﻿Vol. 6 1.] NORTH-EASTERN TEBRTTORIES OF THE CONGO EREE STATE*. 649 



The Limonitic Conglomerates. 



In travelling through the Uelle district, the observer cannot 

 fail to be impressed by the enormous development of limonitic rock. 

 As soon as one leaves Buta and the Rubi basin, this rock is most 

 frequently seen in isolated blocks or sheets of a dark-brown colour, 

 of medium hardness and density. The constituent elements seem 

 to be rounded lumps of iron-ore with clayey impurities, cemented 

 together by oxide of iron. Frequently, small pieces of rounded 

 quartz were seen embedded in the matrix, as well as sand. 



Without in any way exhibiting a stratified appearance, the rock 

 nevertheless offers the aspect of being in many places of the nature 

 of a sedimentary deposit. Its thickness nowhere exceeds 30 feet, 

 and probably averages in most places from 6 to 10 feet only. 



Nearly the whole of the country north of the Uelle presents to 

 view no other rock. In the southern part it is constantly seen in 

 patches on the rises of the undulating country. 



It is a noticeable fact that the mountainous region of Arebi is, in 

 its hilliest part, comparatively free from limonitic conglomerate, 

 and that this is more abundant on the lower elevations, especially 

 below the Mount- Angba hills and Mount Tena. 



At the base of Mount Tena and in the vicinity is an exposure of 

 limonitic conglomerate which deserves special mention. It covers 

 an area which may be described as the more level ground adjoining 

 on the south and west. It is not seen on the north or east side. 

 This conglomerate has the aspect of a surface-formation deposited 

 horizontally, of no great thickness (probably here not exceeding 

 30 feet). It is composed of large waterworn fragments, irregular 

 in size and shape, but of identical nature with the iron- ore found 

 in beds at the summit of the hill, and cemented together by oxide 

 of iron. 



This superficial formation is no doubt the result of the work of 

 denudation and erosion of the iron-ores of the mountain, and its 

 peculiar appearance here has suggested to me the possibility that 

 all the limonitic rocks found so abundantly in the Uelle are the 

 product of denudation and erosion of iron-mountains such as have 

 just been described, or even of other iron-bearing veins, perhaps at 

 present covered up. The work of redeposition took place at a 

 time when the Uelle Basin may have been more or less a succession 

 of shallow lakes and vast swamps. Then the alteration and redis- 

 tribution of the ferruginous materials would have been attended by 

 every condition favourable to the formation of such limonitic deposits. 

 After being deposited over extensive tracts of country, the limonitic 

 conglomerate has been removed by erosion along some of the water- 

 courses, and this accounts for the formation being invariably found 

 upon the summits of the gentle undulations. The process of 

 formation of these conglomerates is not yet at an end, and in close 

 proximity to the iron-mountains it is still in operation. 



In presenting this view of the possible origin of these limonitic 

 conglomerates, I wish it to be understood that it appears to me 



