AUT. 23 DURANGO MINERALOGY AND GEOLOGY FOSHAG 15 



Quartz. — Quartz is not abundant and is one of the later minerals 

 to form. It is commonly observed as nests of small crystals of the 

 orindary prismatic habit in the open spaces of the sepiolite or as 

 crystal coatings in the brecciated ore, cementing broken martite and 

 apatite, sometimes associated with barite. 



In the soft ore of Cueva de Marmaja harder lenses occur in which 

 the ore owes its superior resistance to a cement of quartz. In this 

 ore the delicate branching forms of the martite are inclosed in a 

 quartz matrix. Some small quartz crystals are also found in the 

 cavities of the compact ore of Socavon No. 2. 



Chalcedony. — This mineral is encountered as a constituent of the 

 augite rock found at the contact of the latite and the ore. In other 

 places typical chalcedony coats some of the earlier minerals, but it is 

 not common. It forms an important constituent of the tuffs where it 

 is secondary and impregnates the rock to a large extent. Massive 

 cherty silica is abundant to the north of the hill where it completely 

 replaces the rhyolite porphyry in such abundance that it is quarried 

 and used as furnace linings. 



Opal. — Some hyalite was noted as clear glassy botyroidal crusts 

 on some of the earlier minerals, especially on the colorless apatite 

 crystals in the vugs of the martite ore of Penascos de la Industria. 



Caldte. — Carbonate of lime is sparingly present as a cement of 

 crushed martite and apatite associated with abundant quartz at 

 Socavon No. 2 and as a replacement of crushed porphyry at Labores 

 de la Cueva. 



BaHte. — Tabular crystals of barite, white to pink in color and 

 aggregated into platy masses were identified associated with quartz 

 and calcite cementing broken martite and apatite at Socavon No. 2. 

 It is of the same generation as quartz and calcite. 



Unknoum 'pseudomorphs. — -There was noted in the cavities of the 

 ore from the Penascos de la Industria associated with delicate needles 

 of goethite, a mineral now completely altered to red hematite. These 

 crystals do not exceed a millimeter or two in size and are tetragonal 

 in crystal form. The forms noted are the prism and a steep pyramid, 

 and their habit is much like that of octahedrite, but the original 

 nature of the mineral is unknown. 



Other minerals have been described from Cerro Mercado but have 

 not been confirmed. Rangel mentions a phosphate of iron from 

 Cueva de la Marmaja and showed the writer some such material. 

 Similar specimens collected from this spot had the rusty appearance 

 of the specimens shown to him but carried no determinable phosphate 

 of iron. Chrustschoff reported, in addition to those already de- 

 scribed, amethyst, fluorite, and garnet. Probably these three species 

 come from some near-by point, but it is doubtful that they came from 

 Cerro Mercado itself. 



