THE ERUPDIVES ROCKS OES Vi xaE@; 
Part III of the recently published Bosguejo Geoldgico de 
México consists of a study by Ezequiel Ordofiez of the eruptive 
rocks of Mexico.t' This may be regarded as the most complete 
and satisfactory summary of the present state of knowledge of 
this subject which has yet been published. 
Humboldt’s Essay on New Spain contains many observations 
on the rocks of the silver-bearing regions of Mexico which 
are made with great accuracy and fullness of detail. But the 
science of petrography has made many advances since Hum- 
boldt’s day. It is no longer sufficient to describe rocks as 
primitive schists and porphyries nor can altered andesites and 
tuffs be disposed of as graywackes. These terms are, however, 
an improvement on sazum metalliferum, the name by which 
many of the rocks were earlier known. Modern geologists, 
moreover, can hardly agree with the great savant in his con- 
clusion that the richness of veins is entirely independent of the 
nature of the rocks which the veins traverse. The great sim- 
ilarity found among the rocks of the silver-bearing regions of 
Mexico and their resemblance to those of Nevada and Germany 
in which similar veins occur, indicates that a definite relation 
probably exists between rock and vein. 
Humboldt’s work, however, remains about the only authori- 
tative one on the rocks of Mexico as a whole, which can be 
consulted. The names which he applied to the rocks and the 
opinions which he expressed regarding their origin will be found 
to be those prevailing in Mexico today. Since the publication 
of his work studies of single rocks or of limited regions have 
been published, but little, if any, attempt has been made to 
correlate observations. There is room, therefore, for a compre- 
hensive study of the kind made by Seftor Ordofiez. 
t Boletinz del Institute Geoligico de México, Nums. 4, 5, y 6, Mexico, 1897. 
466 
