F. JV. Guild — Eruptive Boohs in Mexico. 



169 



Microscopically the rock is atypical fine-grained basalt with 

 about equal quantities of olivine and pyroxene, both of which 

 are clear and undecomposed, and rather difficult to distinguish 

 from one another, since the pyroxene is orthorhombic. The 

 remainder of the rock is made up of fine rods of feldspar 

 crowded together and pressed against the larger crystals of 

 olivine and hypersthene, together with magnetite, and consider- 

 able dark isotropic matter. The writer has described basalts 

 of this texture from Arizona,* and they are common in other 

 parts of the United States. 



Fig. 4. Columns in Basalt, Salto de San Anton, Cuernavaca. 



A section was prepared from one of the large inclusions of 

 andesite in the basalt near Xochitepetl (B, fig. 5, p. 172). In 

 general the structure and composition is similar to that of the 

 andesite of Chapultepec. Hornblende and plagioclase are 

 developed in well formed crystals. The hornblende has been 

 the first to decompose, and some crystals are nearly opaque 

 from the separation of magnetite. 



Basaltic outflows similar to the ones described above are 

 found in the vicinity of Cuernavaca, and in places, as at the 

 Salto de San Anton, have a beautiful columnar structure (fig. 4). 

 An analysis of the basalt from this locality shows it to have 

 the composition given below. 



* Petrography of the Tucson Mountains, this Journal, xx, p. 313, pi. ix. 



