116 M. T Hill — Hematite and Martite of Mexico. 



dent that the iron ores are of later age than the rocks in 

 which they occur, and this period was probably near the 

 Eocene, for many of the basin plains of Mexico similar to 

 that of Monclova in which the debris of the iron ore is found, 

 contain the Loup Fork and Equus vertebrate Tertiary faunas 

 of Cope. 



Having now described the geology of the Monclova ores as 

 a typical locality of these peculiar Tertiary Mexican martite 

 hematites, it may be well to examine the distribution of similar 

 occurrences in Mexico. The local geology of Monclova, is 

 typical of the geology of the whole of the Mexican mountain- 

 ous (plateau) region, which may be briefly described as rem- 

 nants of numerous folds of Comanche limestone (also the 

 Laramie sands and clays in the northeast), frequently broken 

 by protrusions of eruptive material, and separated by basin 

 valleys, largely covered by the debris of the mountains. Only 

 along the southern border of the plateau and in the extreme 

 northwestern States are the older rocks of the Archaean or 

 Paleozoic exposed.* All the following localities of iron ore 

 are from this region of Cretaceous limestone and Tertiary 

 eruptives, with the possible exception of Prof. Silliman's locali- 

 ties in Sinaloa, where Carboniferous limestones also occur as 

 well as the Cretaceous. 



The Sierra Oandella Beds. — Seven miles west of the town 

 of Salomon de Botia, and about sixty miles east of Monclova 

 near the Mexican National railway, the writer examined 

 several large deposits of ore similar in composition and occur- 

 rence to that of Sierra de Mercado at Monclova. Although 

 an entirely different group of mountains from those of Mon- 

 clova, the. Sierra Candella consists of the same Comanche 

 limestone, and dioritic eruptives. The Laramie beds however 

 occur as foot hills on both sides of the Sierra Candella. 



The Sierra de Mercado of Durango. — About three hundred 

 miles southwest of the Sierra de Mercado of Monclova is the 

 Sierra de Mercado of Durango, commonly known as the iron 

 mountain of Durango. It was from this locality that the oc- 

 currence of martite was first noted by Prof. Silliman. The 

 mountain has been well described by Mr. John Birkinbinef 

 who says, "The hill is nearly a mile long and a third of a 



mile wide, and from four to six hundred feet high." "1 



am inclined to believe that the Cerro. de Mercado is formed of 

 one or more immense lenses of specular ore, standing vertically, 

 the fragments of which have for ages been thrown down to 



* The writer has reserved for a future paper the discussion of the general 

 geology of the region. 

 f Loc. cit. 



