WEIDMAN — QUARTZ KERATOPHYRE OF BARABOO BLUFFS 37 



The eruptive rock, which was described as quartz por- 

 phyry by the state geologists, is a quartz keratophyre, 

 and occurs in contact with the quartzite along the north 

 side of the north riange in the vicinity of the Lower Nar- 

 rows. This rock was not discovered until 1874 or 1875 by 

 Irving and his co-workers upon the Wisconsin Geological 

 Survey, and all results of work done upon this formation 

 are embodied in the state geological reports. 



Irving ' in 1877 described the porphyry as occurring on 

 the west side of the Baraboo River at the Lower Narrows. 

 He considered the porphyry to be very distinctly bedded, 

 with an east and west strike and a dip of 58° to 60° to the 

 north. He also found that near the quartzite it changes 

 to a distinct schist, which he thought to be allied to the 

 greasy quartz schists at Devil's Lake. Farther west at the 

 northern limit of the porphyry, he found the much 

 fractured area. A schistose structure, which is apparent 

 in places, was taken as bedding and as evidence of the 

 clastic origin of the rock. A specimen w^as analyzed and 

 found to contain 71.24 per cent, of silica and a notably 

 large quantity of soda as compared with potash. 



Chamberlin,' in 1882, mentioned the massive quartz por- 

 phyries which overlie the Baraboo quartzites and referred 

 to their origin as yet in doubt. According to him they 

 might be either a metamorphosed silt-like sedimentary 

 rock; or they might be of eruptive origin, in which case 

 they must have constituted immense overflows of molten 

 rock closely allied in chemical composition to rhyolite. 



Irving," in 1886, in referring to the Baraboo quartzites, 

 stated that " the bedding structure of the quartzite of these 

 ranges and of its associated schists and felsitic porphy- 

 ries, which are taken to have been great eruptive flows, I 

 studied with a great deal of care a number of years since 

 * * *." It is evident from this that he was inclined to 



1 The Baraboo Quartzite Ranges, by R. U. Irving. Geoi. of Wis., Vol. II; pp. 513-515. 

 - General Geology of Wisconsin, by T. C. Chamberlin. Geol. of Wis., Vol. I; p. 87. 

 3 On the Classification of the early Cambrian and pre-Cambrian Formations, by R. D. 

 Irving. Tth Ann. Rept. U. S. Geol. Surv., 1885-6; p. 407. 



