British Somaliland consists of a high plateau, of which the northern scarp is separated from the Gulf of Aden by a belt of low hills and plains, known as the Guban. The southern plateau consists of a vast block of Archæan rocks, mainly gneiss and amphibolites, with intrusive pegmatite–dykes, and it is capped by purple grits, red sandstones, and conglomerates. The more rugged peaks of the coastal belt are outliers of the Archæan plateau; but the main geological interest of this area is due to the occurrence in it of a series of limestones. That the limestones belong to more than one period was obvious from the earliest accounts of them. Thus M. de Rochebrune, who in 1882 first described the Somali limestones, identified the fossils collected by Révoil in the Singeli country (lat. 11° N. & long. 49° E.)as Neocomian; and Miss Raisin, who six years later gave an account of the specimens collected by Capt.King at Mount Eilo, south of Zeila (lat. 10° 30′ N. & long. 43° 35′ E.), suggested, on the evidence of the foraminifera, that the limestone at that locality was late Cretaceous or more probably Kainozoic. The fossils described by M. de Rochebrune were obtained in Eastern Somaliland; the limestone described by Miss Raisin came from the western part of the Guban; while the Neocomian fossils described by Prof. Mayer-Eymar were collected by Prof. Keller on the south-western slopes of the Somali plateau, along the valley of the Faf, a tributary of the Webi