THE PHYSIOGRAPHY OF MEXICO 81 



nevertheless, active agents of erosion when carrying the excessive 

 run-off which follows precipitation on the bare desert slopes. The 

 Sonora, Yaqui, Fuerte, and Rio Grande de Santiago rise farther 

 back in the mountains and have a continuous flow. The canyons 

 which these latter streams have cut through the mountains and 

 foothills are of profound depth, and present to the traveler a 

 sublimely rugged scenery, not to be surpassed on the continent. 



The structure of this province is quite complex on account of 

 the extensive folding, faulting, and vulcanism to which it has been 

 subjected. A cross-section would show folded sedimentaries, 

 upon the beveled edges of which sheets of volcanic material are 

 spread, and into which dikes, plugs, sills, laccoliths, and other 

 igneous bodies have been intruded. The whole mass is broken by 

 numerous faults, chiefly of the N. 45 W. system, though east-west 

 faults are common and occasionally profound, one in the Cananea 

 district having a vertical displacement of at least 1,000 feet. 1 

 The topographic effect of the predominating N. 45 W. faults is to 

 be seen in the scarps of the western border of the Sierra Madre 

 Occidental and of the eastern border of the peninsular mountains. 



Physiographic history. — When this province emerged from the 

 sea after receiving its thick deposit of Mesozoic sediments it was 

 subjected to stresses, which resulted in close folding and in the 

 making of the great synclinal trough previously described. Follow- 

 ing this, in early Tertiary time, there came a period of erosion 

 which beveled the edges of the folded strata and resulted in the 

 making of the "Cordilleran Peneplain," previously described. 



After the country had been base-leveled, vulcanism began to 

 play an important role, and during the Miocene great masses of 

 volcanic material were poured out from numerous vents, completely 

 burying the sedimentaries. Coincident with this a second uplift 

 occurred, giving the streams renewed energy and causing erosion 

 to become the dominant process in the making of the topography. 

 It was during this second uplift, which probably is still in progress, 

 that the Colorado River cut its trench several thousand feet deep 

 through the Pacific trough. 2 The remainder of the province was 

 not seriously affected in the beginning by the erosion of this period, 



1 M. L. Lee, Econ. GeoL, VII, 324. 2 Botsford, op. cit. 



