THE ERUPTIVE ROCKS OF MEXTCO 471 
always contain veins of epidote visible in thin layers. The 
greater number of the mineral veins of this locality occur in 
these rocks. In the state of Sinaloa these greenstones abound 
in many of the mining districts, but are more or less altered by 
the contact of the metalliferous veins, now gold, now silver- 
bearing. These microlitic greenstones pass sometimes to an 
ophitic structure, and even to holocrystalline rocks of clearly 
granitic structure giving types of diorites and diabases. In the 
territory of Tepic green andesites occur in great quantity, 
always with analogous characters. In the state of Jalisco green 
dacites occur at the mines of Los Reyes, San Sebastian, and 
Real Alto. 
In Fresnillo and Sombrerete, in the state of Zacatecas, rocks 
of similar aspect are found covered by an extensive formation 
of rhyolite tuffs.*. In the district of La Luz, state of Guana- 
juato, the small size of the mineral elements and the profound 
alteration which the rocks have undergone prevents usually an 
exact microscopic determination of the minerals or the rock 
structures. It is, however, possible in some cases to observe 
characters which show that the rocks approximate to andesitic 
porphyrites or hornblende andesites.* It is interesting to note 
that there exist great similarities between the rocks of La Luz 
and those associated with the rhyolite tuffs in the mines of 
the state of Zacatecas. The fine grain of the former rock has 
indeed caused it to be called a rhyolite tuff. 
As regards the age of the formations of La Luz and those 
about the city of Guanajuato, various conclusions have been 
reached, owing to the absence of fossil remains in the sediments, 
as well as the complex nature of the eruptive regions of the 
vicinity. The rocks of the latter regions, which include the 
hornblende granites of Santa Ana and the granites of the Ser- 
tThe rocks classified as rhyolite tuffs are some of them andesitic tuffs which 
appeared during the eruptions of andesites, and were later impregnated with silica, 
while others were derived from the eruptions of rhyolites. 
?’'The term porphyrite is restricted in use by Ordonez, but is employed by him to 
designate Tertiary rocks differing slightly in appearance from the common andesites 
and showing peculiar alteration. 
