REVIEWS 519 
Attention is called to the fact that in this same region, volcanoes 
within very short distances of one another were producing at the same 
time, the one basalts, the other andesites. ‘The practical contem- 
poraneity of the outflows is proved by the fact that beneath the lavas 
of each have been found human skulls and evidences of human industry. 
The occurrence is paralleled in modern times by the outflows of 
Jorullo and Colima. The lavas of the former are basalts, of the latter, 
andesites. 
The last pages of the work are devoted to a succinct description of 
the three types of rocks which go to make up the volume. ‘They are 
classified as labradorite basalts, hypersthene andesites, either with or 
without augite, and trachytes. 
The height of the volcano is given as 17,701 feet (5450 meters). 
Observations made by the present writer during a recent visit to 
the volcano, in general go to substantiate the views presented in the 
work under discussion. He can, however, hardly agree with the authors 
in regarding the formation of all the vertical surfaces of the mountain 
as the result of erosion. 
It is especially difficult to understand how the walls of the crater 
could have become vertical in this way. The degradation of cliffs to 
form slopes is a phenomenon of common occurrence, but that the aaah 
reverse can take place seems exceedingly doubtful. ene 
The vertical walls of the crater seem to the writer rather to give 
evidence of the character of the force which produced the latest erup- 
tions. It was an upward force, bringing up from below broken lavas 
and ashes. As the force was sufficient to send these many miles away 
from the volcano, it is not unreasonable to suppose that it was sufficient 
to break through and remove the lavas in the crater. Thus would 
be left a conduit with vertical walls, the only effect of subsequent ero- 
sion upon which would be that of degradation. That this is the process 
going on at the present time, a short stay in the crater makes evident. 
It seems much more probable that the walls of the so-called old 
crater may have been formed, as the authors suggest, by a process of 
erosion, yet their statement of the mode of formation is hardly satis- 
factory. 
Another feature of the volcano which the authors explain as the 
result of erosion, seems to the writer to have had a different origin. 
This is the assemblage of lavas at the point called La Cruz. Here the 
lava lies in great ridges which jut out prominently from the slope of 
