202 Geological Excursion into Central Wisconsin — Hall. 



In this darker colored rock there lies a porphyritic substruc- 

 ture. When fresh fractures are held to the light reflecting 

 surfaces sometimes as large as three-fourths of an inch across are 

 seen. These are cleavage planes of microcline crystals. Within 

 these areas the microcline serves as the matrix in which are em- 

 bedded large numbers of individuals of hornblende, biotite, albite, 

 and oligoclase; also, epidote, apatite and quartz. These minerals do 

 not crystallize perfectly, yet they take on quite strongly in- 

 dividualized forms. There is a vague tendency among them to 

 assume a direction generally parallel to the axis c of the micro- 

 cline matrix in which they lie. This structure is distinguishable 

 by the unaided eye only by the reflection of the microcline cleavage 

 planes; the inclusions within the microcline are like, in all their 

 external features, the same constituents within the matrix rock in 

 which the microclines are in turn embedded. The microscope 

 emphasizes this structure in showing a strong parallel extinction 

 in the microcline areas, a very fresh and limpid condition 

 of this mineral and the very new and unaltered condition 

 of all the minor constituents embedded within it. Figure 

 4, plate III is an attempt to sketch the features described. It 

 seems quite analogous to the lustre mottling among the melaphyres 

 of Lake Superior.* 



Further down the river at Morin's farm, Section 30, Town 

 33, Range 7 E, many knobs of gneiss stand above the surface 

 striking N. 10° E and dipping westerly at 60°. Outwardly this 

 seems to be a typical gneiss in color, texture and structure. The 

 lamination is strong and moderately fine. At the surface the bio- 

 tite has assumed a golden hue in the weathering, but beneath it is 

 bright and clear. The microscope shows that peculiar oil-vitreous 

 lustre which so often characterizes changing acidic rocks. The 

 mineral constituents, particularly the feldspar, are iu two genera- 

 tions, an older with gnawed outlines and corroded and changed in- 

 teriors and a younger with limpid clearness, polarizing strongly 

 and sharply defining each other's boundaries. The former are well 

 sized individuals, the latter generally quite minute. Much epidote 

 characterizes this rock. It lies in small clustered grains or in 

 larger individuals around which are usually grouped minute 



^Raphael Pumpelly, Proc. Am. Acad. Arts Sci., Vol. xiii, p. 260. See 

 also R. D. Irving, Copper bearing rocks of Lake Superior, Mon. v.U. S. Geol. 

 Survey, p. 42. 



