244 HEAVY FORESTS ON ALLUVIAL SANDS. 



gradually becomes curtailed until at length we enter 

 our next division, the Desert Zone, where there are 

 no regular rains. It must be clearly understood, 

 however, that in venturing to assign these appa- 

 rent boundaries to the tropical rains, we merely 

 indicate their approximate limits. Moreover, wher- 

 ever special circumstances occur to favour it, great 

 stretches of heavy forest, thoroughly equatorial in their 

 character, run up, often for long distances, into the 

 Bush Country ; as for instance where rivers of consider- 

 able size pass through it, margined by alluvial plains, 

 or where a damp and fertile soil promotes the growth 

 of heavy timber. 



On the other hand, wherever disturbing causes inter- 

 fere with the equatorial rains, and cause a dry dis- 

 trict to occur within the region of constant rains, the 

 bush extends itself into the equatorial zone, instead 

 of the heavy forest. We shall presently proceed to 

 point out examples of both these exceptional conditions, 

 likewise other instances on the opposite side of the Bush 

 Region, where the desert extends itself across the 

 boundaries where tree growth might be expected to 

 occur. 



Now these differences in the vegetation are merely 

 indicative of climatic peculiarities, the causes of which 

 require to be investigated and understood before they 

 can be accounted for. Notwithstanding these occasional 

 exceptions, however, if we take the map of the 

 world, and study the physical geography of the coun- 

 tries which extend round the earth's terrestrial surface, 

 within the limits which we have indicated as those 

 of the Bush Country it will be found that in the 

 main their character closely corresponds with the 



