F. JV. Guild — Eruptive Rocks in Mexico. L67 



ground mass varies from a pure glass swarming with crystallites 

 to a semi-crystalline condition, frequently containing obscure 

 globulitic and microlitic growths. Hypersthene andesites of 

 this type have been described from Crater Lake, Oregon,* and 

 other portions of the United States. 



Colima de Chapultepec. 



This is a small rocky eminence rising abruptly from the 

 plains, about three miles southwest of the city of Mexico. On 

 the summit is the beautiful castle of Chapultepec, and around 

 its base are artificial lakes and roadways which make it one of 

 the most beautiful parks in the world. Although isolated, it 

 is considered to belong topographically to the Sierra de las 

 Cruces, a range bounding the valley on the sou th west. f 

 Specimens collected from it appear as dark gray mottled rocks 

 with porphyritic hornblendes and feldspars, the latter being 

 most conspicuous. Under the microscope both feldspar and 

 hornblende are found to occur in two generations, the feldspar 

 in large idiomorphic crystals with zonal structure, and symmet- 

 rically arranged inclusions and smaller rod-shaped growths ; 

 the hornblende is in elongated crystals and is of a basic variety 

 possessing strong pleochroism and dark to opaque borders. 

 The groundmass is partly crystallized, containing needles of 

 both hornblende and feldspar with patches of transparent glass. 

 (A, % 5, p. 172.) 



An analysis of this rock was made by the writer with the 

 following results : 



Analysis of Hornblende Andesite from Chapultepec. 



Si0 2 _.. 62-89$ 



A1 2 3 1642 



Fe 2 8 2-64 



FeO 2-24 



MgO : 2-50 



CaO 4-77 



Na 2 4-07 



K 2 2-15 



H 2 0. Above 110 100 



H 2 0. Below 110 ._ '55 



Ti0 2 -83 



P 2 5 -20 



Cr 2 3 -01 



MnO _. -08 



SrO _ -03 



BaO -07 



Li 2 . . _ Trace 



100-45 



* Diller & Patton, The Geology and Petrography of Crater Lake National 

 Park, U. S. Geol. Sur. PP No. 3, plate xv. 

 f Ordonez, Bui. Num. 2, Ins. Geol. de Mex. 



