Oorr6spondence — P. Dixey. 525 



inner margins into the uplands. 'Where, on the other hand, the change 

 from coastal plain to upland or plateau country is abrupt, and the 

 relief of the crystalline rocks is considerable, altered rocks are rarely 

 seen and laterite is practically absent. 



3. Ancient Sedimentary Rocks of the north-west corner of the 

 Protectorate. — These sediments, which do not depart greatly frcnn 

 the horizontal, are capped by a great thickness of dolerite. The 

 sediments and the dolerite, together with the underlying crystalline 

 rocks, form a great scarp over 2,000 feet in height. Laterite is well 

 developed on the dolerite, but it is now undergoing extensive 

 erosion. 



4. The Norite of the Colony. — Laterite is best developed on certain 

 ancient platforms carved into the mountain mass. 



5. The Great Plateau forming the north-eastern part of the 

 Protectorate. — Over a large portion of this plateau, away from the 

 margins, conditions are very different from those considered above, 

 because an extensive area of the crystalline rocks, which make up 

 the plateau, is overlain by a sand formation,^ which so far has not 

 been observed in any other part of the country. This sand formation 

 locally attains a thickness of more than 300 feet. It is undergoing 

 extensive erosion, which has resulted in the central areas of the 

 sand-sheet in the carving of deep valleys, at the bottoms of which 

 the crystalline rocks are again exposed ; towards the margins of the 

 sheet the sands appear as numerous flat-topped and conical hills, and 

 finally, along the limits of the sheet, the sands form only a few caps 

 on the crystalline plateau. This plateau is generally well-defined, 

 although it is for the most part trenched by steep-sided valleys ; 

 locally erosion has proceeded much further. The upper surface of 

 the main sheet of the sands consists of a series of plateaux, which do 

 not differ greatly in height. Laterite is developed on the sands and 

 on the crystalline rocks as follows : — 



(a) The Sands. — The laterite forms a continuous crust, rarely 

 exceeding 2 feet in thickness, on the surfaces of the plateaux and on 

 the flat-topped hills. The laterite of the higher plateaux must be 

 regarded as older than that of the lower. On the sides of the hills 

 laterite does not occur except as fallen blocks, and on the low ground 

 between the hills it is develoj)ed only as occasional thin patches of 

 gravel. 



(b) The CrystaUine Eocks. — Laterite has very rarely been observed 

 on these rocks within the limits of the sand formation. The sands, 

 often containing near their base fragments of the country rocks, 

 frequently rest upon a perfectly fresh surface ; elsewhere they rest 

 upon a surface which is variably, but not deeply, kaolinized. Some- 

 times, far beyond the present limits of the sand-sheet, there occur 

 extensive flat or gently convex areas of the crystalline rocks, chiefly 



^ I have recently given a brief description of these sands and the ferruginous 

 laterite capping them ; see " Primitive Iron-ore Smelting Methods in West 

 Africa " : Mining Mag., October, 1920. 



