the curve of about 500 miles developed between Capes Blanco and Yerd constitutes a separate geological area, fringed north and south of the Senegal by lofty dunes, and describing a regular arc except in the north, where the coastline, eaten away by erosion, is now replaced by a sandbank, over which the surf rolls in long-breakers. Farther south, also, the alluvia deposited by the Senegal has advanced beyond the normal shore-line, forming a convex segment about 120 miles long, with a mean breadth of some 12 miles. But landwards, behind the range of coast dunes, the same geological formations everywhere prevail. Both north and south of the Senegal, towards the Sahara as well as towards the Gambia, the ground consists of ferruginous laterite deposits. Even the two extreme capes, Blanco and Verd ("White" and "Green"), present small prominences which seem to date from the same geological epoch.
The section of the seaboard between Capes Yerd and Roxo develops a curve of about 165 miles with its concave side facing eastwards. The original shore-Line, as revealed by the tongues of sand and submerged bars at the river mouths, is extremely regular, although deeply indented by the fluvial estuaries, whose alluvia are widely spread over the older ferruginous sandstone deposits. On the other hand, the third section between Cape Roxo and Sherbro Island has lost all appearance of regularity, being carved by the waves into a thousand inlets of all sizes, or broken into islets and reefs, now strewn over the neighbouring waters. But in other respects this southern section presents the same alluvial and laterite formations as the seaboard north of Cape Roxo.
The indented parts of the Senegambian coast lie in exactly the same latitude as the Futa-Jallon highlands, and between the two formations it is easy to detect a relation of cause and effect. The streams flowing from the uplands have excavated the valleys and to some extent contributed to the creation of the marine estuaries; but glacial action may also have had its share in the general result. Doubtless the climate is now very different from that which must have prevailed when frozen streams were slowly descending down to the Senegambian plains; but in this respect the African seaboard offers precisely the same conditions as those of Brazil and New Granada, where traces of glaciation have also been detected by Agassiz and other observers. The erratic granite boulders occurring on the sandstone plains of Sierra Leone can scarcely be otherwise accounted for; consequently to the action of glaciers should perhaps be largely attributed the destruction of the Senegambian seaboard, causing it to retire some 60 miles inland.
Political and Social Relations.
Occupying nearly twelve degrees of latitude, and rising to an extreme height of over 6,000 feet, Senegambia naturally presents a great variety of plants and animals, belonging, however, to two distinct domains, that of the neighbouring Saharian savannahs and that of the great Nigritian forests. Great contrasts are also presented by its inhabitants, who possess neither political unity nor social
