114 B. T. Hill — Hematite and Martite of Mexico. 



Dr. Frazer "in order to test the value of this ore severely" 

 took a sample across the face inclosed in the pit inclusive of a 

 horse of limestone there visible, and some scattered masses of 

 sandy material, which gave 53*80 per cent of metallic iron." 

 He gives 0*61 to 4*5 of sulphur in the various analyses of this 

 impure sampling. 



It is not the purpose of this paper to advance an opinion of 

 the origin of this ore, for the writer is unable to satisfy him- 

 self upon this subject, but the opinion of Prof. Frazer that 

 they are altered pyrites is contrary to his observation. The 

 limestone may be looked upon as the most probable source, 

 however, for occurrences of smaller quantities of iron ore have 

 been frequently reported at the contact of igneous and calca- 

 reous rocks, and ores of Jalisco are reported entirely sur- 

 rounded by limestone. It is interesting to note that these 

 masses always occur in the vicinity of eruptive intrusions into 

 the limestone and in some cases are entirely embedded in it. 

 In some of the adjacent Cretaceous limestones was found a 

 small weathered ammonite encrusted with limonite, showing 

 that the limestone contains iron ore and that it concentrates 

 upon the surface. In the silver-bearing fissures and veins of 

 the same limestone which the writer has examined in many 

 parts of Mexico there is always a residuum of excessively 

 ferruginous clay. In the unaltered limestones of the Coman- 

 che series in Texas, notably at Austin, there are many nodules 

 of iron oxide, especially in the Washita beds (the horizon of 

 the Monclova limestones) which are the result of alterations of 

 balls of pyrites. Along the veins and faults of Barton creek 

 are deposits of ferruginous clay derived from the apparently 

 pure white chalky limestone. The fissure at the contact of 

 the Comanche limestone and diorite in which these Mexican 

 ores are deposited can readily be explained by the mountain 

 stresses or chemical solution of the limestone. The diorite is 

 cross or double jointed and the fissures are filling with limonite 

 by hydrous deposition, as can be seen in every crevice. We 

 learn that certain Norwegian geologists have thus explained 

 the origin of iron deposits in that country, but it is not pro- 

 posed to maintain that the Mexican ore bodies so originated, 

 although Barcena the eminent Mexican geologist asserts that 

 in the mineral district of Agostadero, in the State of Jalisco, 

 one can see specular iron ore in process of formation in the 

 feldspathic rocks, through the agency of percolating waters 

 charged with the hydroxide of iron, and percolating into the 

 fissures of the rock.* 



* Tratado Geologica, etc.. por Mariano Barcena; Edicion de la Secretaria de 

 Fomento. Mexico, 1886, p. 90. 



