■\VEIDMAN — QUARTZ KERATOPHYRE OF BARABOO BLUFFS 51 



Magnetite occurs in considerable quantity as shown by 

 the action of the magnet on the powdered rock, and the 

 microscopic sections show it disseminated in small particles 

 throughout the groundmass. Besides the magnetite there 

 is considerable amorphous iron oxide or ferrite present, 

 which appears as a red substance about cavities ^ and in 

 streaks through the groundmass. 



Tlie Ch'oundmass. — The groundmass of the quartz kerato- 

 phyre is holo-crystalline and composed of quartz and feld- 

 spar, stained more or less with oxide of iron. It is crys- 

 tallized in at least three structures which are common to 

 volcanic rocks, viz. ; the fluxion structure, the poikilitic 

 structure, and the spherulitic structure. 



Fluxion structure. — In all the sections examined under 

 the microscope the rock shows clearly sinuous lines of flow 

 in the groundmass. Tbese lines of flow curve and wind 

 about phenocrysts, and give them the appearance of eyes. 

 They also curve about the fragments and between the 

 broken parts of fragments and phenocrysts. The frag- 

 ments about which flowage is apjoarent in the hand speci- 

 men have also a flow structure of their own and in one 

 case - a fragment within a fragment showed the typical 

 flow lines under the microscope. One section ' (Plate 3, Fig. 

 2), shows quite well the flow structure in a volcanic breccia, 

 in which the lines of flow in the fragments are at right angles 

 to those in the surrounding mass. In the sericite schists 

 the fluxion structure is clearly defined, which as well as 

 the field relations proves them to be the metamorphosed 

 equivalents of the quartz keratophyre. 



Poikilitic Structu7-e.— This structure, which has recently 

 been described by Haworth,' Williams,' and others, is quite 



1 Section 30r6. 



2 Section 3084.1 



3 Section 3096. 



* A Contribution to the Archean Geology of Missouri, by Erasmus Haworth. Am 

 Geol., Vol. l;p. 368. 



s On the use of the terms Poikilitic and Micropoikilitic in Petrography, by G. H. Wil- 

 liams. Jour, of Geol., Vol. I; pp. 170-179. 



