90 WARREN N. THAYER 



the second beginning in the Pliocene and continuing to the present 

 time and limited to this province. 1 This second period appears 

 to have been divided into three well-defined epochs: first, that 

 which produced the high peaks and cones; secondly, that which 

 produced the pit craters; thirdly, that which is manifested in the 

 hot springs and geysers of the present time. Between the first 

 and second epochs there was a time of lesser activity during which 

 were produced the minor cones which have since been reduced to 

 slight eminences. 2 



An exception is found in Colima, which according to the classi- 

 faction given would belong to the first epoch of the second period. 

 This volcano has erupted at intervals during the life of the present 

 generation. The eruptive material has not consisted of lava, but 

 of glassy fragments of various sizes. It has been assumed that the 

 eruptive forces at work during this latter epoch of activity have 

 not been sufficient to overcome the altitude of more than 12,000 

 feet, and that therefore the lava rises to a much lower level in the 

 crater and only the hot scoriae are ejected over the sides. 3 



As we have indicated, the second erosion cycle in this province 

 was interrupted by widespread and long-continued vulcanism. 

 Since the cessation of volcanic activity, erosion has again become 

 the dominant factor in making the topography of the province. 

 Shall we call this a continuation of the second cycle or the begin- 

 ning of the third ? Inasmuch as the topographic effects of the 

 second erosion cycle were largely obliterated, and as early youth 

 is manifested over a large part of the province, I believe we are 

 justified in stating that a third cycle is beginning, and although 

 it is by no means certain that the titanic forces which built up this 

 area are exhausted, erosion seems to have the better of vulcanism 

 in the contention for supremacy. 4 



THE SIERRA DEL SUR PROVINCE 



Definition and boundaries . — This is the southernmost division 

 of the old Mexican Plateau and is in all essential respects, save 



1 T. Flores, Guides des excursions, 10th Inter. Geo!. Cong. Mex., iqo6, No. 9. 



2 Ordonez and Aguilera, op. cit., p. 80. 



3 J. M. Arreola, Jour. Geol., XI, 749. 4 Ordonez and Aguilera, op. cit., p. 80. 



