LOWER CEETACEOUS. 589 



cases by means of fossils, both plant remains and vertebrates ; in other cases it has been necessary 

 to base an opinion upon the presence or absence of modern eruptive rocks in the conglomerate. 



E-H 12-15. MEXICO IN GENEBAL. 



Cretaceous rocks, chie% limestones, have great thickness and wide distribution 

 in Mexico and Central America. Two classifications have been applied to them. 

 One is that of Hill, who recognized the Comanche series in its characteristic develop- 

 ment and also the Upper Cretaceous. The Mexican Survey, on the other hand, 

 divides all the strata of the Cretaceous system into three series, which it designates 

 as Eo-, Meso-, and Neo-Cretaceous and which are correlated on paleontologic evi- 

 dence with European divisions. The Eo- and Meso-Cretaceous of this classifica- 

 tion correspond with the Comanche of Hill's classification, and the Neo-Cretaceous 

 is Upper Cretaceous, later than Dakota. Aguilera®^ refers to the synopsis of the 

 geology of Mexico^ and says: 



We divided the Cretaceous into Lower, Middle, and Upper divisions, which, according to 

 the resolutions adopted at the International Geological Congress, should be called respectively 

 Eo-Cretaceous, Meso-Cretaceous, and Neo-Cretaceous. In practice this arrangement has 

 proved advantageous, although it has been abandoned elsewhere, as it is here very useful from 

 the double point of view of hthology and paleontology. The limit between the Eo-Cretaceous 

 and Meso-Cretaceous series should correspond to the bottom of the compact limestones which 

 form the mountains of Mexico, where it is impossible for us to separate the Albien from the 

 Cenomanien, as the former has not yet been identified by its fossils in Mexico; and the Eo- 

 Cretaceous series thus ends with the Aptien. The Meso-Cretaceous extends from the Albien to 

 the Cenomanien inclusive, and the Neo-Cretaceous is composed of the Turonien, Emscherien, 

 Aturien, and Danien. 



The Meso-Cretaceous series is very extensively developed throughout the country and 

 forms almost all of the Hmestone mountains of Mexico. It is found from Sonora, Chihuahua, 

 and Tamauhpas in the north to Chiapas and Tabasco in the south, and from the Gulf of Mexico 

 to the Pacific Ocean. The predominant facies of the Meso-Cretaceous are those of the Rudistes 

 and Caprinides. Up to the present time we have not succeeded in clearly separating the Albien 

 from the Cenomanien. To the north the series is composed of marly rocks and in certain places 

 carries a sufficiently rich fauna. 



The following notes on the stratigraphy and distribution are condensed from 

 Aguilera's descriptions, and arranged from south to north. For fossils and Euro- 

 pean equivalents, the original should be consulted. 



Eo-Cretaceous strata occur as follows : 



In the State of Oaxaca, at Tlaxiaco, the Eo-Cretaceous is represented by cal- 

 careous to gray calcareous argillaceous shales and by compact limestones of gray 

 and grayish-black colors. 



Near Cualac, in the State of Guerrero, are quartzose sandstones, gray to red 

 in color, with grayish and yellowish argillaceous and marly shales. At the base 

 are gray sandstones with impressions of plants. Red and yellowish quartzose 

 sandstones and argillaceous schists occur near Tomixlahuaca. 



In the State of Michoacan argillaceous shales of black and grayish colors alter- 

 nating with yellowish fossiliferous sandstones are found at Cochixtla, and argil- 

 laceous marly shales alternating with calcareous fossiliferous shales at Parota, in 

 the district of Coalcoman. 



