EFFECTS OF VARIATION IN RAINFALL 299 



The frequent occurrence of foothills at the base of the main ranges, 

 both on the eastern face and in the valleys of this section of the Cordil- 

 leras, is noteworthy. Here, as elsewhere, these low ridges have notice- 

 ably accordant heights ; they are, moreover, much less dissected than 

 are the mountain at whose feet they lie. Wherever these foothills are 

 well developed the talus slopes are inconspicuous or wanting. If the 

 view previously expressed, to the effect that foothills of this type result 

 from the exposure of a portion of the gently sloping bed rock of a valley 

 which took its first shape while the process of refilling was going on, 

 then the process of baring and dissecting must have been a concomitant 

 of an increased rainfall which has served to destroy the ancient talus 

 slopes or benches. In other words, we have to suppose that an arid 

 period, during which extensive taluses were built, was followed by a 

 humid period, in which they were to a great extent swept away. 



Possible Cause of increased Rainfall in eastern Cordilleras 



At first sight it may seem that the last Glacial period, with its in- 

 creased precipitation, could sufficiently account for the augmented rain- 

 fall which the features just above noted appear to demand for their 

 explanation ; but it is probable that the increase in erosive action 

 in this eastern section of the Cordilleras is much more ancient than the 

 ice-time. Although the dissection of the upper part of the rock slope 

 is often of the canyon type, the incisions are deep and apparently must 

 have required far more time than elapsed since the beginning of that 

 humid period. In view of the facts, I have been led to a hypothesis as 

 to the cause of this ancient temporary increase of rainfall and of the 

 aridity which preceded it. This view I shall now set forth. 



It is eminently probable that the source of the larger part of the rainfall 

 that comes to the drainage basin of the Mississippi valley, including the 

 eastern portion of the Cordilleras, is to be found in the basin of the gulf 

 of Mexico and the Caribbean sea. Depending on the evaporation from 

 this basin as the source of precipitation, we may reasonably suppose 

 that any great variations in the area of these seas would be attended by 

 changes of rainfall which might, if the alterations were great enough, 

 range through a wide scale. Although we can not as yet clearl^^ trace 

 the alterations of the Caribbean group of seas in the later geological 

 periods, we know enough to be sure that these have been great. Devel- 

 oping mainly about the eastern and western mountain systems, this con- 

 tinent has, since the Paleozoic age, had a broad, low interval between 

 the Appalachian and Cordilleran high ground — a region so little elevated 

 above the average level of the ocean that it has in many alternations been 

 dry land and shallow sea. Still further, in southern Louisiana, where 



