THE ERUPTIVE, ROCKS OF MEXICO 469 
mentary rocks, nearly to the coast of the Pacific and the Isthmus 
of Tehauntepec. 
By far the larger number of the eruptive rocks of Mexico 
_are, however, of post-Cretaceous origin. Among the rocks 
which began this prolonged eruptive epoch, granites and granu- 
lites predominate, syenites are rare, andesitic diorites are abun- 
dant, and diabases sometimes occur. The different varieties, 
however, pass from one to another by insensible gradations, and 
frequently exhibit as well an ophitic or trachytic structure which 
leads them to resemble the true porphyrites and andesites. The 
frequent recurrence of these phenomena serves as a corroboration 
of Iddings’ theory of the differentiation of magmas. The post- 
Cretaceous eruptive rocks which approximate in structure most 
nearly to those of pre-Cretaceous age, occur chiefly in the 
central regions of the northern and northeastern portions of the 
country, and are usually associated with Cretaceous limestones. 
The granulitic structure is that which predominates, but it may 
be modified to that of the granites, or even descend through the 
micro-granulites to the orthophyres and rhyolites. 
The rocks which come after the granites have usually been 
known in Mexico as greenstones or green porphyries on account 
of their characteristic color and porphyritic appearance. To 
these rocks great interest has long been attached since they 
lodge the most important metalliferous veins of the country. A 
chart showing the mines of Mexico well indicates the distribu- 
tion of these rocks. Each metalliferous district presents in the 
mass of its rocks a similar series of eruptions, thus indicating a 
certain contemporaneity and analogy of circumstances of out- 
flow. The three types of rocks found in these districts in the 
order of their appearance are: (a) andesites and green dacites, 
(6) rhyolites, and (c) labradorites and basalts. 
Those of the first type have already been referred to as 
greenstones. They may also be described as andesitic porphy- 
rites, chiefly of hornblende, and orthophyres, while some more 
nearly resemble the amphibole and pyroxene-andesites. All 
present similarities to the rocks described by von Richthofen 
