JURASSIC AND CRETACEOUS 605 



age are found in Cliilmaliua, Durango, Nnevo Leon, Zacatecas, San Luis 

 Potosi, Puebla, Guerrero, Oaxaca, and the Isthmus of Tehuantepec, and 

 include Oxfordian, Kimmeridgian, and Portland ian. According to 

 Burckhardt, the faunas are related to those of central Europe q,nd the 

 Mediterranean, but also include elements derived from the faunas of 

 India, of the boreal region, and of the Andes, together with a character- 

 istic element which may be called Mexican. This Mexican element seems 

 to be represented in the Jurassic of western Cuba, which is the oldest 

 known Mesozoic of the West Indies. A reasonable interpretation includes 

 the Mexican and Texan Upper Jurassic in the Atlantic or Gulf of Mexico 

 sedimentation, with one or more temporary connections with the Pacific 

 to admit the boreal and Indian elements of the fauna. 



Cretaceous 



Cretaceous limestones with minor shales and sandstones have a great 

 thickness and wide distribution in Mexico and Central America. Much 

 the larger part of them are referred to the Comanche series by American 

 geologists, and assigned to the Lower Cretaceous. Both the rocks and 

 the faunas are of the same f acies as the typical Comanche series of Texas, 

 though it . is recognized that some of the beds in southern Mexico are 

 probably older than any of those in Texas. The Mexican geologists have 

 divided these rocks into Lower and Middle Cretaceous, and all are agreed 

 •in assigning the rocks overlying the great limestones to the Upper Cre- 

 taceous. It is well known that the Comanche fauna is related to the 

 Cretaceous fauna of the Mediterranean province and is totally distinct 

 from any of the Shasta faunas of the Pacific coast of the United States, 

 which must be in part contemporaneous with it. The difference is so 

 great that a connection between the Pacific and the Gulf of Mexico has 

 not been thought possible, in spite of the fact that the Comanche type of 

 sediments and faunas seems to reach the present Pacific coast in southern 

 Mexico. The difficulties in placing a land barrier west of the present 

 west coast of Mexico and Central America are realized, for the 100 -fathom 

 line is only 10 to 100 miles off the coast, and the 1,000-fathom line is 

 approximately parallel to it and only a few miles farther out. The fact 

 that the 1,500-fathom line extends on the Equator out beyond the Gala- 

 pagos Islands may have some significance, and the 2,000-fathom line 

 sweeps far out opposite the Gulf of California as well as on the Equator. 

 Though the difference in facies of both sediments and faunas in the Co- 

 manche as compared with the Shasta may be again mentioned, by way 

 of caution, as partly accounting for differences in the faunas, it is be- 

 lieved that such a complete lack of common species must have been caused 



