308 MR. J. PARKINSON ON THE POST-CRETACEOTJS [Aug. 1907^ 



17. The Post-Cretaceous Stratigraphy of Southern Nigeria. 

 By John Parkinson, B.A., F.G.S. (Read December 19th, 1906.) 



[Map on p. 310.] 



The account given in the following pages, of the stratigraphy of the 

 more recent deposits of Southern Nigeria, is the result of work 

 carried out between the winter of 1903 and the summer of 1906. 



Owing to the lamentable lack of fossils, and (I may add also) 

 owing to the lack of time available for a lengthened search, that 

 defmiteness of conclusion which is desirable in regard to the age of 

 given beds is wanting. Correlation from point to point is accordingly 

 based on lithological resemblances, aided in places by the establish- 

 ment of an unconformity. 



The following is the stratigraphical succession, in descending order : 



1. Alluvium of rivers and lower terraces. 



2. Benin Sands. 



3. Ijebu Beds. 



4. Lignite Series. 



Omitting the mangrove-swamps, almost universal on the South 

 Nigerian coast, the traveller finds, on penetrating a certain distance 

 into the hinterland, a great number of sandbanks, now in course of 

 formation. Whether the river be great or small these sandbanks 

 are, during the dry season, the prominent feature which imprints 

 itself on the memory. Sometimes spreading out over almost the 

 entire bed of the river, covered by a few inches of water ; sometimes 

 fringing the river-bank, and half overgrown with grass and reeds, 

 rising step by step many feet above low-water level, a yellow glaring 

 expanse a mile or more in length, these accumulations are a sad 

 hindrance to travel. 



Intermediate in age between these sandbanks and the alluvium 

 of the same valleys, unrelated to the present water-levels, we find 

 extensive areas occupied by alluvial flats, usually raised above the 

 level of any (save exceptionally-high) floods, and actively cultivated.. 

 Such flats are especially well seen along the rivers emptying into 

 the Lekki Lagoon (Ijebu district of Lagos Province). 



Of the various beds described, the sands and clays which I have 

 grouped together as the Benin Sands are stratigraphically the 

 most important, on account of their almost universal occurrence 

 to the south of the basement-platform of crystalline rocks. In 

 their remarkable content of iron they differ from the subjacent rocks, 

 although it should be noted that locally the Cretaceous beds are 

 distinctly ferruginous and resemble also the Benin Sands, as regards 

 their lower beds, in being practically arkoses. The explanation 

 is doubtless the same in both cases, and we must seek it, I believe, 

 in the rapid disintegration of crystalline rocks concomitant with 

 equally-rapid deposition. In the occurrence of fragments of semi- 

 decomposed felspar, also of flakes of muscovite, and in the angularity 

 of the quartz-grains we find evidence of this erosion, while garnet and 

 zircon with various iron-oxides occur in the pan-concentrates. No 

 fossils have yet been found ; the exposed surfaces of the sands often, 

 harden to form a kind of crust. 



