Q PROCEEDINGS OF THE NATIONAL MUSEUM vul. 74 



Two main faults have been noted, although it is probable that 

 there are more that are less evident. The more prominent fault 

 strikes northeasterly, bordering the ore bodies of Cordon Rangel. 

 Picacho Sur, and Picacho de la Cruz on the south and is responsible 

 for the steep southern slope of the hill. This fault is postulated in 

 large part upon physiographic evidence and upon the fact that the 

 small patches of tuff at the foot of Picacho de la Cruz may be best 

 explained as small down-faulted portions of the tuff of the northern 

 slope. 



A second fault strikes in an almost exactly northerly direction, 

 bounding the iron-ore bodies of Picacho Socavons 4, 1, and 2 on the 

 east and separating the ore body of Picacho Socavon 1 from that of 

 Cordon Rangel. This fault has been encountered in Socavon 1 at a 

 distance of 150 meters from its portal, where it abruptly terminates 

 the ore body. The fault plane is sharp, without much slickensiding 

 but with a considerable brecciation of the ore. 



PETROLOGY 



Latite. — The latite forms a fine-grained rock of a vinaceous gray 

 color (92142).*^ The groundmass is aphanitic, often with numerous 

 irregular gas cavities that contain minute corroded quartz crystals or 

 small plates of specular hematite. The phenocrysts, seldom exceed- 

 ing 5 millimeters in length, are in part glassy feldspar and in part a 

 feldspar partially or completel}?^ altered to sericite or kaolin. An- 

 other mineral that may be detected with a hand lens is iron ore, pre- 

 sumably magnetite, the result of the magmatic alteration of the 

 pyroxene of the rock. No phenocrysts of quartz were observed. In 

 thin sections of the rock the groundmass, while indistinctly bire- 

 fracting in places, appears to be largely glass. Throughout this 

 groundmass there are the usual numerous microlites, the larger ones 

 being chiefly the magnetite residues of resorbed pja'oxene. The 

 phenocrysts are orthoclase with a small optical axial angle or feld- 

 spars now completely or partially altered to a birefracting mass. In 

 the specimens studied most of the feldspar phenocrysts have the op- 

 tical properties of orthoclase but smaller amounts are andesine. No 

 twinning was noted in the plagioclase. Martinez-Quintero ^ describes 

 the latite as being somewhat variable in the proportion of its feld- 

 spars, some specimens having the orthoclase predominant, in others 

 plagioclase is the chief feldspar. Besides the small altered pyroxene 

 crystals in the groundmass there are numerous larger ones in the 

 form of phenocrysts but the original mineral is now entirely de- 

 stroyed leaving only magnetite. Near Pozo de Aguacera is a glassy 



8 The numbers refer to the catalogue number of the spocimens. 

 ' Inst. Geol. Mexico, Col. 44, p. 23, 1923. 



