THE PHYSIOGRAPHY OF MEXICO 



65 



The southern boundary of this province has been placed at the 

 canyons of the Rio Grande de Santiago and the Rio Lerma; not 

 arbitrarily, but because at this place there is a sharp change in the 

 strike of the massive folded sediments. 1 North of this line the 



Sierra. /7fac/re 

 Occidental 



~tTi e sa 



Sierra ltio.drc 

 Oriental 



Lot. z£ 



Sierra. ?7la.e/re 

 Oee-Lcterttai 



TTlesa. 

 Central 



SlerrG.)Tl4.d,re 

 Oriental 



Lai. 2 3 



Lat 21 



/S/>ocaieffetl 



Ut./9° 



Fig. 2. — Profile of Mexican Plateau. (After J. G. Aguilera.) 



structural trend is generally northwest-southeast, while south 

 of it the trend is more nearly east-west. South of this line, also, 

 erosional down-cutting in the plateau ceases to be the controlling 

 factor in the making of the topography, and gives way to volcanic 

 piling-up on it. 



1 Ordonez, op. cit. 



