THE EROPTIVE ROCKS OF MEXTCO 467 
In his paper the rocks are described in terms of modern 
petrography and with as much detail as the plan of the sketch 
permits. This plan he states to be an endeavor to give some 
idea of the petrographic provinces of the country, indicating 
for each one of them the predominating species, without enter- 
ing into minute details concerning the extent of each. Com- 
mencing with the pre-Cretaceous rocks, the granites, pegmatites, 
granulites, syenites, and dicrites of this age are described in 
order in their respective provinces. The post-Cretaceous rocks 
are discussed next, beginning with the granites and passing on 
in order through the granulites, diorites, diabases, andesites, 
dacites and rhyolites. Then are taken up what are called the 
andesites of the second invasion, which were produced by the 
volcanic eruptions which began near the end of the Tertiary 
period. Finally the labradorites and basalts which largely 
characterize the latest eruptions are considered. Of especial 
interest is the account given of the rocks of the great silver- 
bearing regions of Zacatecas, Guanajuato and Pachuca, regions 
which though widely separated, the author finds to present 
remarkable uniformity as to kinds of rock and circumstances 
of outflow. 
The work is marred by some errors, such as calling hyper- 
sthene a monoclinic pyroxene (p. 264), and the indiscriminate 
use of the terms amphibole and hornblende. The punctuation 
and paragraphing also admit of much improvement. The char- 
acter of the work is as a whole, however, so admirable, that I 
have thought that to give a résumé of it by means of a transla- 
tion of extracts would assure a wider circulation to some of the 
facts enunciated by Sefior Ordonez than they would perhaps 
otherwise attain. Of such extracts the remainder of this paper 
is made up. 
The first indication of the part of the American continent 
which forms the country of Mexico, was given in Paleozoic 
time by the emergence of a narrow, elongated backbone which 
uniting with the beginnings of the Rocky. Mountains to the 
north and of the Andes to the south constituted the foundation 
