R. T. Hill — Hematite and Martite of Mexico. Ill 



Art. XT. — The Occurence of Hematite and Martite Iron 

 Ores in Mexico ; by Kobert T. Hill. 



The iron ores of Mexico have been frequently mentioned, 

 but the writer is aware of no attempt to define their position 

 and origin. The admirable paper of Mr. John Birkinbine, 

 the well-known mining engineer, has made known the chemi- 

 cal composition and extent of the famed iron mountain of 

 Durango,* and Prof. B. Silliman has described the peculiar 

 occurrence of martite at this locality.f 



The writer has recently studied several deposits of similar 

 iron ores in Mexico, and in August, 1892, gave special atten- 

 tion to the study of certain beds situated on the line of the 

 Mexican International railway, near the city of Monclova, in 

 the State of Coahuila. The mountain in which the ore occurs, 

 like the iron mountain of Durango, is known as the Sierra de 

 Mercado, and is one of the elongated, isolated masses charac- 

 teristic of the Great Basin region of the United States, of 

 which the so-called Mexican plateau is a geographic continua- 

 tion, and is surrounded by the customary basin plains, or val- 

 leys, tilled in by debris of the mountains. 



In structure the mountain is composed of sub-vertical strata 

 of a hard blue and gray limestone, very much resembling the 

 familiar Paleozoic mountain limestones of the Appalachian 

 region, but which, in fact, as will be shown is of Mesozoic 

 age. . Through this limestone, in a direction usually corre- 

 sponding with the strike of its stratification are vast masses of 

 eruptive diorite. which simulate the limestone in color. Inas- 

 much as this diorite includes large fragments of the lime- 

 stone in its substance there can be no doubt of its later origin. 



The talus which imbeds the base of the mountain, and is 

 widely distributed over the plain, is composed principally of 

 limestone cobble, with a large admixture of rounded lumps of 

 iron ore, black upon the surface, strongly resembling magne- 

 tite, but which reveal _a lustrous (specular) surface structure 

 upon fracture, and give the streak of hematite^ This iron 

 ore is so abundant in the talus that it suggests the fact that 

 the beds from which it was originally derived hav,e long been 

 exposed to denudation. 



In ascending one of the numerous lateral canons at the 

 south end of the mountain, many alternations of the limestone 



*"The Cerro de Mercado (Iron Mountain) at Durango, Mexico, by John 

 Birkinbine," Trans, of the Am. Inst, of Wining Engineers, 1SS4, pp. 1-19. 



f " Martite of the Cerro Mercado, or Iron Mountain of Durango, and Certain 

 Other Ores of Durango, by B. Silliman, this Journal, November, 1882. 



