J. Parkinson — Geology of the Gold Coast Littoral. 267 



the alluvium at Krobo, but a short distance from the mouth, and again 

 at Eiposo, two and a half hours' journey by canoe, or roughly ten 

 miles. Here occurs a pegmatitic granite, containing a few flakes of 

 biotite, which make a foliation just discernible. Fine-grained 'pepper 

 and salt ' gneisses are also conspicuous, in which the content of biotite 

 varies irregularly, producing locally a concretionary appearance. 

 These rocks outcrop at intervals from the river-bed at low-water 

 ai'ound the village of Dabuassi, but the best exposures are at the 

 Falls. Here many varieties of gneiss occur, amongst them one 

 containing biotite, subordinate green hornblende, and paler epidote. 

 Quartz, exhibiting some crush shadows, preponderates amongst the 

 colourless minerals, which include a little microcline, a good deal of 

 untwinned felspar, presumably orthoclase, and rare triclinic felspar. 

 Very coarse pegmatite veins and a fine-grained white biotite-granite 

 are associated with the foliated rocks. The black coating, super- 

 ficially somewhat resembling graphite, which covers these rocks, and 

 effectually masks their structure, is exceedingly well developed at 

 these Falls of the Prah. 



Some three and a half miles from Chama (going west) the purple 

 grits come on again in force, dipping almost due south. They are 

 often green and mottled, as are our English slates in some parts of 

 Cornwall, and, approaching Sekondi, are iu turn again succeeded by 

 a hornblende-gneiss, cut as before by an acid rock. The more basic 

 type of the former consists for rather more than half its bulk of 

 a green hornblende. A few flakes of elongated biotite, of earlier 

 growth, cut these. The felspar, an acid to intermediate plagioclase, 

 preponderates over the quartz, which, as at Chama, shows crush 

 shadows. Biotite-hornblende-gneisses, in which the proportion of the 

 two-coloured constituents varies considerably, come from the same 

 locality. In these quartz is usually common, exhibiting crush 

 shadows as before, and quartz vermicule is locally conspicuous. 

 Large orthoclases, often twinned, a little microcline, and triclinic 

 felspars, having symmetrical extinction angles up to 15°, are normal, 

 but, in regard to the last named, it is noticeable that the twinning is 

 often partial. Sphene, apatite, specular iron, pyrites, and iron oxides 

 are the usual accessory minerals. 



It is clear that along this part of the Gold Coast littoral, a distance 

 of approximately forty miles, we have — 



1. A group of biotite- and hornblende-gneisses, associated with later 

 granites, pegmatites, and aplites. 



2. A series of purple grits laid down upon the eroded surface of the 

 crystalline rocks. 



3. Evidence of considerable later elevation of the coastline. This 

 elevation, remarked upon by Ellis 1 and suggested by the present 

 writer in the case of Western Liberia, is, in the same manner, clear 

 around the mouth of the Sassandra River, and may tentatively be 

 correlated with the local elevation of the Ijebu Beds of Southern 

 Nigeria (Pleistocene) and Benin Sands (Late Pleistocene to llecent) 

 of the same Protectorate. 



1 Ellis, The Tslii- speaking Peoples of the Gold Coast of West Africa, p. 2. 



