4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I23 



but the material was unfortunately not diagnostic enough to permit 

 a close age assignment. The results of the two summers of field 

 work have been summarized in a manuscript submitted to the Geo- 

 logical Survey and the Instituto de Recursos Minerales, covering the 

 Guanajuato, Taxco, and Zacatecas areas, which is being prepared for 

 publication as a chapter in Professional Paper 264 of the U. S. 

 Geological Survey. 



SKETCH OF THE GEOLOGY AT GUANAJUATO 



The following resume of the pre-Cenozoic rocks and the somewhat 

 detailed description of the red conglomerate and overlying rocks are 

 partly abstracted from the unpublished report by Edwards on the 

 Guanajuato area and are partly the result of observation and inter- 

 pretation by Fries over a decade during which he visited the area a 

 number of times. The Guanajuato area, as shown in figure i, is near 

 the geographic center of Mexico and near the midline of the high 

 plateau. Mining has been carried on there along the Veta Madre 

 gold-silver lode for about four centuries. The ores are in a fault zone 

 trending northwest, whose southwest side has apparently been down- 

 thrown more than 1,500 meters (see fig. 2). The northeast side of 

 the fault is composed of metamorphosed sediments (mes in fig. 2), 

 mainly phyllites, and some igneous intrusive rocks. These sedimen- 

 tary rocks are believed to be in part of Triassic age, because of their 

 lithologic similarity to rocks in the Zacatecas mining district where 

 some poorly preserved Triassic fossils were found by Burckhardt and 

 Scalia in the early part of the present century. Edwards and Ortiz 

 found fragments of limestone in the overlying conglomerate of 

 Cenozoic age that contained lower Cretaceous fossils, and hence the 

 area of pre-Cenozoic rocks (mes) must have contained outcrops of 

 early Cretaceous limestone and probably some Jurassic sediments as 

 well. The southwest side of the fault is composed of red conglom- 

 erate of Cenozoic age, which is the host rock to most of the ore in 

 the district and also to the vertebrate fossils described in this paper. 



All the exposed contacts of the red conglomerate with the older, 

 underlying rocks are faults, and hence the complete thickness of the 

 formation and the lithology of the basal part are not known. Edwards 

 has estimated the thickness to be at least 1,500 meters and probably 

 nearer 2,000 meters. The conglomerate is preserved because it has 

 been faulted down to depths where it has been protected from ero- 

 sion. The formation may be divided into three principal parts ; the 

 first two parts are grouped into one in the map in figure 2. The 



