POPOCATEPETL AND IXTACCIHUATL—FARRINGTON. 8I 
from the valley of Puebla in favorable weather the western wall 
of the crater may be seen as a thin, black streak standing above 
the snow-mantled eastern slope. 
When approached nearer the apparent regularity of the cone dis- 
appears, for while the upper portion, clothed with sand and snow, 
presents a nearly uniform slope, into the middle portion descending 
waters have cut numerous channels, and the lower portion is involved 
in the tortuous folds which make up the Sierra. Onthe northwestern 
slope, too, the sharp, jagged peak known as the Pico del Fraile rises 
to a height only about 1,000 feet less than that of the principal cone, . 
and cutting an amphitheater-like section out of the mountain destroys 
its symmetry. 
Popocatepetl belongs to the class of volcanoes known as strati- 
fied, because of the resemblance to sedimentary formations which is 
presented by the successive layers of lava of which it is made up. 
In the character of the lava of these successive outflows Aguilera 
and Ordofiez* have traced a marked gradation corresponding to a 
gradual decrease in the heat’and energy of the volcano. Thus the 
lavas of the lower currents they assert to be granular, devitrified and 
of trachytic structure, while those of the upper are vitreous and 
amorphous, with, however, a few phenocrysts. 
This difference in structure they explain on the ground that the 
upper currents being farther from the source of heat were more nearly 
cooled when poured out and thus became solid before time sufficed 
for crystallization. A difference in mineralogical composition of 
these lavas is also noted. Those of the lower areas are basalts con- 
taining a large amount of olivine; those of the upper, hypersthene- 
andesites. These authors also believe that three periods in the life of 
the volcano, marked by the character of its products, can be distin- 
guished and that these correspond with a gradual diminution in its 
heat and energy. These periods they characterize as the period of 
lava eruptions, of breccia eruptions and of ash eruptions respectively. 
This view has been justly criticised by a reviewer + as presum- 
ing conditions so extraordinary as to be hardly probable. It is far 
more likely that the change in products from lavas to breccias and 
breccias to ashes has been many times repeated in the life of the vol- 
cano. | 
The activity of the volcano in modern times has been confined to 
the emission of steam and sulphur from the crater. It is, in other 


*Expedicion cientifica al Popocatepetl, p. 23. 
+Neues Jahrbuch, II Bd., Thiel III, p. 270, Stuttgart, 1896. 
