The continent of Africa is something like a dish turned upside down, having a high and flat central plateau, with a higher rim of hills surrounding it; from below which, exterially, it suddenly slopes down to the flat strip of land bordering on the sea. A dish, however, is generally uniform in shape—Africa is not. For instance, we find in its centre a high group of hills surrounding the head of the Tanganyika Lake, composed chiefly of argillaceous sandstones which I suppose to be the Lunae Montes of Ptolemy, or the Soma Giri of the ancient Hindus. Further, instead of a rim at the northern end, the country shelves down from the equator to the Mediterranean Sea; and on the general surface of the interior plateau there are basins full of water (lakes), from which, when rains overflow them, rivers are formed, that, cutting through the flanking rim of hills, find their way to the sea.