504 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



In a later article ^^^ Sapper stated that he had not been able to observe the con- 

 tact of the Todos Santos beds with the underlying Carboniferous and could not be 

 sure that the two were unconformable, although that conclusion was probably correct. 



Bose^" in 1906 repeats Sapper's description and adds that the Todos Santos 

 strata appear to belong to the " Triassic- Jurassic," as they are similar in position and 

 lithologic character to strata of those ages which are known in other parts of Mexico 

 (PUebla and Oaxaca) and also in Honduras. 



E-F 14. CENTRAL AND NOBTHEBN MEXICO. 



Aguilera ^"^ in 1906 thus summed up the occurrences of the Triassic in central 

 and northern Mexico under the heading " Neo-Triassic " : 



Marine deposits of the Carnien stage are represented in the Yicinity of the city of Zacatecas 

 by sihceous shales and argillaceous and sandy rocks which contain a number of species of 

 Paleoneilo, Halobia, Avicula, Cassianella (Burckhardtia), and some ammonites: Serenites 

 smitJii Burckh., Anatomites mosjvari Burckhardt, Clionites, and Trachyceras. The Triassic beds 

 of Zacatecas are intimately associated with spilitic rocks (greenstones). We may mention in 

 the same connection the marsh and lacustrine deposits of Juravien age which correspond probably 

 to the Keuper and extend up to the Rhsetic. They are composed of quartzose sandstones of 

 gray or reddish-brown color, gray, black or varicolored slates, and some beds of red conglomer- 

 ates with small pebbles in the region of Los Bronches, at the Barranca of San Marcial and San 

 Jose de Pimas in Sonora. 



A list of fossils comprising 30 species follows, and Aguilera continues : 



Conglomerates and red sandstones which alternate with gray and black slates and which, 

 inasmuch as they include fossil plants of the same genera and species as those of Los Bronches, 

 are of the same age, have been found in Puebla and Oaxlaca, and probably also beneath the beds 

 of Miquihuana, in TamauUpas. 



In the geologic map of North America the occurrences of the Triassic in central 

 and northern Mexico have generally been mapped with the Jurassic in accordance 

 with the manuscript map furnished by the Mexican Survey. (See also quotations 

 under Jurassic in Chapter XIII.) 



The Triassic of Zacatecas was described by Burckhardt and Scalia."' After 

 describing ancient sericitic schists, whose age they say is probably pre-Triassic, 

 as they are unconformably overlain by beds of the upper Triassic, the authors 

 continue : 



The upper Triassic strata rest in discordance upon the ancient schists forming the central 

 portions of the two recumbent synclines that are open toward the east and which may be observed 

 in the valley of the Pimienta. The existence of these synchnes is proved by the arrangement of 

 two bands of Triassic strata intercalated in the ancient schists. One may observe quartzites 

 and quartzitic sandstones of dark-gray color containing fragments of quartz and feldspar in the 

 center of the two Triassic occurrences, while these rocks are limited on both sides by quite a 

 thickness of siliceous and argillaceous schists which are black or dark blue and alternate with 

 green sandstones and greenish or gray argillites. All of these rocks dip toward the east. * * * 

 The Triassic fossils occur in two different localities ; the first is situated close to the point where • 

 the Arroyo Calavera or Pimienta is crossed by a bridge, the bridge of Ahogado. At that point, 

 in the northern flank of the eastern syncline of the Trias, fossils occur in the black siliceous slates 

 as well as in a gray argillaceous rock. These beds are filled with fossils, especially with small 

 bivalves, while ammonites are rare and fragmentary. 



