PLATE XX. 
Fig. a. — Perlitic parting in an aporhyolite. Specimen 279. Slide G. H. W. (un- 
numbered). Raccoon Creek, Franklin County. In ordinary light, X 88. 
Fig. b. — The same. In polarized light, X 225. A quartz-feldspar mosaic obscures 
all trace of glassy structures. 
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