WEIDMAN QUARTZ KEEATOPHYRE OF BARABOO BLUFFS 53 



pear to be composed of grains of quartz of smaller size than 

 that which replaces the feldspar, and which is otherwise 

 aggregated in the groundmass. Within the secondary 

 spherulite, too, the quartz seems to be associated with 

 sericite and iron oxide, whereas the secondary quartz other- 

 wise aggregated is free from these minerals. One section 1 

 which contains secondary quartz partly surrounding one of 

 the secondary spherulites shows very plainly both in or- 

 dinary and polarized light the difference between the two 

 structures. 



The aggregations and general appearance of the second- 

 ary quartz in the groundmass is of considerable interest in 

 these rocks. The secondary quartz of the groundmass 

 most generally forms interlocking areas of grains of var- 

 ious sizes, and it is possible that much of it may be chal 

 cedony. Large reticulating veins of quartz up to two inches 

 in width have been described as occurring in large numbers 

 in the eruptive rock in the field. The thin sections like- 

 wise show many minute veins of secondary quartz ramify- 

 ing throughout the rock. The quartz often occurs in an- 

 gular forms, when it very probably is a replacement of 

 ilmenite and feldspar. The spherical shape which the sec- 

 ondary quartz assumes is a very noticeable and interest- 

 ing feature (Plate 3, Fig. 1). These spheres vary in size 

 from one-tenth of a millimeter to one and five-tenths mil- 

 limeters across, and usually have an elliptical outline, with 

 the boundary sometimes slightly crenulated. The quartz 

 in these spheres differs from that in the secondary spher- 

 ulites in being of coarser grain, and in being also unasso- 

 ciated with sericite and iron oxide. Some of the sec- 

 tions, 2 show very large areas of secondary quartz. This 

 secondary quartz is also apparent in the hand specimen, 

 where it appears in areas about an inch in diameter resem- 

 bling greenish brown chert, and the rock is considered a 



i Section 3085. 

 8 Section 3077. 





