18 VEGETATION IN CENTRAL AMERICA. 



the present time in the denuded mountains of northwestern Guate- 

 mala, between Santo Tomas and Jacaltenango. Terraces of the sec- 

 ond type cover many square miles of unoccupied land in the semiarid 

 plateau region of the Mexican State of Chiapas, between the Guate- 

 malan boundary and Comitan, as well as between Comitan and Oco- 

 singo. This terraced region is now covered partly by fire-swept 

 grass lands and partly by scattered growths of pines and oaks, where 

 the fires are less severe. In some localities to the east of Comitan the 

 terraced lands are now used for agriculture, but the present occupa- 

 tion is obviously recent and has nothing to do with the building of 

 the terraces. Indeed, it is difficult to convince the natives that the 

 terraces are not natural features of the country. 



The motive of the prehistoric people in building these broad ter- 

 races is not easy to understand unless they are considered as a method 

 of dry farming. The walls are only a few feet high, though the area 

 inclosed may be several acres in extent. Even without the terraces 

 the land would often appear quite level, the slopes being extremely 

 gentle. This makes it quite improbable that the terraces had the 

 object of avoiding erosion, like the system of terracing followed in 

 our Southeastern States. There is also no indication that they were 

 intended to impound running water or that artificial irrigation was 

 applied. The walls do not rise above the level of the land inclosed. 

 The j)robability is that the complete leveling of the land was found 

 useful to prevent the running off of any of the rather slight rainfall 

 which comes to these semiarid districts. While the terraces are not 

 high, the walls are often half a mile or more in length. The leveling 

 of the land must have required vastly more labor than the building 

 of the walls, but the work may have been done gradually if a regular 

 practice of hoeing the earth toward the wall were followed. 



EFFECTS OF AGRICULTURE ON FORMS OF EROSION. 



That the ancient occupations of the humid mountain regions of 

 eastern Guatemala by agricultural civilizations were Very prolonged 

 or were repeated in several prehistoric ages is indicated by the very 

 severe erosion which this region has suffered. It is not likely that 

 such deeply dissected contours would have been formed if the country 

 had not been kept in a denuded condition for long periods of time. 



There is practically no erosion at all on slopes covered with dense 

 tropical forest. The soil is never loosened by frost, but is held in 

 place by the matted roots of trees and thatched over with a compact 

 layer of fallen leaves. On the steep slopes this covering sheds the 

 rain so effectively that the subsoil often remains permanently dry and 

 dusty. 



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