GEOLOGY. 285 



man from nature, a higher degree of intellectual culture brings us back and 

 nearer to its wonders. 



GEOLOGY OF CENTRAL AFKICA. 



The following interesting facts respecting the geology and topography 

 of Central Africa have been gleaned from the reports of Dr. Vogel, who, it 

 will be remembered, has succeeded Dr. Earth in the work of exploration. 



The great plain of Central Africa presents nowhere as far as 9-J- north lati- 

 tude (a few isolated small granitic cones excepted) an elevation exceeding 

 950 feet. Dr. Vogel says that in about 11 north latitude, 120 miles from 

 Kuka, he found, at a depth of 20 feet under the surface of the ground, the 

 same layer, consisting of limestone and freshwater shells, which he met with 

 at Kuka 6 feet under the ground, and he suggests that the whole region ex- 

 tending thus for upward of 100 miles S.S.E. from Kuka, was at one tune 

 occupied by Lake Tsad, when its limits extended greatly beyond its present 

 ones. But whether this assumption be correct or not, the well-ascertained 

 fact as to the slight elevation of that region, together with the results of the 

 previous hypsometrical observations of Dr. Vogel and Dr. Overweg, as well as 

 of the discoveries and acute estimates of Dr. Earth, relating to altitudes, are 

 well worthy consideration, as they completely upset our previous notions of 

 African geography. It is well known that all our best authorities represent 

 the G-reat Desert of Sahara, and nearly the whole of Northern Africa, as one 

 vast plain, if not a dead level, at least one of very little elevation ; whereas, 

 immediately to the south of Lake Tsad, the existence of mountain ranges, alp- 

 ine groups, highlands, and mighty table-lands of many thousand feet eleva- 

 tion was asserted and taught us as well-established facts. Now, from the 

 observations made by the members of the Expedition to Central Africa, this 

 is found to be quite the reverse, and both features may be truly said to have 

 changed places an extensive table-land, from 1,000 to 2,000 feet average 

 elevation, occupying the Sahara ; whereas, on the other hand, the extensive 

 basin of Lake Tsad and the River Shary forms a great interior depression, 

 which attains its minimum elevation in the lake with 850 feet. On every 

 side the basin of Lake Tsad is fringed with more or less elevated tracts which 

 separate it from the other hydrographical systems, as, for instance, those of 

 the Nile and the Kowara. These new facts of the relative elevation of In- 

 ner Africa also explain to us many features connected with the physical con- 

 figuration, the climate, botany, and zoology of the regions they refer to. 



The countries round Lake Tsad form an immense alluvial plain. Dr. Vogel, 

 after leaving the oasis of Aghadem, situated upward of 250 geographical 

 miles north from Kuka, did not see a single rock or stone till he came to "Waza^ 

 which lies upward of 100 miles S.S.E. of Kuka, thus leaving an alluvial tract 

 between the two points of upward of 350 geographical miles in the heart 

 of Africa. At Waza an isolated group of granitic cones rises almost perpen- 

 dicularly out of the alluvial plain to the height of 400 feet above their basis. 



