24 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 14 



Forming an ultrabasic phase at the contact, a part of the iron must have 

 accompanied it and upon solidification the globules of oxide of iron which were 

 first suspended became imprisoned in the pyroxeniie, as one sees in the samples 

 tliat have been studied. 



Later, when the magma had solidified, the gaseous emaviations of water vapor. 

 P^Ob, CO2 CI, and some others, as well as the magnetic solutions, contributed 

 to the enrichment of the mass of iron already formed, transporting iron from 

 the lower to the higher portions and being deposited in the same oxides through 

 simple crystallization or through substitution of the adjacent igneous rocks. 

 At the Cueva de la Marmnja one encounters vestiges of the rock which was not 

 completely substituted but altered and which appears to belong to the por- 

 phyritic rock in which the K2O predominates over Na^O. 



Salazar-Salinas's ^*^ own views are expressed in the following 

 citation : 



There was formed, at a profound depth, a magmatic chamber of basic nature, 

 which moved through a fold of sedimentary formation, this same fold accen- 

 tuating its effect upon the pressure was able to bring about the fusion of the 

 deeper siliceous sediments, giving a new magmatic chamber which, through 

 the lesser density of its constituents penetrated the basic chamber and ter- 

 minating in superposition, staying in part intermixed, one with the other; 

 or admitting the existence of the acid effusive rock, this was penetrated by 

 the magmatic products leaving traces of this passage in the phenomena of 

 metamorphism and in the mixture of the elements. 



To properly explain the origin of the ores of Cerro Mercado 

 account must be taken of the following features: 



1. The occurrence of iron ore in Tertiary acid volcanic rocks, 

 including rhyolites, latites, and tuffs. 



2. The occurrence of iron ore in broad tabular bodies. 



3. The occurrence of iron ore as martite, the pseudomorphous 

 form of hematite. 



4. The association of the minor mineral constituents, especially the 

 apatite, with the ore. 



o. The alteration of the wall rock to pyroxene, chert, and mont- 

 morillonite. 



6. The numerous anastamosing veiniets of ore in the wall rock. 



The objections to the ores of Cerro Mercado having originated 

 as a flow of magnetite are quite evident. The ore bodies are without 

 doubt a later introduction and not contemporaneous with the inclos- 

 ing rocks. Alteration of the vvall rock has taken place to a per- 

 ceptible degree and the ore bodies are not concordant v>dth the posi- 

 tion of the flows of volcanic rock but are inclosed in all the types 

 of rock exposed. 



The explanation that the ore deposits are dikes seems at first a 

 logical one but continued exploration under Seiior Rangel's direc- 

 tion has demonstrated that they are not in the form of dikes, but 

 that their mass has no great downward extension and that they are 

 more in the nature of flat lying tabular bodies of ore. With this 



^^^Salzar-Salinas. Inst. Gcol. Mex., Bol. 44, 1923, p. 44. 



