56 BULLETIN OF THE UNIVERSITY OF WISCONSIN 



also contains spherulites, a large fan- shaped one being 

 quite well defined in the upper right-hand corner of the 

 figure. 



PLATE 3. 



Photographs of Thin Sections. 

 Fig. 1. 



Fragment of Volcanic Breccia. Section 3085. Polarized 

 light, x 25. In the upper part of the figure is seen a 

 crystal of ilmenite, partly replaced by quartz. Through 

 the groundmass, which shows fluxion structure, are ellip- 

 tical areas of secondary quartz. 



Fig. 2. 



Volcanic Breccia. Section 3096.1. Ordinary light, X 25. 

 This section was prepared from a block and shows in the 

 figure a fractured rock fragment, which has flow lines at 

 right angles to the flow structure of the groundmass. The 

 space between the broken parts of the fragment is filled 

 with secondary quartz. The groundmass of the rock and 

 that of its imbedded fragment are essentially the same. 



