622 KEPOKT— 1882. 



discovery in the same line of country. Probably as fast as one line is made good 

 geographically fresh lines may present themselves, and new vistas will be opened 

 to the astonished gaze of geographers. At length, with all the constancy and 

 courage which arduous travel never fails to inspire, the inquirers of the future will 

 doubtless explore this plateau till it becomes as well known as the Alpine regions 

 of Europe. 



The following Papers were read : — 

 1. Some poinfs of Physical GeograpJiij ohserved during a recent tour round 

 South America, By John Ball, M.A., F.B.S. 



2. On the Geographical Evolution of the Tanganyika Basin. 

 By Joseph Thomson, F.B.G.S. 

 The keynote of this paper is struck by a reference to a recent lecture of Dr. 

 Archibald Geikie, to the Royal Geographical Society, in which he points out that 

 the days are now oA'er in which the scientific geographer is content with the simple 

 description of the superficial aspects of the various regions of the globe. He must 

 also know how they came to be, and what they have been in the past. This line 

 of inquiry is applied by Mr. Thomson to the lake regions of Central Africa, but 

 more particidarly to the Tanganyika Basin. In the first place he presented a bird's- 

 eye view of the lake regions from the Indian to the Atlantic Ocean, bringing into 

 relief only the most prominent features of the geography, but describing more in 

 detail the aspect of the Tangan-s-ika Basin, round which the chief interest centres. 

 From a description of these purely superficial matters he proceeded to describe what 

 these have been in the remote past, and the manner in which they liave been 

 evolved, being of course compelled to call in the assistance of the sister science 

 geology. 



The conclusions he arrived at as to the primary origin of the region were, from 

 purely hypothetical considerations, based on the theory of a shrinking nucleus, and 

 the necessary eflects on the earths crust arising therefrom. At a later stage, 

 however, he was able to appeal to the rocks themselves as to the aboriginal con- 

 ditions of the African continent south of the Equator, These, according to 

 Mr. Thomson, prove the existence of an immense central sea cut ofl" from 

 the ocean by the elevation of the continent, and which was almost conterminous 

 with the present drainage area of the Congo. An elevated ridge was upheaved 

 along the eastern boundary of this sea, the origin of the trough of Tanganvika, 

 by the collapse of the centre of this ridge and the central sea, subsequently drained 

 away to the west, leaving Tanganyika isolated. Mr. Thomson then proceeded 

 to describe how its secondary characters arose, and its scenery was moulded, by 

 the action of sub-aerial denudation on rocks of different powers of resisting 

 the decomposing and eroding agents, and explained the curious marine-like type 

 of its shells, the origin of its outlet, the Lukuga, the freshening of the water of 

 the lake, and finally the curious intermittency of the outflow. 



The various stages in the evolution of the Tanganyika Basin were summarised 

 as follows : — 



The first appearance of the future continent, he had been led to believe from 

 various theoretical considerations, was the appearance of a fold of the earth's crust 

 boimded by two lines of weakness converging towards the south, which fold 

 gradually rose till it appeared above the ocean, first along these two lines of 

 weakness, in the form of a series of islands, which finally joined, enclosing in their 

 centre a large part of the ocean. This enclosed water area formed a great central 

 sea, and the enclosing land along the lines of weakness is now indicated by the 

 east and west coast ranges. 



In the second stage the continent of Africa south of 5° N. latitude presented the 

 outline of the continent of to-day. 



The third stage showed the central plateau with the great central sea very much 

 diminished in size and almost coinciding with the present Congo Basin. There was 

 as yet no e^adence of the existence of Tanganyika. 



