ABSENCE OF ACCUMULATED VEGETABLE MATTER 307 



tation, by which their current is checked and their effectiveness as an 

 eroding agent correspondingly reduced. 



The decay of vegetable matter is so rapid that there are no consider- 

 able accumulations of such matter either in the forest generall}" or in the 

 lagoons and swamps. In boring through the alluvial floodplains,many 

 of which have once been open lagoons, while an occasional log was en- 

 countered, nothing was found in the nature of peat, and the silt contains 

 only a relatively small proportion of finely comminuted organic matter. 

 On well drained surfaces, such as moderately steep hillsides, there is 

 generally no humus layer. The red soil, practically free from incorpo- 

 rated organic matter, forms the surface, only in part covered by the forest 

 litter. 



Western division. — In the western division, particularly that portion -of 

 it lying west of the lake, the distribution of the rainfall produces a dis- 

 tinctly different type of vegetation. This region is characterized by open 

 savannas, in which the trees are small and grow in isolated patches, 

 the greater part of the surface being open and covered with grass or small 

 bushes. These savannas are probably due to deforesting, in part by 

 clearing for cultivation and grazing, and in part by fires. Wherever a 

 forest covers the surface its character is entirely different from that in 

 the eastern division. It has the thorny habit and scant foliage which 

 characterizes the vegetation of a semi-arid region. The light is not cut 

 off by the foliage of the higher trees, and hence the smaller herbaceous 

 vegetation is much more abundant than in the eastern division. Fires 

 prevail in the dry season, so that the forest litter does not accumulate, 

 and at the beginning of the wet season, before the vegetation is renewed, 

 the surface is entirely unprotected from the effects of the heavy rainfall 

 which inaugurates that season. 



Red soil is rarely seen west of the lake, the prevailing colors being blue, 

 bluish gray, or black, and this is quite independent of the character of 

 the rock from which it is derived, since the rocks are essentially the same 

 as those which yield red soils in the eastern division. Toward the end 

 of the dry season the surface is intersected by many deep cracks, often 

 2 or 3 inches wide and as many feet deep, which effectually destro}' the 

 coherence of the clay. This alternate saturation and baking of the soil 

 therefore effects somewhat the same result as that accomplished else- 

 where by frost. It also permits the incorporation of much organic matter 

 with the upper portions of the soil, forming an exceptionally thick humus 

 layer. From these and perhaps other con4itions it results that the smaller 

 rainfall of the western division is a very much more efficient agent of 

 erosion than the greater rainfall of the eastern division. 



