92 WARREN N. THAYER 



however, it is developing a typical dendritic system of drainage, 

 and is rapidly dissecting the plateau in Oaxaca. The stream was 

 probably originally consequent upon a fault rift in the steep slope 

 of the plateau edge flanking the isthmus, but has since lengthened 

 its course and developed a drainage basin by processes characteris- 

 tic of subsequent streams. 1 



The structure on the whole is similar to that described for the 

 northern provinces, namely, folded, faulted sedimentaries, overlain 

 with superficial volcanic material, and intruded by various igneous 

 bodies; but the trend of the folds here is generally east- west. 

 Over the greater part of the province the Miocene volcanic cap has 

 been removed, laying bare the metamorphosed sediments beneath. 

 In addition to having the sedimentaries clearly exposed, we know 

 the character of the actual basement rock — a condition which may 

 only be surmised over the rest of Mexico. The basement rock in 

 this province is Archean crystallines, which may be observed in the 

 deepest canyons not far above sea-level and upward to positions on 

 mountain flanks 6,000 feet above tide. 



Physiographic history. — In the other Mexican provinces we 

 were obliged to begin the history at the first post-Cretaceous cycle. 

 Here, although we have Archean rocks determining the topography 

 in places, history must again begin with the Mesozoic era, for the 

 Paleozoic record is a blank. We have Mesozoic sediments resting 

 unconformably on Archean crystallines. How many erosion cycles, 

 or what length of time was necessary completely to remove all 

 traces of Paleozoic rocks and carve deeply into the crystallines, is 

 indeterminable. There is little evidence to warrant the belief that 

 the Mexican Plateau was an island during the Paleozoic era, and 

 consequently without marine sediments. Moreover, there are 

 Paleozoic rocks exposed in small patches in Sonora, and again in 

 Chiapas and Central America 2 and I believe we may safely infer 

 that they once covered the entire plateau and were removed. After 

 the uplift of the plateau following the deposition of the Mesozoic 

 sediments, the history of this province becomes coincident with 

 that of the northern provinces. 



1 Bose, Guides des excursions, 10th Inter. Geo!. Cong. Mex., iqo6, No. 31, p. 21. 

 a Hill, Eng. and Min. Jour., LXXXIV, 63. 



