THE PHYSIOGRAPHY OF MEXICO 91 



the direction of its folds, a counterpart of the northern provinces, 

 from which it has been cut off by the volcanic flows of the province, 

 previously described. It would no doubt fall under the discussion 

 of the Sierra Madre Occidental province were it possible to connect 

 with that. The volcanic area, however, has completely isolated it, 

 and it therefore requires separate treatment. It is an area of odd 

 shape, aligned roughly with the folds, and lies between the volcanic 

 area on the north and the Pacific Ocean on the south. 



The northern boundary has been described, and for the greater 

 part of its length is a natural dividing line separating two regions 

 of widely different types of topography. The Pacific Ocean forms 

 a natural southern and western limit and the remainder of the 

 boundary line is drawn at the foot of the escarpment, where the edge 

 of the plateau rises above the lowlands of the Isthmus of Tehuante- 

 pec. (Compare the accompanying sketch map, Fig. 1, with the 

 Bartholomew topographic map previously referred to.) 



Topography. — In the general literature of Mexican geology and 

 geography this area is frequently described as though it were a part 

 of the volcanic area. Such description is erroneous, however, for 

 here again the characteristic feature of the topography is plateau 

 dissection. It duplicates the erosional features of the northern 

 provinces and is unaffected by the extrusions which characterize 

 the Volcanic province. 1 



The interior is a labyrinth of mountain ranges trending in diverse 

 directions, with here and there an occasional peak, more resistant 

 than its neighbors or by its configuration less exposed to erosion 

 agencies, rising to a height comparable to that of some of the 

 volcanic cones. 



On the Pacific side, head-working streams are rapidly degrading 

 the plateau, leaving its front a tattered edge, with long headlands 

 jutting out between the stream courses. The Rio Balsas, which 

 in its upper course is known as the Rio Mexcala, is one of the most 

 important streams in Southern Mexico, and drains a large portion 

 of the interior of both this and the Volcanic province. The Rio 

 Tehuantepec of the isthmian slope is remarkably straight in its 

 lower course, and without tributaries there. In the upper course, 



1 William Niven, Eng. and Min. Jour., XC, 672. 



