388 H. FOSTER BAIN 



uncovered and resting upon crystalline schists, granites and sim- 

 ilar rocks, or are intercalated between the Huronian rocks and 

 the arenaceous marls and slates of the Upper Jurassic. The lat- 

 ter is especially true south of Acatlan and about Tezoatlan. In 

 some instances the Triassic is covered only by the Cretaceous or 

 the Tertiary. 



The position of the rocks along the Gulf of California and in 

 the territory of Puebla and Oaxaca indicates that after their 

 deposition they were subjected to an elevation which, continuing 

 to the present time, has placed them more than a thousand meters 

 above sea level in Puebla and more than two thousand in Oaxaca. 

 In spite of the evidence of the invasion of the sea in the Triassic, 

 the absence of marine sediments makes it impossible to trace 

 the old shore lines, but the same absence indicates that the land 

 then extended notably more to the west than at present and 

 that the deposits then made along the coast have since been 

 buried. 



While the deposits of the Triassic were made in marshes and 

 lagoons, some of which perhaps communicated with the sea, 

 those of the Jurassic are in general such as denote continental 

 seas and deep water. Certain of the Jurassic rocks, however, 

 seem to have been deposited in an interior sea of slight depth. 

 At the close of the Triassic the northwestern and southern parts 

 of the country were elevated, draining the marshes. At the 

 same time the southeast sank beneath the Lower, and later the 

 Middle Jurassic seas. At the close of the Middle Jurassic there 

 was a further land movement and the Upper Jurassic sea crept 

 in over large portions of Coahuila and Oaxaca. The Jurassic 

 rocks are conformable with the Cretaceous and are intimately 

 folded with them. 



The Cretaceous rocks cover much the greatest portion of 

 Mexican territory. They include three well-marked series of 

 beds, the Lower, Middle, and Upper Cretaceous. These corre- 

 spond respectively the Lower to the Neocomian, the Middle to 

 the Cenomanian, Turonian and a portion of the Senonian, and 

 the Upper to the Danian, and a part of the Senonian of Europe. 



