CHAP. XII THE BEGINNING OF THE RIFT VALLEY 231 



and Naivasha, to the southern end of the volcanic region near 

 Lake Nyasa. Throughout the whole of geological history the 

 dominant lines of weakness in East Africa have run from 

 north to south, and it is therefore only natural that the ridge 

 should have trended in the same direction. As the move- 

 ments of this epoch raised the line which formed most of the 

 existing watershed between the Atlantic and Indian Oceans, 

 it may be called the " Doenyan," from the Masai name for 

 mountain. 



The regions on either side of this central ridge probably 

 continued to subside. The central arch was therefore left 

 unsupported, and parallel cracks opened along its flanks, as 

 they do on a bridge when its buttresses give way. As the 

 summit of the ridge was thus weakened it sank, making the 

 first step in the formation of the Rift Valley. 



As we have seen in the Introduction, Suess has shown 

 that the Rift Valley extended from the north of Palestine to 

 Lake Nyasa, but the whole of it was not formed simultaneously. 

 Moreover, each part was probably formed by a series of earth- 

 movements at distant intervals. 



The first of the series of north and south faults which 

 formed the Rift Valley happened in the Eocene. This is 

 determined by the occurrence of marine fossils, which must 

 have lived in a gulf that ran into the valley from the Indian 

 Ocean. As the earth -movements of this epoch formed the 

 Nyasa region of the Rift Valley, we may call it the " Nyasan." 



After the subsidences of this epoch, volcanic disturbances 

 of the plateau type began again. In the foot-hills of Kamasia 

 there are lake deposits, in which occur beds of pebbles of the 

 Kaptian lavas. Above the lake deposits there are flows of 

 basalt, which have baked the clays into a porcelain as hard 

 and white as the purest china. We may call this second series 

 of plateau eruptions the " Laikipian," as its basalts form the 

 main cap of Laikipia, where they lie on eroded surfaces of the 

 older lavas. After these eruptions had again disturbed the 

 equilibrium of the region by removing the foundations in some 

 places, and piling up accumulations of volcanic materials over 

 others, earth-movements again set in. Probably it was at this 

 time that the plateau which occupied the site of the Victoria 

 Nyanza began to subside. The head streams of the rivers 



