Notices of Memoirs — Dr. 0. Lenz — Geology of W. Africa. 173 



assumes a metallic brightness. The crust is most conspicuous on 

 the gneisses and on the mica-schists with garnets in the Apinshi region. 

 It is absent as well on the portions above the contact of water as on 

 the rocks along the banks where the water flows quietly. 



Similar facts have been ascertained on the Eapids of the Nile, 

 on the Cataracts of the Congo (by Captain Tuckey), and on the 

 S3 r enite rocks along the Orinoco (by Alex, von Humboldt). Dr. 

 Darwin observed a black crust, similar in appearance to graphite, 

 on the banks of several Brazilian rivers which open into the 

 Atlantic. Berzelius found this crust to be composed of oxyds of 

 iron and manganese. 



The older crystalline rocks of North-west Africa are everywhere 

 overlain by a (probably Diluvial) deposit of intensely yellow, loamy, 

 and highly ferruginous sands, including large blocks of brown 

 hydroxyd of iron. These blocks are aggregated concretions of the 

 size of beans or peas, similar to the pisiform iron -ore of Europe. 

 "When such blocks are decomposed, the concretions, bearing distinct 

 marks of having been rolled, are spread over an extensive surface. 

 The rivers carry with them enormous quantities of fine white quartz 

 sand, with flakes of mica, which every year during low water are 

 deposited in enormous sand-banks, rising several metres above the 

 sea-level. The waters, whirling violently against the rocks, keep 

 in suspension the quartz grains and the concretions of iron-oxyd, 

 and the last material, comminuted by the friction of the hard sand, 

 is deposited on the surface of the rocks in the form of a thin crust. 



3. Geology of the Gold Coast, Guinea. — The gold here appears in 

 the shape of dust or granules, seldom of the size of a pea. The 

 natives wash it out of the clays and sands in a most primitive way ; 

 and they frequently adulterate it, by boring holes into the granules, 

 filling up the cavities with copper or brass, and carefully closing them 

 again. The primary locality of the gold is still unknown. 



It is washed everywhere also in the region of the Senegal and 

 Gambia Kivers, as near Cape Palmas, out of a red clay, of probably 

 comparatively recent origin, including layers of rolled ferruginous 

 fragments. Near Accra, close to the sea-shore, there is a coarse- 

 grained, intensely red, and somewhat argillaceous sandstone, with 

 intercalated layers of large rolled fragments of quartz, and without 

 any traces of organic remains. At first view it has a resemblance 

 to some of the Triassic beds of Germany. Further inland are 

 gneisses and granites ; and in the Ashanti region and along the river 

 Volta there are fine black amphibolic schists, abounding with 

 garnets, locally of rather large size. Possibly, as in the Ural, these 

 schists may be the primary locality of the gold. 



4. Itabirite {Iron mica-schist) of the Okande Begion. — The Okande 

 region is situated some sixty geographical miles inland, amidst the 

 rapids of the Ogowe. This river breaks its way westward through 

 a chain of schistose rocks, with a main strike N.-S., and with a very 

 steep dip from W. to E. The Itabirite rests on a very thick stratum 

 of white and red quartz, the same which appears, at a lower horizon, 

 intercalated among the mica-schists with garnets of the Apinshi 

 region. In its upper horizons it passes gradually into quartz. 



