518 REVIEWS 
lower lavas is also noticeable; the lower lavas are basalts containing 
a large amount of olivine; the upper, hypersthene andesites. 
Considering the products of the volcano as a whole, the life of the 
volcano can be divided into three periods, indicating a gradual 
decrease in its heat and energy. These may be denominated the 
period of lava eruptions, the period of breccia eruptions and the period 
of eruptions of ashes. 
Between the successive outflows of the first period, varying inter- 
vals of timeintervened. During the intervals the surface of the outflow 
became covered with masses of cooled and broken lava which were 
cemented by the next outflow into a breccia. These layers of breccia 
are prominent through the lavas and serve to mark the boundaries of 
each outflow. 
During the second period of the life of the volcano, solid material 
was ejected in the form of bombs, blocks of lava and pumice. These 
were more or less assorted by running water and consolidated by argil- 
laceous and ferruginous cements. Their character and relative position 
may now readily be studied in the Barranca of ‘Tlamacas. 
During the last period of the life of the volcano, the coarse, 
black sands or ashes which cover the surface of the upper part 
of the cone were extruded. These have been moved about by 
winds and waters till now they are deposited over the surface of the 
cone in masses of very unequal thickness. Between these deposits are 
intercalated thin lenses of alluvium, indicating periods of repose in 
this phase of the volcanic activity as well. 
Attention is called to the similarity of the material poured forth 
by the volcano to that produced by eruptions in the valley of Mexico, 
and on this fact is based a suggestion as to the age of the lavas. In 
the latter place, the first eruptions of hypersthene andesites are covered 
by sediments in which are found remains of eguus, cariacus, elephas, 
etc., showing that the eruptions must have taken place in the early 
Pliocene. 
The early eruptions of Popocatapetl which produced olivine basalts 
are believed to have been contemporaneous with these, inasmuch as they 
underlie andesitic lavas similar to those of the Santa Catarina volcano. 
The lavas of the latter mountain are known to have been ejected at the 
time when the Pliocene sediments were forming over the valley of 
Mexico, hence the andesite lavas of Popocatapetl may be regarded as 
late Pliocene in age. . 
