M. Fergusson — Notes on Geology of Tanganyika. 369 



' of the felspars were ophitic plates of quartz. In its quartz contents 



and general character the rock is strikingly similar to quartz-dolerites 

 (diabases) of the Transvaal (e.g. rock from near Potchefstroom)."] 

 To the west of Lake Victoria sandstones overlie the gneiss and 

 continue round the lake shore for some distance. This gneiss 

 formation is again in evidence on the eastern shore, and continues 

 up the Nandi plateau till the volcanic disturbances from the eastern 

 rift valley occur at Mau. 



[" The rocks collected in this volcanic region consist mainly of 

 phonolites and phonolitic trachytes. Most of them contain pheno- 

 crysts of anorthoclase and asgirine-augite in a trachytic groundmass 

 of felspar-laths, through which are distributed irregular grains and 

 feathery patches of augite, and of soda-hornblendes closely related 

 to, if not identical with, Cossyrite and the Catophorite of Brogger. 

 Some of them contain olivine in small amount, and in this respect 

 as well as in their more glassy character approach closely to the 

 pantellerite-like rocks or Kenytes of Mt. Kenya described by Gregory.^ 

 An anorthoclase phouolite from Mau differs from the phonolites on 

 the east side of the rift valley in containing large crystals of a 

 remarkably pleochroic (colourless to deep rose-red) sphene, which 

 appears to have crystallized about the same time as the augite 

 phenocrysts, since in some cases it includes augite and in others is 

 included by that mineral. Besides the phonolites there is also 

 a specimen of andesitic hornblende-enstatite-basalt, somewhat similar 

 in character to the so-called enstatite-porphyrite from Schneide- 

 miillerskopf, Thuringia."] 



North of Lake Naivasha I found a deposit of very pure diatomaceous 

 earth containing fragments of obsidian, and on account of this 

 I imagined it to be a volcanic tuff till Mr. J. J. H. Teall examined 

 it and found it to be full of diatoms. [" The obsidian chips appear to 

 be worked implements similar to those from the same neighbourhood 

 described and figured by Gregory ('The Great Eift Valley,' 

 p. 324)."] 



The diatomaceous earth has been examined by Mr. Thomas Comber, 

 who finds that it consists of fresh-water species, of which he has 

 identified the following : — 



1. Neidium affine, CI., forma major, 



V. H. 



2. AnomcBoneis sph^rophora, CI. 



3. Diploneis ovalis, CI. 



4. Finnularia (icrosphairia, E,bh. 



5. P. legumcn, Ehr. 



6. Navicula bacilUformis, V. H. 



7. JV. pseudohacillum, Grun. 



8. iV. pupula, Kutz. 



9. N. Tuscula, Grun. 



10. N. slesvicensis, Grun. 



11. N. radiosa, Kutz. 



12. var. tenella, Breb. 



13. N. miitica, Kutz. 



14. Cymbella lanceolata, V. H. 



15. C. cistula, V. H., var. maculata, 



Kutz. 



16. C. cymbiformis, V. H. 



17. G. parva, CI. 



18. C. leptoceras, Kutz. 



19. C. tnrgida, Greg. 



20. C. amphicephala, Naeg. 



21. Gomphonema gracile, Ehr. 



22. var. aurita, Breb. 



23. G. intricafum, Kutz. 



24. G. subclavatum, Grun. 



25. G. montanum, Schum. 



26. Amphora ovalis, Kutz. 



1 Quart. Journ. Geol. Soc, 1900, Ivi, p. 205. 



DECADE IV. — VOL. VIII. — NO. VIII. 



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