WEIDMAN — QUARTZ KERATOPHYRE OP BARABOO BLUFFS 55 



eruptive area, the quartz keratophyre is probably the top- 

 most member of the Huronian rocks in this region, and was 

 upturned by the orographic movement which tilted the 

 quartzite beds into their present i)Osition. 



EXPLANATION OF PLATES. 

 PLATE 1. 



Geological Map of a Portion of the North Range of the Bara- 

 boo Bluffs. The scale is one and one-half inches to the 

 mile. 



PLATE 2. 

 Photographs of Thin Sections. 



Fig. 1. 



Volcanic Breccia. Section 3079. Polarized light, x 25. This 

 figure shows a large number of fractured soda-orthoclase 

 phenocrysts. Near the lower side is a fragment of for- 

 eign rock. The figure also shows crystals of ilmenite 

 partly altered to leucoxene. The groundmass of this slide 

 shows the fluxion structure, though less plainly than many 

 of the others. 



Fig. 2. 



Black Quartz Keratophyre. Section 3080. Polarized light, 

 X 25. This section was prepared from a block and shows 

 a large broken phenocryst of soda-orthoclase. In the lower 

 left-hand corner are several small crystals of soda-ortho- 

 clase, belonging to a younger generation than the large 

 broken one. Quite near the large phenocryst in the 

 upper part of the figure are a few radial fibres of biotite. 

 The groundmass shows clearly the lines of flow passing 

 between the parts of the phenocryst. The groundmass 



