4 PROCEEDIlSrGS OF THE NATIONAL MUSEUM vol. 74 



tion of Seilor Manuel Rangel, became the chief iron ore producer of 

 Mexico. 



GENERAL GEOLOGY 



The geology of the northern plateau region of Mexico falls into 

 two main divisions: (1) Mesozoic sedimentary rocks with rare and 

 scattered intrusions of granodiorite, diorite, and other ilijected 

 forms, and (2) Tertiary volcanic flows and tuffs with minor areas 

 of later sediments. Along the line of the Mexican Central Kailroad 

 both types are encountered, this being in a rough way the line of 

 demarcation of those areas predominantly^ of Mesozoic rocks and 

 those of Tertiary rocks. To the east Cretaceous sediments abound 

 almost to the entire exclusion of the eruptive rocks of later age, 

 while to the west the sedimentary formations rapidly give way to 

 flows and tuffs of Tertiary age. 



Along the railroad from Torreon, to Durango limestone is promi- 

 nently exposed and can be found at Torreon and for some distance 

 to the west. At Velardena, limestone with intrusions of granodio- 

 rite, diorite, alaskaite, and other rocks are abundant and Tertiary 

 eruptive rocks cap many of the hills and ranges. Farther west 

 the sediments become less and less prominent until at Durango they 

 have given way entirely to rhyolite, latite, and tuff. From Durango 

 west, across the Sierra Madre Kange. the rocks are all eruptive and 

 are largely rhyolitic in character. The Sierra Madre Mountains are 

 built up of successive flows of eruptive lavas. On the east they lie 

 directly upon folded slates ; upon the west they overlie older andesite 

 and more rarely granite. These eruptive rocks are capped in many 

 places throughout the Sierra Madre by later basalts. The order of 

 succession as given by Weed* is (1) andesite, (2) trachyte, (3) 

 granite, (4) dacite, (5) rhyolite, and (G) basalt. 



The rocks in the immediate vicinity of Cerro Mercado are entirely 

 volcanic and include, according to A. K. Martinez-Quintero,^ crystal 

 tuff (Cerro Santuario and Cerros de los Presos), rhyolite tufi (Cerro 

 de los Kemedios, Cerro Frey Diego), lithoidal rhyolite (Cerro de los 

 Remedies), lithophysal rhyolite (Cerro del Calvario) and latite 

 (Cerro de San Antonio). 



SPECIAL GEOLOGY 



The rocks of Cerro Mercado can be divided into three types: (1) 

 latite, (2) rhyolite tuff, and (3) rhyolite. (See fig. 1.) Of these 

 latite is the lowest in the sequence, tuff intermediate, and rhyolite the 

 topmost member. The latite is well exposed on the western and 

 southern slopes to the crest of the hill but is absent from the northern 



*Amer. Inst. Mln. Eng. Trans., vol. 32, p. 458, 1902. 



6 El Cerro de Mercado, Durango, Bol. 44, Inst. Geol. Mexico, pp. 21-39, 1923. 



