THE PHYSIOGRAPHY OF MEXICO 69 



of the preceding flows. Erosion has affected unequally these tuffs 

 and the harder lavas (rhyolites and dacites), and in consequence 

 there have been produced, on many of the slopes, several terraces 

 or "shoulders." 1 



Physiographic history. — The history of the present Sierra Madre 

 Occidental begins properly with the uplift which brought the Cre- 

 taceous sediments out of the sea. This uplift continued to a con- 

 siderable altitude and was accompanied by profound deformation, 

 which took the form of close folds and overthrusts passing into 

 faults, the strike of the entire system being approximately north- 

 south. This uplift, which was an event of early Tertiary time, 

 inaugurated the first erosion cycle, which, when completed, had 

 removed some thousands of feet of rock and resulted in the forma- 

 tion of the "Cordilleran Peneplain." 2 



Following this peneplanation, toward the close of the Miocene, 3 

 another uplift occurred, accompanied by widespread vulcanism. 

 Numerous vents were opened over the province, from which flowed 

 an enormous quantity of slightly viscous lava. Uplift continued; 

 in fact, is still in progress. In the meantime, a second erosion cycle 

 began, during which there has been removed at least two thousand 

 feet of volcanic rock, and which has seen the larger westward- 

 flowing streams incise their canyons a mile below the top of the 

 plateau. 4 



Recent work on the Grand Canyon of the Colorado has indi- 

 cated a third erosion cycle, intermediate between the two just 

 described. 5 It is impossible to determine from the data at hand 

 whether it included the entire Mexican Plateau. It is certain that 

 in some places the early eruptives (andesites) as well as the intru- 

 sives (diorites and granites) were deeply eroded before the later 

 eruptives (rhyolites and dacites) appeared. However, the evidence 

 of this third cycle is obscure, even where displayed to the best 

 advantage, whereas the evidence of the two cycles described above 

 is perfectly clear to experienced physiographers. 



1 Hovey, Bull. Am. Geog. Soc, XXXVII, 531. 3 Ordonez, op. tit. 



2 Hill, Eng. and Min. Jour., LXXXV, 6S1. « Hill, Science, N.S., XXV, 710. 

 5 L. F. Noble, Am. Jour. Science, XXIX, 374-80. 



