78 BvUetin American Museum of Natural History. [Vol. XXXVI, 



The Cauca semi-arid region, begins slightly south of the head of navi- 

 gation in the Cauca river, in Antioquia and extends up that much con- 

 stricted region lying between the Western and Central Andes, through 

 which the Cauca flows, to the Cauca Valley. The topographic isolation 

 of this valley is therefore faunally increased by the aridity of the region 

 which lies between it and the humid lower Cauca-Magdalena region. 



The Cauca Valley, as elsewhere remarked, is far from arid. Neverthe- 

 less, its forests are largely restricted to the banks of streams and low-lying 

 areas where a natural subsurface irrigation gives them the water the insuffi- 

 cient rainfall does not directly provide. 



It must, howfever, be remembered that the Cauca region has been 

 settled for many years, and that deforestation to create grazing for cattle 

 has long been practiced. It is probable, therefore, that the forested area 

 was formerly more extensive than it is at present. 



Mountain Forests. — Under the general head of Mountain Forests we 

 may include the forests of the Subti-opical and Temperate Zones. Both 

 may be classed as cloud forests, the lower limit of the first being determined 

 by the altitude of condensation, the upper limit of the second by that of 

 the temperature at which tree-growth ends. 



Depending for their continuity on topography, or the relation between 

 slope exposure and prevailing air currents, on the relative altitude of con- 

 tiguous ridges and other comparatively local conditions, a detailed survey 

 would be required to make anything approaching an accurate map, showing 

 the distribution of mountain forests in Colombia. It may, however, be 

 said in a general way that Subtropical forests, or those lying between approx- 

 imately 5000 and 9500 feet, because of their lower altitude, are more or less 

 continuous on one slope or both, of all three ranges of the Colombian Andes. 



Temperate Zone forest, both because of the higher altitude required, 

 and also because of the lower rainfall which usually prevails on this zone, 

 is less continuous than that of the Subtropical Zone. 



Unforested Mountain Areas. — As indicated by the statements just made, 

 the forestless tracts above the tropics are found largely in the Temperate 

 Zone, where on the Bogota Savanna, or in that region lying south of Popa- 

 yan, for example, there are wide areas which are treeless. 



What is believed to be an approximation to existing facts is expressed, 

 semi-diagrammatically, in the accompanying map in which the yellow color 

 employed represents not only the arid coast region, the savannas, and 

 plains of the Tropical Zone, but also treeless, grass-covered slopes and 

 paramos of the higher summits. 



