162 A NATURALIST IN MID-AFRICA. 



coast formations, one enters on the Karoo sand- 

 stones, and then in the Transvaal on a curious 

 mixture of Archaean rocks and porphyritic diorites. 



In East Africa, on this last expedition, one 

 enters, soon after leaving Mombasa, an Archaean 

 series of gneisses, which in the Masai highlands 

 are covered by masses of lava flows, and reappear 

 in the Victoria region, extending to the base of 

 Ruwenzori and, as shown in the map, ending on 

 its eastern side. 



These are overlaid by a probably unconformable 

 series of clay slates, sandstones and quartzites, 

 which seem to extend very far to the south, 

 probably reaching South Tanganyika, and which 

 form what I call the Central Watershed". There 

 is, besides this, Ruwenzori, which is not a pure 

 volcanic mass like Elgon, Kenia or Kilimandjaro, 

 but probably a "block 1 mountain, consisting 

 mainly of schists and a central core of gneiss or 

 granite, and which also possesses round the base 

 another quite different series of comparatively 

 recent volcanic rocks. 



Neither the Karoo sandstones, the Nubian sand- 

 stones, nor the sedimentary rocks of the Central 

 Watershed are, at any rate to an ordinary observer, 

 in the least similar to one another, but appear to 

 be quite distinct and separate formations. 



It is excessively difficult, therefore, to under- 

 stand in what condition the continent of Africa 

 was in any preceding geological age, and without 



