The Geology of the Oban Hills (Southern Nigeria)

Abstract

The country worked over comprises some 1800 square miles adjacent to the Kamerun frontier, lying in the form of an ellipse, with its major axis running north and south, bounded respectively by the Cross River and the swamps east of Calabar. (see map. p. 314). I arrive at the following conclusions :— (1)A study of the Eastern Province of Southern Nigeria shows the Oban Hills to be part of an extensive spur or inlier of crystal-line rocks, surrounded in British territory by Cretaceous and later sediments. (2)The crystalline rocks are broadly divided into two groups— the one characterized by the presence, the other by the absence of foliation. (3)The former consists in part of acid orthogneisses, which tend to lose their foliation and pass into granites, and when coarsely crystalline they have a pegmatoidal habit. The commonest rock of the series is a fine-grained biotite-gneiss, but many varieties are found, and it is considered that these are the members of a differentiated magma, the various products of which took up their present positions at different times : that is, they differ slightly in age. Mica and less frequently hornblende-schists, together with sphene-sehists and rare sillimanite-gneisses (Awofong), occur throughout the district, and it is contended that these rocks are older than the gneisses. At Abuton (Kwa River) a garnet-tourmaline-orthogneiss is associated with a fine-grained mica-schist, so closely as to produce a banded composite gneiss by a process, apparently