Notices of Memoirs — Dr. 0. Lenz — Geology of W. Africa. 175 



coast of Gaboon, on both banks of the Ogowe, even to the outposts 

 of the West-African Schist-formation. Its surface is covered with 

 innumerable granules of pisiform iron-ore, resulting from disaggre- 

 gation of the concretions of hydroxydated iron. The whole deposit 

 may be compared with what is generally called Diluvium, and may 

 be coeval with the origin of the Ogowe Lakes, on the withdrawal 

 of the waters into their existing beds. Still, a number of larger 

 and smaller lakes exist on both sides of the Ogowe, connected with 

 it by channels, a narrow wall of loam, at most ten to fifteen metres 

 in height, standing between lake and river. Many blocks of schis- 

 tose rocks, probably transported by the waters once filling up the 

 whole region from Gaboon to Ncomi (Kamma), are spread over the 

 surface of these natural dykes. The analogy of this loam with the 

 Laterite of East India is conspicuous. At present the Ogowe, in its 

 whole course, down to a few miles above its mouth, carries and 

 deposits only enormous quantities of purest quartz sand, without 

 any trace of loam ; while the adjacent loam dykes are completely 

 devoid of arenaceous beds. 



The foremost chains of an extensive range of crystalline schists, 

 spreading from the inmost corner of the Gulf of Guinea, Southward 

 to Angola, appear in the Okota region about forty miles inland. 

 The range is composed of many parallel chains, dipping eastward 

 with a steep angle. The lowermost horizon (Okota) has a group of 

 thin-bedded, light-coloured, fine-grained schists, with some mica, 

 locally talcose, and in one place containing a great lenticular inter- 

 calation of steatite. Subordinate beds of red and white quartz are 

 not rare, both in the schists, and in the typical granatiferous mica- 

 schists. A ferruginous schist, closely resembling the Itabirite of 

 Brazil, exists along the frontier of the Okande region. Great beds 

 of black siliceous schists extend from the River Okne to the Cataracts 

 of Ndume, at the commencement of the inner plain. Granite, in 

 fine varieties, appears only in large erratic blocks, brought probably 

 from the interior when the Ogowe had a far larger bed. 



The massif here described appears on the maps as Sierra Complida 

 and Sierra do Cry stall, and may be conveniently designated as the 

 West-African Schist Mountains. Globular segregations, including 

 fine crystals of yellow and reddish quartz, and covered on their 

 surface with a peculiar network much resembling honeycomb, are 

 found in both original and derivative situations, in the latter case 

 having probably been transported out of the black siliceous schists 

 mentioned above. The volcanic region of the Cameroon and Rumbi 

 Mountains extends over more than 100 German miles; its highest 

 summits, ascended by MM. Burton and Mann, exceed 13,000 feet. 

 The existence of twenty-eight craters has been ascertained. All the 

 lava-currents have gone southward; and, in this direction, the 

 marginal ashes and slags of the craters are lower and are cut 

 through. Emanations of smoke prove all this region to be still in 

 the condition of a Solfatara. An eruption is said to have taken place 

 between 1830 and 1840 ; details, however, are wanting. This region 

 is south-westward of the volcanic islands of Fernando Po, Principe 



