54 BULLETIN OF THE UNIVERSITY OP WISCONSIN 



pseudo- breccia. These secondary quartz areas, as seen 

 under the microscope, appear to be composed of much finer 

 grains of quartz than that which is aggregated in spheres 

 and angular shapes. The groundmass outside of these 

 areas shows the fluxion structure, is not mingled with the 

 fine-grained secondary quartz, and there is no character- 

 istic arrangement of the minerals at the boundary between 

 the two. 



CONCLUSION. 



The eruptive rock of the Baraboo region is thus shown 

 by chemical analysis to be a quartz keratophyre, whereas 

 its mineralogical composition alone would class it rather as 

 a keratophyre. Typical volcanic structures of the ground- 

 mass are clearly shown. The broken phenocrysts and the 

 marked fluxion structure indicate the usual motion of a 

 lava flow. In the field are found outcrops of volcanic brec- 

 cia, some of the fragments of which are likewise volcanic 

 breccia. The quartz keratophyre, like the bedded quartz- 

 ite, lies unconformably below the horizontal formations of 

 the Upper Cambrian. The dip of the cleavage planes of 

 the schists conforms to the dip of the underlying quartz- 

 ite beds, which is at a high angle to the north, and the 

 general direction of the contact agrees with the strike of 

 the quartzite beds and the strike of the schists of eruptive 

 origin. The changes in the general direction of the con- 

 tact in the western portion of the area indicates that the 

 quartzite beds were eroded previous to the deposition of the 

 porphyry. It may be that the quartz keratophyre eruption 

 took place when the underlying quartzite beds were hori- 

 zontal, in which case the eruptive material must have had 

 the enormous thickness of 3,300 feet, as shown by its areal 

 extent from north to south. Or it may be that one or more 

 elevations had taken place in the quartzite previous to the 

 eruptive flow, in which case its actual thickness cannot be 

 estimated. Since no quartzite lies to the north of the 



