16S F. JV. Guild — Eruptive Rocks in Mexico. 



Normative Mineralogical Composition and Classification. 



Quartz, Si0 2 . 17'52< 



Orthoclase, K a O.Al 9 3 .6Si0 3 __ 12 -79 



Albite, Na o 0.Al a 8 .6Si0 8 34-58 



Anorthite,"CaO.Al 9 3 .2Si0 3 20-02 



( MgO.SiO, * -90 ) 



Diopside \ FeO.Si0 2 -13 \ 2*18 



( CaO.Si0 2 115 ) 



Ilype^ene ) J^. ™ \ ,03 



Magnetite, FeO.Fe 2 O q 371 



Ilmenite, FeO.Ti0 2 . .' 1-50 



Apatite -30 



Rest s ... 174 



100-27 



Sal 84-91 7 \5 T7 



<[ - > — , Class 11, Dosalane 



Q 17-52 3 . 1 ~ , A ' . 



"F = WW ' < 5~ ^ 7 ' ' ' Austrare 



KO + NaO 89 5 ' ' 3 _, ^ , 



= — -, < - > — , Rang 3, Tonalase 



CaO ~~72' ^3 



o 



K.0 23 3 .. 1 



= — , <C^ ^> 7T > Subrang 4, Tonalose 



Na.,0 66' ^5^7 



Basaltic lava sheets. 



The extreme southern portion of the valley of Mexico in 

 the vicinity of Tlalpam is flooded with an enormous outflow 

 from the craters of Zeutli, Xicalco and Xitli. Although no 

 records are preserved in the ancient Aztec's writings regarding 

 eruptions from these volcanoes, there are abundant evidences 

 showing their recent origin. The lavas are in a very fresh 

 condition and human bones and implements are found beneath 

 them. Their surfaces are very rough, consisting of bowlders 

 and loose fragments of all sizes. Compact and scoriaceous 

 modifications are both represented. The most common type is 

 an exceedingly fine-grained black rock lacking phenocrysts, 

 but filled with numerous small cavities, and containing frequent 

 inclusions of other rocks, usually andesitic in character, 

 especially where the lava stream has encroached upon eleva- 

 tions representing older outflows, as in the vicinity of Cerro de 

 Xochitepetl. 



