Vol. 56.] AND CAMrTONTTIC DYKES IN BRITISH EA.ST AFRICA. 227 



they would answer for the common East African Jurassic type ; 

 and Stuhlrnann, 1 in his geological map, marks Umba as Jurassic. 



Thomson's identification however, appears to rest in part on litho- 

 logical resemblances. He remarks that lithologically the sandstones 

 agree with those described by Thornton at Mombasa ; but, as the 

 only sandstones on Mombasa Island are Pleistocene, any lithological 

 resemblance to them is not a proof of Carboniferous age. Thomson also 

 compares the Usambara limestones to those of Mombasa ; but this does 

 not strengthen his case. For, though I examined many of the coast- 

 sections of Mombasa Island, and also traversed the island in several 

 directions, I saw nothing but Pleistocene deposits, such as alluvium, 

 raised reefs, and red sandstones. Some of the limestones, however, 

 are so full of Archaean debris and garnets that they appear very 

 ancient. One of these limestones from near the Administrator's 

 house at Kilindini is full of garnets, orthoclase, microcline,plagioclase, 

 quartz, tourmaline, 2 etc., and its aspect in hand-specimens resembles 

 that of a calciphyre. This apparently ancient limestone may well 

 be the rock that Thomson had in mind when correlating the Mombasa 

 with the Usambara limestones. But the former are certainly late 

 Kainozoic, and their age may be merely a matter of a few centuries. 



A further blow at the Carboniferous age of the coast series has 

 been given by the discovery of an ammonite at Kessa, west 'of 

 Bagamoyo. This ammonite, though badly preserved, has been 

 determined by Putterer 3 as a member of the group of Perisphinctes 

 Martinsi. I am not sure of the exact position of this locality, which 

 is simply described by Putterer as west of Bagamoyo. The most 

 probable name marked on the Langhans' Deutscher Kolonial-Atlas 

 (1897, map No. 22) is Kwa Kiuisa, which is almost due west of 

 Bagamoyo on the meridian of 38° ; and Thomson, in his map at 

 the end of vol. ii. of ' To the African Lakes and Back ' marks 

 his belt of limestone only about 5 miles west of the 38th meridian, 

 and at the western end of the sedimentary series. The ammonite 

 occurred in a micaceous quartziferous limestone, which rests con- 

 formably on the sandstones of Usaramo, these being in all probability 

 a continuation of those of Duruma. And Putterer concludes, ' it 

 appears very probable that the Usaramo Sandstones also are Jurassic, 

 and their supposed older date cannot be maintained for at least the 

 greater part of them' (l,oc. cit.). 



The same conclusion is maintained by Stromer von Eeichen- 

 bach, 4 who points out that ' it is still very problematical whether 

 all these rocks are of the same age, and whether they belong 

 to the Carboniferous. The similarity of their stratification and of 

 their petrographical characters with the Jurassic beds, and especially 



1 ' Mit Emm Pascha ins Herz von Afrika' Berlin, 1894 : geol. inset-map on 

 large map. 



2 The occurrence of some of these minerals in the existing shore-sands of 

 Mombasa has been recorded by Mr. Walcot Gibson in bis ' Geological Sketch of 

 Central East Africa ' Rep. Brit. Assoc. 1893 (Nottingham) p. 758. 



3 Zeitschr. Deutsch. Geol. Gesellsch. vol. xlvi (1894) p. 49. 



4 'Geologie d. deutschen Schutzgebiete in Afrika' 1896, p. 28. 



