174 Notices of Memoirs — Br. 0. Lenz — Geology ofW. Africa. 



The Okande Itabirite is hard and heavy, its colour is reddish- 

 purple ; its texture granular-schistose ; its components are quartz, 

 specular oxyd of iron, and magnetic iron. The quartz is con- 

 spicuously prevalent, in the form of whitish-grey granules, in 

 coherent parallel layers. The specular iron-ore appears in bright 

 black lamellae, scattered in the quartz, and frequently bearing a 

 crust of red oxyd of iron. This oxyd appears, likewise, in coherent 

 layers, parallel to those of quartz, and alternating with them. Thus 

 a tranverse fracture offers an alternation of rather broad red and 

 white stripes, interspersed with bright lamellae of iron-ore. 

 Magnetic iron-oxyd is scattered in minute particles throughout the 

 whole, so that large specimens of this rock cause a marked irritation 

 of the magnetic needle. Atmospheric agents give rise to a thin out- 

 ward crust of hydroxyd of iron, and make the surface irregularly 

 rough, the quartz offering a greater resistance to these agents than 

 the portions impregnated with oxyd of iron. 



The Itabirite of Okande differs from that of Brazil by not including 

 such accessory minerals as gold, talc, chlorite, iron-pyrites, and 

 actinote. Itabirite and analogous rocks, associated with Itacolumnite, 

 were first found in the Schist-formation of Brazil, subsequently in 

 South Carolina, the South-east of France (Departement du Var), 

 Portugal (Tras os Montes), and Germany (Soonwald). 



The Okande Itabirite is a normal and important constituent of the 

 West- African Schist-formation. It assumes generally the shape of 

 low and ragged cliffs in the river-courses. In the plain of Lope it 

 disappears, almost entirely, beneath beds of Diluvial loam of com- 

 paratively recent origin. 



5. Geology of West Africa. — The islands in Corisco Bay, some- 

 what north of the Equator, rise about ten metres above the sea, and 

 are composed of a light-coloured calcareous sandstone, in horizontal 

 strata, overlain with vegetable soil, and extending eastward as far as 

 the mainland at the mouths of the Eivers Muni and Munda. These 

 strata contain many casts of very large, inflated, knotted, and 

 carinated Ammonites, closely allied to Ammonites inflatus, a character- 

 istic fossil of the Upper Gault. A well-preserved fragment of this 

 species has been found farther south, in Fish Bay (Benguela). 

 Small, badly preserved Bivalves are of rare occurrence : and car- 

 bonized stems of indeterminable plants are frequent. Numerous 

 fissures in every direction are filled up with an uncommonly hard 

 and solid ferruginous sandstone, occasionally also with oxide of 

 iron. 



Near Gaboon, white limestones, about two metres thick, with 

 many veins of calcite, and with local accumulations of Gasteropods, 

 Bivalves, fragments of Crustacea, Echinida, etc., rest on Cretaceous 

 Sandstones. The above-mentioned deposits bear an Eocene fades, 

 and are strictly local. 



The horizontal Tertiaries of the Loango Coast are overlain by an 

 unstratified deep yellow loam, with nodules of white marl and 

 hollow concretions of hydroxyd of iron, thus nearly resembling 

 Loess. This loam, destitute of organic remains, extends along the 



