22 PROCEEDINGS OF THE NATIONAL MUSEUM vcr.. 74 



4. The anastamosing veinlets that cut and brecciate the country 

 rock. 



5. The alteration of the wall rock to a pyroxenic skarn. 



6. The tabular form of the ore bodies. 



The resemblances are much greater in those portions of the Kiiruna 

 district ^Yhere the original magnetite has been secondarily changed 

 to hematite corresponding to the martite ore bodies of Cerro Mer- 

 cado. One may reasonably conclude that the hematite ores of Cerro 

 Mercado are forn:ied by the secondary oxidation of magnetite, such 

 as those Geijer describes at " Professorn," the oxidation having been 

 carried to the almost complete alteration of the magnetite. 



ORIGIN 



A number of writers have expressed views on the origin of the 

 great ore bodies of Cerro Mercado but only two are the result of a 

 sufficiently detailed examination to merit serious attention. These 

 are the works of Manuel Rangel ^^ and Leopoldo Salazar-Salinas 

 and his coworkers.^^ We may, however, mention very briefly some 

 of the earlier views. 



Frederico Weidner ^° one of the first to visit the locality and to 

 report upon its iron resources held that the ores were the result of 

 volcanic activity: 



One finds rounded ore masses from Cerro Mercado of the shape and figure of 

 projectiles scattered on all the immediate terrain, not only on the lower por- 

 tions, as is natural, but also on some of the higher hillocks which seems ex- 

 plainable only that the volcanic force of the hill to have flung and transported 

 them to these points. 



In the porphyry hills which encircle Cerro Mercado one finds the most ap- 

 parent vestiges of volcanic action of the mass of Cerro Mercado, because there 

 one finds that the porphyries are altered in color, luster, and texture as if they 

 were smelted and fragments of the porphyry rock are found inclosed in the 

 crystallized magnetic iron ; on the south side of the Cerro the porphyry incloses 

 particles of micaceous iron, which could have penetrated it only through sub- 

 limation ; to the southeast side the porphyries are wrapped in oxide of iron to 

 the extent of being partially converted to almagre; in the central part of Cerro 

 Mercado and its folds lies pieces and banks of- destroyed porphyry evidently 

 lifted by the iron ; all of which proves that the Cerro Mercado is of an origin 

 later than the porphyry in which it rests, that the porphyry previously occupied 

 the place now of metal. This last, impelled by volcanic force has burst the 

 floor of the valley, breaking through the porphyry, dislocating, lifting, and 

 breaking some of it in its path and involving in its mass many fragments of 

 the same rock which it has just destroyed. 



1* Los criadoros de flerro del Cerro de Mercado, Durango. Bol. 16. Inst. Geol. Mexico, 

 1902. 



" El Cerro de Mercado, Durango. Bol. 44, Inst. Geol. Mex. 



^Anales del Ministerio de Fomento. Mexico. Vol. 3, pp. 169-170, 1877. 



