212 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



At their western end the old mountain ranges are buried under vast deposits of volcanic 

 tuff forming a broad plateau. On the \^•estern border of this a line of splendid volcanoes, 

 10,000 to 13,500 feet high, extends along the Pacific side of the country at a distance of 

 30 to 40 miles from the coast. The plateau itself, at an altitude of from 5,000 to 8,000 

 feet above the sea, is relatively flat, although divided, as it were, into terraces at various 

 elevations. It would be easy to traverse except that it is frequently cut by deep ' ' barancos" 

 or canj^ons with nearly vertical sides. In a state of nature most of the plateau would be 

 covered with an open, grassy pine forest, but its healthfulness and coolness have caused 

 it to be almost entirely cleared and to be the home of one of the densest agricultural popu- 

 lations to be found in any part of the world. 



Southwest of the plateau the lower slopes of the volcanoes are densely forested and 

 feverish, for rain falls whenever the trade-winds weaken and a movement of the air takes 

 place from the Pacific. Coffee is here raised on a large scale, but were it not for this the 

 population would be relatively scarce. The lower edge of the coffee country extends into 

 the Pacific plain. While the volcanoes were building up the plateau, the rivers were busily 

 carrying volcanic materials down toward the Pacific Ocean, and forming a smooth pied- 

 mont plain 20 to 30 miles wide. The inner edge of the plain Hes in the zone of dense jungle, 

 500 to 1,000 feet above the sea. The seaward edge is relatively dry and bears a wearisome 

 growth of thick bushes. Everywhere the plain is unhealthful and sparsely populated, for 

 in the rainy seasons its flat surface holds the water in stagnant swamps, where mosquitoes 

 breed by the million. 



Turning now to the terraces, our first line of evidence as to changes of climate, we find 

 an interesting contrast between the Pacific and Atlantic slopes. On the Pacific slope I saw 

 more or less of at least a dozen small river systems, some in their head-waters before they 

 had passed through the great line of volcanoes, and some farther downstream. None 

 are well terraced like certain valleys in southern INIexico, but many show evidence that a 

 terracing process has not been wholly absent. On the river between Tecpam and Gaudines, 

 for instance, the valley was first eroded to nearly its present level, was then filled to a depth 

 of at least 30 to 40 feet with coarse alluvium, and has since been re-excavated. Before the 

 re-excavation the stream was displaced to one side of the valley, so that when it began 

 cutting downward it soon encountered solid rock. In this it has cut only a narrow gorge, 

 although both upstream and down it has so broadened the valley floor that only faint traces 

 of terraces are seen. Many other phenomena indicate that as a rule the valleys of the 

 Pacific slope have recently experienced two periods of terracing and that the terraces occur 

 both above and below the volcanoes, but are generally better developed above. As might 

 be anticipated, however, the evidence is not strong, partly because the streams fall rapidly 

 and the slopes are well covered with vegetation, and still more because where volcanoes 

 have been active so recently it would be idle to look for marked cUmatic terraces of any 

 great age. If the scanty terraces now visible are of climatic rather than volcanic origin, 

 they may enable us to date various events in the volcanic historj^ of the country. For our 

 present purpose, however, they possess no special importance. 



On the Atlantic slope the case is far different, for volcanic action either ceased long 

 ago or never prevailed, and large areas are so dry that almost anj' cUmatic change would 

 have an appreciable effect upon vegetation. I was able to see only two drainage sj-stems, 

 those of the large IMotagua River and the small Santa Toma just north of it. They suffice, 

 however, to show that here, just as in southern Mexico, Aj-izona, Persia, and many other 

 places, the most recent geological times have been characterized by at least four terrace- 

 making epochs. The reasons for believing the terraces to be of chmatic rather than of 

 tectonic origin are the same here as elsewhere. The verj' fact of their occurrence here 

 with the same character as in far-distant regions is in itself a strong reason for assigning 

 to them an origin of world-wide application, such as climate, rather than local earth-move- 



