May, 1904.’ GEOLOGY: OF WESTERN MEXICO—FARRINGTON. 227 
from below and filled a fissure, or that, on the other hand, a separa- 
tion took place into an acidic and basic portion while the magma was 
still in a fused condition. If the former action took place one would 
expect to find evidences of contact metamorphism such as have not 
yet been observed. The latter supposition seems equally untenable, 
’ however, owing to the difficulty of conceiving of the separation of 
a magma into proportions so acidic on the one hand and so basic on 
the other. That masses of magnetite of considerable bulk may be 
of eruptive origin seems to be generally conceded, several occurrences 
being usually so explained. In most of these cases, however, the 
accompanying rocks are basic in character, so that it is not difficult 
to conceive of the separation of iron from sucha magma. As already 
noted, however, the rocks accompanying the iron oxide at the Cerro 
Mercado are acidic in character. 
At many points on the mountain the line of demarkation between 
eruptive rock and iron oxide is sharp and distinct, while at other 
points a gradation is traceable. Thus Rangel says,* ‘‘at the base is 
seen an enormous bank of compact rhyolite; a little higher up appears 
a deposit of fragments of rhyolite; higher up yet the fragments of 
rhyolite are covered on some of their sides with ferruginous material 
crystallized in small octahedrons and mixed with pieces of the mineral. 
The proportion of mineral increases as the height until it constitutes 
exclusively the material.”’ 
Witherbee also describes} “large lumps of what appears to be 
solid iron ore, which, upon breaking, prove to be only pieces of can- 
tera covered with a superficial coating of iron oxide, often highly crys- 
talline, and only from 0.25 to 0.5 in. thick, as though the rock had 
been immersed in molten iron oxide.’’ Moreover, the studies made 
in the preceding pages show a change in the substance of the rock 
itself in approaching the mountain. It becomes more basic, though 
not highly so, and shows a gradual increase in content of iron. Do 
such phenomena as these represent primary or secondary changes? 
If primary, is the iron oxide younger than the undoubted eruptives? 
If so, has it flowed over them, or has the separation been of a mag- 
matic nature? If secondary, can the phenomena be referred to the 
action of infiltrating waters? Such are some of the questions regard- 
ing the origin of the mass of iron oxide which remain yet to be 
answered. Metasomatic processes, to which, for several reasons, the 
origin of the iron might be ascribed, should, it would seem, more 
extensively impregnate the adjoining rocks than has been the case. 

*Translation by Kirby Thomas, Mining World, Feb. 12, 1904. 
{Trans. Am. Inst Min. Eng., Vol: XXXII.. p. 156. 
