R. T. Hill — Hematite and Martite of Mexico. 115 



The most interesting fact is the unusual geological age of 

 the ore, and its occurrence different from any of our own 

 country, unless the ore at Yan Horn, in Trans-Pecos Texas, a 

 part of the same geological province, is similar.* 



The principal specular ores of the United States are from 

 the older Paleozoic and Archsean rocks, and as we proceed 

 upward in the geological column they decrease in compact- 

 ness and in luster. In fact the Mexican ores have a strong 

 resemblance to those of the Superior district, and practical 

 miners often speak of them as identical, a mistake which is 

 further instigated by the Paleozoic aspect of the Mesozoic and 

 Tertiary phenomena of the region. The diorite is called 

 "granitico" by the natives, and is spoken of as granite by 

 one of our prominent mining engineers, f The coal fields of 

 the Sabinas, which extend to within thirty miles of the iron 

 deposits have been often referred to as Carboniferous and 

 Triassic;}: until their Laramie age was shown by Dr. C. A. 

 White, in 1886. The massive Paleozoic-looking limestone is 

 undoubtedly of Lower Cretaceous age, and contains the char- 

 acteristic Terebratulas, Pectens, Limas, Aviculas, Monopleuras, 

 Ammonites, Radiolites and Nerineas of the Comanche Series 

 of Texas, and is the result of alteration of the same beds 

 which have a chalky aspect in the non-mountainous portion of 

 the latter state. This limestone was published as Silurian by 

 Rock,§ and Frazer received conditional opinions from Prof. 

 Angelo Heilprin and James Hall to the effect that it was 

 probably of Carboniferous age.| 



The diorites, as shown by their intrusion through the 

 Comanche limestone at the Sierra Mercado of Monclova, and 

 through both the limestone and the Laramie beds in the Sierra 

 Candella, are clearly of post-Cretaceous age. Mr. Whitman 

 Cross, of the U. S. Geological Survey, has kindly studied these 

 for me, and his mineralogical notes are appended. His remark 

 that they strongly resemble certain post-Cretaceous eruptives 

 in Colorado, is an important point. These diorites have a 

 wide distribution over the Mexican plateau. It is very evi- 



* The writer has seen masses of ore from Van Horn which resemble in 

 mineralogical characters the Mexican ore. 



f Dr. Frazer in the paper already cited, speaks of the diorite both of the Sierra 

 de Mercado of Monclova and of the Carrissal and Yilladama regions as " granite." 



% Mr. W. H. Adams, in an otherwise excellent paper refers these coal fields to 

 the Trias. See " Coal in Mexico : Santa Rosa District," Trans. Am. Inst. Min- 

 ing Engineers, vol. iii, pp. 25-28. 1874. 



§ Keport on Examinations of the San Rafael Mining Company's Mines, by 

 Adolphe Rock, Mobile, 1876. 



I Both of these geologists stated that the paleontologic evidence furnished 

 them by Prof. Frazer was insufficient to accurately determine the age of these 

 limestones, and that their opinions were largely based upon their lithologic 

 characters. See Dr. Frazer's paper, previously cited. 



