52 BULLETIN OF THE UNIVERSITY OF WISCONSIN 



common in acid volcanic rocks, and occurs to some extent 

 in these rocks. This structure is not apparent in ordi- 

 nary light, but in polarized light on revolving the stage, 

 small areas in the groundmass appear alternately dark 

 and light in patches, giving a mottled appearance. These 

 areas are composed of irregular grains of quartz in close 

 contact with feldspar. They do not have sharp extinction 

 individually, but gradually pass into one another as the 

 stage is revolved. Sections 1 taken from the fractured rock 

 show this structure better than the others. 



Splierulitic Structure. The spherulitic structure is well 

 developed in many of the sections examined. Some of the 

 spherulites are a primary crystallization and are composed 

 of radial fibres of feldspar associated with quartz, and 

 occur in small circular and semicircular areas, 2 and in larger 

 fan-shaped forms 3 (Plate 2, Fig. 2), like those described 

 by Iddings 4 from the Yellowstone Park rhyolites. They 

 are scarcely perceptible in ordinary light, but in polarized 

 light the usual distinct dark cross is seen. Besides 

 those showing the radiating structure there also occur 

 some spherulites which have a granular texture, and are 

 of secondary origin. Similar secondary spherulites have 

 recen ly been described 5 from the acid volcanic rocks of 

 South Mountain. In the Baraboo quartz keratophyre these 

 secondary spherulites as they appear in the thin section 

 are much larger than those of primary origin. In the hand 

 specimen, too, the only sphe^ ilites found are the secon- 

 dary ones. One of the specimens 6 shows many such spher- 

 ulites one-fourth inch across, with lines of flowage passing 

 around them. Under the microscope these spherulites 7 ap- 



i Section 307^ . 

 Section 3074. 

 Section 3080. 



* Obsidian Cliff, Yellowstone National Park, by J. P. Iddings. 5th Ann. Rep. U. S. 

 Geological Survey; pp. 276-279. 



6 The Structures, Origin, and Nomenclature of the Acid Volcanic Rocks of South Moun- 

 tain, by F. Bascom. Jour, of Geol., Vol. I; pp. 817-819. 



8 Specimen 3094. 



7 Stctions 3085, 3094a and 3094b. 



