WEIDMAN — QUARTZ KERATOPIIYRE OF BARABOO BLUFFS 43 



Quartz keratopJnjre.— The quartz keratophyre, which forms 

 the normal rock type of the whole eruptive area, as it oc- 

 curs in the narrow strip at tbe eastern extension of the area 

 is not much fissured, but is jointed and cut by reticulat- 

 ing veins of quartz. It forms rounded ledges and grades 

 into the sericite schist zone, which lies to the south of it. 

 The rock' is very fresh, and breaks with a conchoidal frac- 

 ture. It is red in color, containing in about equal num- 

 bers many beautiful red and white feldspar phenocrysts, 

 some of which are three-eighths inches in diameter. These 

 are imbedded in a brownish red matrix. The rock on the 

 eastern side of the Lower Narrows contains fewer pheno- 

 crysts than that on the western side. Both phenocrysts 

 and groundmass become red on weathering. 



Farther west, at the large northward projecting ridge 

 which occurs in the northeast i of Sec. 21 (Fig. 1, Sec. 

 Ill), the keratophyre ' is found to be unlike that farther 

 east, in that it is much fractured, and the feldspar crystals 

 are rarely apparent in the hand specimen. Reticulating 

 quartz veins from a fraction of an inch to three or four 

 inches in thickness are quite numerous. The fractures 

 which cut the rock run in all directions, so that it breaks 

 and weathers in small fragments, bounded on all sides by 

 plane surfaces. On the weathered surface the rock is a 

 reddish brown, but within it is of a darker hue. 



Within the eruptive area are many detached blocks of 

 quartz keratophyre. Some of these blocks are quite differ- 

 ent from the quartz keratophyre found in place. A de- 

 tached block ' which was found upon the summit of the 

 range in the northwest i of Sec. 21, unlike the quartz 

 keratophyre found m situ, is black in color, and on close 

 examination it shows fluxion structure. Several blocks of 

 similar black colored quartz keratophyre were found near 



1 Specimens 3089 and 3090. The specimen and thin section numbers referred to in this 

 paper are those of the University of Wisconsin Collection. 



2 Specimens 3075 and 3002. 

 s Specimen 3080. 



