164 



F. N. Guild — Eruptive Rocks in Mexico. 



oped in unusually large phenocrysts. The tracliites of Popo- 

 catepetl might better be described as andesites with trachitic 

 texture since plagioclase seems to predominate. 



The chemical composition of the hypersthene-andesites from 

 Popocatepetl is shown by two analyses made by the writer. 

 The first (No. 1, described above) represents a recent outflow, 

 the other (No. 3, from near Tlamacas) is representative of an 

 earlier period of activity. 



3i0 2 



Al.O, 



Fe 2 O s 



FeO 



MgO 



CaO 



Na 2 4-28 



K 2 0. 

 H,0. 

 H 2 0. 

 TiO„ 



Above 110 

 Below 110 



PA 



Mi.O . 

 SrO.. 

 BaO . 

 Li O . 



No. 1. 



No. 3. 



62*51 



58-07 



16-62 



15*83 



1-12 



2-97 



3*75 



3-89 



3-30 



5-56 



5*10 



6-70 



4*28 



3-89 



1-86 



1-73 



•53 



•23 



•15 



•18 



1-02 



1-27 



•23 



•29 



•015 



•01 



•10 



•06 



•03 



•04 



•14 



•07 



trace 



trace 



100-75; 



100-79 



The normative mineralogical composition and position of 

 the rock according to the quantitative classification is given in 

 the table below. 



No. 1. 



Quartz, Si0 2 13-98$ 



Orthoclase, K 9 O.Al 9 8 .6SiO a 11*12 



Albite, Na 2 0. Al 2 3 .6Si0 2 36-16 



Anorthite, CaO.A] 0,.2Si0 20-57 



•52 1 



Diopside 



Hypersthene 



f FeO.Si0 2 

 CaO.Si0 2 1 



MgO.Si0 2 1 



L MnO.SiO, 



j MgO.SiO, 7 



j FeO.Si0 2 3 



Magnetite, FeO.Fe 2 3 1*86 



Ilmenite, FeO.Ti0 2 1-82 



Apatite -31 



Rest -86 



62 ! 

 00 f 

 18 J 



20 | 

 70 X 



3-2' 



10-90 



100-85 



