500 Geological Society : — 



The structure of the phonolites is trachytic, the groundmass in- 

 cluding abundant small, lath-shaped plagioclases ; nepheline is not 

 very abundant, and occurs in larger crystals than the other con- 

 stituents, the chief of which is aegyrine. The basalts are often 

 vesicular and columnar, and coutain olivine, augite, magnetite, and 

 apatite often in a glassy groundmass. 



The pyroclastic rocks vary in texture from coarse to fine ; they 

 contain blocks of all types of the lavas, and also large crystals of 

 anorthoclase. 



The nepheline-syenite is the most deeply-seated, holocrystalline 

 rock found on Kenya; and the mountain appears to represent a 

 single, ancient, dissected, volcanic mass, with a shorter geological 

 history and a narrower range of rock-types than its great companion 

 volcano, Kilima j^sjaro. The author gives reasons for considering 

 that all the different rock-types present may have resulted by 

 differentiation from the olivine - anorthoclase - nepheline - syenite 

 magma, although this involves the conclusion that in some cases 

 the basic materials must have concentrated in the glass and solidified 

 after the formation of the felspars. 



4. ' Contributions to the Geology of British East Africa: Part III. 

 — The Elseolite-Syenite and Fourchites intrusive in the Coast Series.' 

 By J. W. Gregory, D.Sc, F.G.S. 



The rocks described in this paper were given to the author by 

 Mr. C. W. Hobley Mount Zombo, situated in long. 89° 13' E. and 

 lat. 4° 26' S., and 1519 feet high, is a massif of coarse-grained 

 elssolite-syenite. consisting of anorthoclase, elaeolite, usually allo- 

 triomorphic, and aegyrine. The rock must occur in the belt of 

 Duruma Sandstone, unless the fossiliferous Jurassic shales run 

 westward up the low valley of the Umba River. Associated with 

 this massif is a series of dykes belonging to the olivine-less variety 

 of monchiquites known as fourchites. Their phenocrysts consist 

 of plagioclase, probably oligoclase, hornblende, and aegyrine, but 

 augite and ilmenite are also present : the groundmass contains a 

 certain amount of analcite. 



Unfortunately there is some doubt as to the exact age of the 

 Duruma Sandstone into which the fourchites are intrusive. The 

 author gives an account of the different opinions as to the age of 

 these rocks ; the evidence at present available is only sufficient to 

 prove that they are post-Carboniferous and pre-Callovian. The 

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

 Usambara, in his opinion, may be provisionally arranged as 

 follows : — 



5. Pleistocene reef's, limestones, alluvium, and laterites. 



4. Jurassic shales and sandstones; Kimeridgian, Oxfordian, and CalJovian. 



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



2. Magarini sandstones : ? Triassic. 



1. Sabaki shales : Upper Carboniferous - 



It is, therefore, probable that the igneous rocks are not older 

 han the early Mesozoic, and may be Jurassic or post-Jurassic. 



