166 



F. N. Guild — Eruptive Rocks in Mexico. 



especially in the vicinity of the city, they are composed of rocks 

 presenting the appearance of volcanic breccia with cementing 

 material of a dark vitreous andesite, with sometimes small 

 phenocrysts of feldspar; the fragments are of very much the 

 same material with occasionally lighter colored and more por- 

 phyritic varieties. The most common type is that of a dark 

 brown to black compact rock, with a few visible phenocrysts 

 of green pyroxene. These rocks are nearly all opal-bearing, the 



Jfetp^^^iw .^g»r*g? - {■ 



V Q9t& 



Fig. 3. A view of the Sierra de Guadalupe, 

 the foreground. 



Aqueduct of Guadalupe in 



mineral occurring in cracks and occasionally in amygdaloidal 

 cavities.* It is usually colorless (hyalite), but sometimes pre 

 sents various shades of yellow, red and blue. More porphyritic 

 types of these andesites occur in other portions of the series. 

 Microscopically these rocks are similar to those from Popoca- 

 tepetl and are hypersthene andesites. While there is great 

 mineralogical similarity in the specimens from different places, 

 there is also great diversity in the arrangement of the con- 

 stituents, variations in texture due to more or less rapid cooling. 

 The feldspar appears as rods, broken fragments, and larger 

 crystals with zonal structure frequently possessing a clear 

 border, but containing dark inclusions in the interior. The 



* For an excellent description of the occurrence of opal in the eruptive 

 rocks of Mexico, and how they are frequently found filling decayed spheru- 

 litic growths, see, Las Rhyolitas de Mexico, Bui. Num. 14 y 15, Inst. Geol. 

 de Mex. 



