Yol. 63.] THE GEOLOGY OF THE OBAN HILLS. 313 



18. The Geology of the Oban Hills (Southern Nigeria). 

 By John Parkinson, B.A., E.G.S. (Read December 19th, 1906.) 



[Abridged. — Map on p. 314.] 



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-schists 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 asso- 

 ciated with a fine-grained mica-schist, so closely as to produce a 

 banded composite gneiss by a process, apparently, of lit-par-lit 

 injection. Examination of numerous gravels shows that the 

 foliated rocks contain monazite. 



(4) The unfoliated group, developed principally on the western 

 side of the Oban Hills, but sporadically over the entire district, 

 consists of many varieties of granite, of tourmaliniferous pegmatites, 

 and of aplite-dykes. 



(5) The youngest rocks are a series of olivine-basalt dykes, 

 which, from a comparison with dykes and sills cutting Cretaceous 

 rocks on the Cross and Aweyong Rivers, are considered to be of post- 

 Cretaceous age. 



(6) A group of phyllites and subordinate grits altered by acidic 

 intrusions, now gneissose, into various forms of Eruchtschiefer 

 (spotted slates), micaceous and staurolite-schists, and of garnet- 

 and andalusite-hornfels, occur near Uwet on the Calabar River. 



