﻿RESEARCH IN BRITISH WEST AFRICA. 143 



As has been shown, the only rivers of any size in the Colony are the Niger 

 and Cross Rivers, but neither of these is dependent on the rainfall of Southern 

 Nigeria to any great extent. The rise of the Cross River is due almost entirely 

 to the heavy rainfall in the German Kamerun hinterland, while that of the Niger 

 is associated with the increased supply from its higher reaches in French Guinea 

 and Northern Nigeria and also from the Benue River. The Benue in turn 

 receives its supply from the Kamerun hinterland. The heavy rainfall of 

 Southern Nigeria is confined practically to the Coast, or delta region, and is 

 consequently almost coterminous with the area of tidal influence. 



Generally speaking, the whole of the shore area is low-lying and fringed with 

 mangrove swamp. It consists of a large alluvial plain which extends for 

 hundreds of miles, except on the extreme east, where the Oban Hills almost 

 touch the foreshore and extend northwards to the Kameruns. This alluvial low- 

 lying land reaches its greatest dimensions in the delta of the Niger, which 

 projects so far into the Gulf of Guinea as to form two distinct bays known as the 

 Bights of Benin and Biafra. 



Beyond this zone the land gradually rises, and the mangrove gives way to the 

 open grassy plains of the hinterland, which extend to Northern Nigeria. 

 Superimposed on these are the various small mountain ranges already mentioned, 

 which separate the different drainage systems. 



The soil is mainly red clay, but the hills are intrusions of metamorphic rocks, 

 granitoid, schistose or quartzite. Here and there beds of limestone occur, and, 

 more commonly, large outcrops of laterite. 



0) Vegetation. 



As has been shown in my previous reports, the nature of the vegetation in any 

 part of a colony has a distinct bearing on the insect fauna. For this reason it 

 might be well to discuss briefly the main types found in Southern Nigeria, their 

 general character, and their distribution. I am indebted to an admirable paper* 

 by Mr. H. N. Thomson, Conservator of Forests in Southern Nigeria, for a 

 considerable part of what follows in this chapter. The quotations given are 

 taken from that report. 



The type of forest found in any particular region depends almost entirely on 

 the rainfall, but occasionally in the drier regions one comes across patches of a 

 type generally associated with a heavy rainfall ; these occur as outcrops due to 

 permanent and abundant telluric moisture. 



The forest growths of Southern Nigeria may be divided roughly into : — 

 (a) tropical rain forest, (b) fresh-water swamp forest, (c) monsoon or mixed 

 deciduous forest, (d) savannah forest and (e) mangrove forest. 



Generally speaking, where there is an abundant supply of moisture and little or 

 no differentiation into wet and dry seasons, the tropical rain forest predominates ; 

 where the soil is permanently moist, even if there is a moderately long dry 



* Journ. Afr. Soc. X, p. 125, 



