting, roundish granules of quartz and feldspar in nearly 

 equal quantity. Contained within this gronndmass are numerous large crystals of horn- 

 blende, which in transverse sections often have well developed crystal outlines, the forms 

 being usually the pinacoid and unit prism. Facli individual of hornblende includes many 

 grains of quartz and feldspar. Tlir hornblende must have been here the last mineral to 

 develop, since it includes so large a proportion of the other minerals. Its relations to the 

 other minerals taken in connection with its crystal forms make its occurrence analogous 

 to such metamorphic minerals as garnet and staurolite. 

 478 



