VOLCANIC PHENOMENA ABOUT CITLALTEPETL AND PO- 
POCATEPTL. 
A 
BY CHARLES R. KEYES. 
(Abstract.) 
Two noble mountains fired the fancies of the native races of 
Mexico before the Aztecs. Quiescent at present they were within 
historic time quite active. They are the ‘“Star Mountain,” or 
Citlaltepetl, and the “Smoking Mountain” or Popocatepetl. Both 
tower above the great Mexican tableland, itself 7,000 feet above 
tide-level; the first named on its eastern margin and the last men- 
tioned in the central part overlooking the capital city. 
Those scientists who, during the past summer attended the ses- 
sions of the Tenth International Geological Congress held in the 
city of Mexico, and who took part in some of the special excur- 
sions, had ample opportunity to inspect these volcanoes in con- 
siderable detail. 
These mountains are respectively 18,314 feet and 17,876 feet 
above the sea. 
Popocatepetl is the more widely known of the two mountains. At 
the present time the volcano exhibits little signs of activity. How- 
ever, a few solfataras exist in the bottom of the chaldron and 
continue to exhale some aqueous and sulphurous vapors. There is 
still enough heat to continually melt the snows, in consequence of 
which the center of the crater contains a diminutive lake. The 
snow-cap begins at about 14,000 feet. 
The lavas appear to be chiefly dark-colored hypersthene andesites 
with an occasional basaltic flow. The latter are the older. Miner- 
alogically the andesites consist mainly of albite, anorthite, hyper- 
sthene and quartz with some orthoclase and diopside. Magnetite 
illmenite and apatite occur in small amounts as accessories. 
The chemical compositions of the andesites, as recently made out 
by Guild, are as follows : 
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