PLATE XYI. 
Fig. a. — Microperthic structure in a Carlsbad twin of anorthoclase in quartz-por- 
phyry. Specimen 181. Slide 181 D. Near the old viaduct on tlie Tapeworm Hail- 
road. In polarized light, X 85. 
Fig. b. — Broken feldspar crystal in quartz-porphyry. Specimen 166. Slide 166 D. 
Near Gum Spring on the Old Furnace road. In polarized light, X 28. The phenocryst 
has been broken and pulled apart and the cracks cemented with sericite scales. Tbe 
ground mass shows the micropoikilitic structure and the micropoikilitic areas are 
surrounded with sericite scales. 
98 
