F. N. Guild — Eruptive Rocks in Mexico. 163 



which is doubtless due to more acid plagioclases in the ground- 

 mass. Pyroxene is present in both orthorhombic and monoclinic 

 varieties. Some are slightly pleochroic. The groundmass is a 

 dark, nearly opaque glass. 



No. 2 was also taken from the rim of the crater. It is more 

 compact and contains much less conspicuous phenocrysts. The 

 microscope shows the same mineralogical composition as in 

 No. 1. The feldspar is developed in much smaller crystals and 

 twinning and zonal structure is not marked. Nearly all of the 

 crystals are tilled with inclusions of the groundmass. 



No. 3 was taken some distance below the snow line from a 

 large ledge near Tlamacas and extending towards the north. 

 Hand specimens appear as medium dark gray rocks of non- 

 porphyritic structure. The material is very uniform in dif- 

 ferent portions of the same mass and probably represents an 

 outflow of considerable extent. Green crystals of pyroxene 

 and occasionally a small fragment of feldspar can be seen with- 

 out the microscope. An optical study shows that the crystals are 

 well formed and possess sharp angular outlines. Zonal structure 

 and twinning are both developed as in No. 1, but the pyroxenes 

 are much more abundant and hypersthene greatly predominates 

 over augite. The groundmass is a felt of magnetite, pyroxene 

 microlites and obscure feldspar rods. An optical determination 

 of the feldspar crystals makes them mostly labradorite of 

 medium acidity, though a few belong to less basic types. 



No. 4 was taken from the Barranca de Tlamacas. It may 

 be classified as a dark yellowish gray to brown andesitic pumice. 

 It is filled with large cavities one or two centimeters in diam- 

 eter, at or near the center of which is frequently to be found a 

 small rounded mass of dark glassy material sometimes contain- 

 ing a few phenocrysts and supported by radiating threads of 

 delicate glass which appear not unlike spiders' webs. The 

 microscope shows this rock to be made up mostly of threads 

 and fragments of light yellow glass with only an occasional 

 crystal of feldspar or pyroxene. 



Many other specimens, collected from various points about 

 the volcano, show practically the same mineralogical composi- 

 tion as those described but with variations in structure and 

 texture due doubtless to more or less rapid cooling rather than 

 to difference in chemical composition. Basalts and trachites 

 have been described from Popocatepetl by Aguilera and 

 Ordonez. The basalts occur as representatives of older out- 

 flows and are not at all abundantly represented. The writer 

 did not observe any occurrences of this type of rock during 

 his recent expedition to the volcano. Specimens, however, 

 studied at the Instituto Geologico de Mexico, through the 

 kindness of the Director, appear as typical fine-grained basalt 

 with abundant olivine. The latter mineral is sometimes devel- 



