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



under favourable circumstances, of iron leached from the beds them- 

 selves. A typical example of the ordinary red grit was composed of 

 about 50 per cent, of angular quartz-grains, the greater part of the 

 remainder of felspar, twinned and untwinned, often kaolinized, and 

 similar to the felspars of the surrounding crystalline rocks. 



Thin laminae of kaolin also occur at Elmina, and occasionally 

 pebble beds. The series continues along the sea-front as a broken 

 ridge, and about six miles from the sea, to the northward, rests on the 

 edge of the granitic platform. Here the coast is characterized by 

 isolated flat-topped hills, which appear clearly to owe their structure 

 to marine erosion, and are separated by serai-marshy flats of blackish 

 soil, crowded with shells and impregnated with salt. The shells 

 disappear about three miles from the sea. In these flats the natives 

 dig pits, which fill naturally with water. This on being heated 

 3 T ields the salt. A slight depression would thus reduce the littoral to 

 a series of islands, arranged in rows, approximately parallel to each 

 other and to the sea-front. 



The crystalline rocks include a biotite-garnet- granite, with muscovite 

 as an accessory mineral, and having associated with it an aplite, locally 

 pegmatoidal. In this rock microcline and quartz are the two principal 

 constituents, the latter mineral giving evidence of a slight amount of 

 crush. Triclinic felspar is rare and shows, Avhen noticed, symmetrical 

 extinction angles of about«13°, but the twinning is, in many instances, 

 very irregular and partial. 



At Chama, both on the shore and in the town, a massive gneiss is 

 admirably exposed, and amongst the many varieties is one conspicuously 

 banded. Gneissose veins or dykes cross the foliation of this rock, and, 

 in thin sections, are seen to be identical in structure and composition 

 with the more acid bands. The banding has, in fact, been produced 

 by the injection, in parallel 'sills', of an acid magma, along the 

 foliation planes of a more basic rock. The rocks are of granulitic 

 structure, and usually contain a few flakes of biotite, with sphene and 

 epidote as rare accessory minerals. Untwinned felspar, presumably 

 orthoclase, is common, and occasionally a considerable proportion of 

 microcline. An acid plagioclase is subordinate. The basic part of the 

 gneisses contains much biotite, in flakes occasionally 6 mm. in length. 



Two and a half miles west of Chama I found a garnet-hornblende- 

 biotite-gneiss, granulitic in structure and well foliated. Into this 

 rock gneissose dykes are clearly intrusive, and differ only in un- 

 essential points from those of Chama itself. It seems clear that at 

 Chama and in the neighbourhood we have a group of biotite-gneisses, 

 of which the acid member is the later, and which are associated with 

 intrusive biotite-granites, aplites, and pegmatites. The strike of the 

 planes of foliation is approximately east and west true. 



The JPrah River, which empties into the Atlantic about a mile to 

 the east of Chama, flows, as regards its lower course, through a well- 

 graded plane of granites and gneisses. 1 These rocks first emerge from 



1 In 1908 the Prah broke through the strip of sand which, as in many West 

 Coast rivers, extended almost entirely across its month and enclosed a lagoon. 

 When I saw it in the spring of 1910 this had been repaired by the building-up 

 action of the surf and stood some 15 feet above sea-level. 



