THE PHYSIOGRAPHY OF MEXICO 89 



places its gradient is very steep. Near Guadalajara there is a 

 beautiful falls with a sheer drop of 100 feet, and above La Juntas 

 the stream descends 1,800 feet, over a series of cascades, in a dis- 

 tance of 15 miles. 1 



The structure of this province is difficult to determine, owing 

 to the concealment of all early formations by the more recent 

 extrusives. Where it has been possible to obtain data, however, 

 it has been interpreted as follows: bedrock of massive Cretaceous 

 limestones and shales, folded along east- west axes, badly faulted, 

 cut by granitic intrusives, and covered by a great cap of lava and 

 tuff. 



This is the first province to be discussed where the structural 

 trend is east-west. Ordonez thinks that the number and disposition 

 of the volcanic craters along the Rio Grande de Santiago suggests 

 the idea of its marking the line of an important fracture, corre- 

 sponding to a place of dislocation in the general elevation of the 

 plateau, and where it has brusquely interrupted its direction 

 (northwest-southeast) and effected a movement in a different 

 direction (east- west). 



Physiographic history.- — -The early history of this province is 

 similar to that of the rest of the plateau, as far, at least, as the making 

 of the Cordilleran peneplain and the second uplift; beyond that it 

 is obscure. We may infer, however, that the second erosion cycle 

 had proceeded to the extent of cutting rather deep valleys in the 

 sedimentaries before the first eruptions occurred. The first flows 

 filled these valleys, leaving hills of sedimentaries standing out above 

 them. Erosion continued, and in time these hills were reduced to 

 valleys, and the lava-protected valleys became hills and moun- 

 tains. Over the greater part of the province later vulcanism com- 

 pletely effaced all traces of these earlier events, and built the great 

 volcanic plain and the peaks which now characterize the province, 

 and it is only around its borders that this part of its history may 

 be determined. 2 



It appears that there have been two periods of vulcanism in 

 this province: the first in the Miocene and affecting all Mexico; 



1 H. A. Horsfall, Eng. and Min. Jour., LXXXVIII, 665. 



2 Hill, Eng. and Min. Jour., LXXIX, 410. 



