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THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



numbers and in a highly developed condition in Sonora, and also in the vicinity of Monterey 

 in the northeast of Mexico, along the railway line from Laredo to Mexico City. Through 

 the courtesy of Dr. Jose G. Aguilera, Director of the Geological Institute of Mexico, one 

 of his assistants, Mr. Ygnacio S. Bonillas, was permitted to spend some days with me in 

 studying the region around the City of Mexico. Thanks to Mr. Bonillas's thorough 

 knowledge of the local geology, I was able in a short time to see things which it would 

 have taken weeks to find alone. Northwest of the city the volcanic hills are deeply seamed 

 with rugged ravines descending from high mountains. There in four small valleys we 

 found terraces of the kind under discussion. The presence of revolutionists within 3 or 

 4 miles of the places where we were at work, and in all the country round about, prevented 

 us from examining others or from following any of the four up into the mountains, where 

 the maximum development is to be expected. Nevertheless, the places pointed out by 

 Mr. Bonillas were sufficient to indicate that, as a general rule, valleys of sufficient size and 

 coming from mountains of sufficient height contain alluvial terraces of the type which 

 elsewhere seems to be climatic. In various places the cross-section of the valleys is like 

 that shown in figure 8. The calcareous caliche or "tepetate" on the top of the main 



Fig. 8.— Cross-section of Alluvial Terraces in Mountain 

 Valleys near the City of Mexico. 



1 = Volcanic tuff. 3 — First alluvium. 



2 = Caliche. 4 = Second alluvium. 

 A, B, C = Successive gorges. 



volcanic deposits suggests a long dry epoch; the rapid cutting to form the gorge A indicates 

 a pronounced uplift or else a period of comparative moisture, during which the streams 

 were either of large volume or else were not overloaded with detritus because of the 

 covering of the slopes with vegetation. In either case they were able to erode rapidly. 

 The alluvial filling, 3, indicates either a tilting of the earth's cnist back towards its original 

 position or a period of aridity which would cause deposition either by diminishing the 

 streams, or, more likely, by increasing their load through the death of vegetation and 

 consequent releasing of the soil. The process of cutting and filling was repeated at least 

 twice, and may have been repeated several times, although the evidence is now concealed 

 or has been worn away. 



Similar phenomena on a much larger scale occur farther south, especially in the valley 

 of the Papaloapam River, nearly 200 miles southwest of Mexico City, between Puebla and 

 Oaxaca. Here the terraces reach a height of at least 200 to 300 feet, and are developed 

 to the number of four over long distances. Still farther south, in Guatemala, only 15° from 

 the equator, terraces are found in an equally well-developed condition, as will be described 

 later. They are of the same tyi^e as those in regions hundreds and thousands of miles 

 away, and appear to be due to a common cause which can scarcely be anything but climatic 

 pulsations. The constant occurrence of such terraces from Utah on the north through 

 Mexico to the far south, and their high development even at the southern limit to which they 

 have yet been traced, seem to be strong indications that climatic changes have taken place 

 in Mexico as well as in the United States. The lakes of Mexico and the traces of ancient 

 cultures in the strata forming the floor of the Mexican basin suggest that here, as elsewhere, 

 the later changes have taken place since man reached a stage of comparative civilization. 



