228 PROF. J. W. GREGORY ON NEPHELINE-SYENITE [May I0,0O, 



the already-mentioned discovery of an ammonite in the limestone 

 of Kessa, indicate a much less remote age for at least a part of 

 the sandstone-formation.' With the evidence at present available 

 it is clear that we cannot go far beyond that author's cautious 

 conclusion. It is certain that approximately parallel to the coast- 

 line runs a belt of Middle and Upper Jurassic shales, resting upon 

 a series of shales and sandstones, which are said to be con- 

 formable to them. In one place the Jurassic rocks are proved to 

 extend across almost the whole sedimentary belt ; but it is possible 

 that the lower beds of this belt may be pre-Jurassic, though the 

 evidence on which they were claimed as Carboniferous is valueless. 

 It is, therefore, probable that Thomson's marine 'Carboniferous' 

 shells are Jurassic, while his corals are probably specimens of 

 weathered recent coral-limestones. 



If any part of the series is pre-Jurassic, then we might expect to 

 find in it some representatives of the Cape Series, which have been 

 recorded as far north as Nyasa, and then, after a great break, in the 

 Sabaki River by the ' Second Stockade ' (lat. 3° 3' south, long. 39° 5' 

 east), that is, about 100 miles west-north-west of Mombasa. I there 

 collected a considerable number of specimens of a mollusc which has 

 been determined by Prof. Amalizky as Palceanodonta Fischeri 

 (Amal.). These Sabaki Shales are therefore Upper Carboniferous. 1 



The sedimentary series on the coast-lands of British East Africa 

 and Usambara may, then, be provisionally arranged as follows, in 

 descending order : — 



1. Pleistocene reefs, limestones, alluvium, and laterites. 



2. Jurassic shales and sandstones: Kimeridgian, Oxfordian, and Callovian. 



3. Possibly a pre-Jurassic part of the Duruma Sandstones. 



4. Magarini Sandstones : ? Triassic. 



5. Sabaki Shales : Upper Carboniferous. 



The attention of travellers in East Africa may be called to the 

 desirability of endeavouring to collect specimens of the fossil ter- 

 restrial plants which various travellers have seen in the Duruma 

 Sandstones, in order to determine the exact age of that series ; and 

 in the second place to observe into precisely which member of this 

 sequence the camptonite-dykes are intrusive. 



In the Sabaki Valley the great flow of lava that forms the Yatta 

 plateau is no doubt younger than the Sabaki Shales ; but, so far as 

 I know, the only case of actual contact of the post-Archaean igneous 

 rocks of British East Africa with the sedimentary series is afforded 

 by the nepheline-syenite of Jombo and its camptonite-dykes. And 

 though the evidence is not conclusive, it is probable that these rocks 

 are not older than the early Mesozoic, and may be Jurassic or post- 

 Jurassic. 



Further evidence of the Jurassic age of the Duruma Sandstones 

 is given by a coral collected by Mr. Hobley west of Buni Hill (lat. 

 3° 58' south, long. 39° 35' east), a few miles west of the ammonite- 



1 See my work on ' The Great Eift Valley,' 1896, p. 229. 



