2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I23 



from place to place, and learning the age of different periods of 

 faulting, intrusion, and metallization. 



The western part of the Mexican plateau has been a land mass 

 since early Mesozoic time or before; the southern and eastern parts 

 were covered by the sea as recently as late Cretaceous time. The 

 lower Mesozoic rocks consist largely of sandstone and shale, now 

 strongly metamorphosed. The middle Mesozoic rocks (late Jurassic 

 and early Cretaceous) are mainly limestone, and upper Cretaceous 

 rocks are largely shale. All these rocks were strongly folded, faulted, 

 intruded by igneous bodies, and eroded before the earliest nonmarine 

 Cenozoic rocks were deposited. 



The rocks of earliest Cenozoic age in the plateau region are gen- 

 erally reddish elastics composed largely of eroded Mesozoic rocks 

 and, in smaller part, of pene-contemporaneous volcanic materials, 

 except locally where the volcanic rocks predominate. A few of the 

 outcrop areas of these elastics are shown in figure i. The youngest 

 rocks on which these elastics rest are strongly folded, intruded, and 

 eroded marine sediments of latest Cretaceous age. The elastics are 

 in turn intruded, faulted, and strongly tilted, but they are not folded 

 except perhaps as very broad, broken arches. They are overlain by 

 volcanic rocks and interbedded clastic rocks, which reach great thick- 

 nesses locally. These latter rocks are in turn overlain by middle and 

 upper Pliocene conglomerates and volcanics. All these younger over- 

 lying rocks are affected by normal faulting, but they are only slightly 

 tilted locally, in marked contrast to the structure of the older clastic 

 rocks. 



BACKGROUND OF THE RED CONGLOMERATE STUDY 



As part of the cooperative scientific and technical assistance pro- 

 gram of the United States Government, geologists of the U. S. Geo- 

 logical Survey in collaboration with geologists of the Instituto Na- 

 cional para la Investigacion de Recursos Minerales and the Instituto 

 de Geologia of Mexico have for some years been mapping red clastic 

 deposits of early Cenozoic age incidental to mineral-district studies. 

 In 1949 John D. Edwards began field work on a study of the deposits 

 of red clastic rocks with the encouragement and counsel of personnel 

 of the Survey and the Instituto de Geologia. Brief investigations 

 were made at Zimapan, Guanajuato, Zacatecas, and Taxco, all mining 

 districts that have been exploited actively for more than three cen- 

 turies. During that field season the need was recognized for a more 

 intensive study and fossil search in at least one of these areas, pref- 



