172 



F. JSf. Guild — Eruptive Rocks in Mexico. 



A B 



Fig. 5. Description of Photomicrographs illustrating the types of ande- 

 sites in the valley of Mexico. 



A. Porphyritic hornblende andesite from Chapultepec, showing both feld- 

 spar and hornblende in two generations. The groundmass is nearly all 

 crystallized material. 14 diameters. 



B. Hornblende andesite from Xochitepetl. Zonal structure in feldspar. 

 Crystalline groundmass. Crossed nicols. 14 diameters. 



C. Andesite of basaltic aspect from Popocatepetl. Abundant feldspar 

 and less pyroxene in a vitreous groundmass. Pyroxene cannot be dis- 

 tinguished from feldspar in the cut. 14 diameters. 



D. Andesite of basaltic aspect from Sierra de Guadalupe showing a large 

 hypersthene with smaller feldspar crystals in a semi-vitreous groundmass. 

 14 diameters. 



A and B are light-colored porphyritic andesites representing an old out- 

 flow of great extent. C and D are megascopically non-porphyritic and 

 belong to an outflow of recent date. 





