94 WARREN N. THAYER 



The low mountain chain running through Tarifa Chivela is 

 probably a spur of the Mexican Mountains — a southward extension 

 of the Sierra Madre Occidental perhaps — that came into existence 

 with the Cretaceous-Eocene uplift; the plicated and metamor- 

 phosed sediments are to us as a mutilated page torn from an 

 important but now missing chapter in our historical account of 

 Mexican physiography; and the low swells and undulations of the 

 Atlantic slope are the result of the uplift and deformation that 

 " constructed the bridge between the two American continents." 1 



Summing these events up in chronological order we may say 

 that upon the fundamental Archean complex were deposited a 

 series of Paleozoic rocks of unknown thickness and areal extent. 

 These were so profoundly deformed as to become highly meta- 

 morphosed. Following the deformation came a period of erosion 

 which probably continued until the Middle Cretaceous sub- 

 mergence. In other words, the pre-Mesozoic history of this 

 province probably was coincident with that of the Mexican Plateau, 

 of which the isthmus was no doubt an integral part. At the time 

 of the uplift referred to a great structural movement took place which 

 cut this province off from the remainder of the plateau, the place 

 of rupture being still visible in the great scarp in Oaxaca. The 

 dropping of the block on the east of this fault line brought about 

 the inundation of what is known as the isthmus. The character 

 and distribution of the Tertiary deposits would indicate that the 

 submergence thus produced was not to great depths. In late 

 Tertiary and early Quaternary time an orogenic movement took 

 place which produced a second series of folds, and raised the isthmus 

 above the sea. This movement is to be regarded as an event 

 peculiar to Central America, at all events to that part of the country 

 south of Oaxaca. The Mexican Plateau was not affected. 



' Bose, op. cit., p. 26. 



