THE PHYSIOGRAPHY OF MEXICO 75 



A little speculative interest is attached to the making of this first 

 peneplain. In humid regions the processes of erosion tend ulti- 

 mately to reduce the land surface to a base-level which is at or near 

 sea-level; but in an arid region of basin-drainage type erosion 

 cuts down hills and aggrades the basins, so that the plane of refer- 

 ence to which both hills and basins will be brought ultimately is 

 not sea-level, but some higher indeterminate level. 1 Now if 

 aridity characterized this early Tertiary base-leveling period in 

 Mexico (a condition of which we are not certain) , it is possible that 

 the peneplain resulting from the erosion of the first cycle was not 

 made at sea-level. 



Following this peneplanation another uplift took place; in fact, 

 is still in progress. This was accompanied in the beginning, prob- 

 ably Miocene time, 2 by orogenic movements which altered the entire 

 structural trend to a general northwest-southeast direction. 3 

 There also occurred at this time widespread volcanic activity, both 

 extrusive and intrusive. 



In the second erosion cycle inaugurated by this uplift there were 

 probably a number of stages, for there exist in many places old 

 erosion surfaces made on both sedimentaries and volcanics, covered 

 and filled by later volcanic flows. At the end of the volcanic period 

 erosion became the dominant factor in determining the topography, 

 and has remained so until the present time; and it is this epoch in 

 the history of the province that holds the greatest interest from a 

 physiographic viewpoint. 



All of the features of the topography of this province which we 

 have previously discussed are due to erosion agencies peculiar to an 

 excessively arid region. In humid climates water in some form is 

 the chief agent of erosion, but in this province nothing is more 

 scarce than water, and the excessive denudation which has taken 

 place must therefore be attributed to other agents. Chief among 

 these is a large diurnal temperature variation. A midday tempera- 

 ture of 130 F. is not uncommon over a large part of the province, 

 and a midnight temperature of 50 F. is usual throughout most of 

 the year in the same places. This gives a maximum daily variation 



1 W. M. Davis, Jour. Geol, XIII, 381. 



2 Ordonez, op. cit. 3 Hill, Science, N.S., XXV, 710. 



