230 THE GEOLOGY OF BRITISH EAST AFRICA part in 



fossil is not free from doubt, but as the conclusions it suggests 

 are probable and in accordance with other lines of evidence, 

 and as it fixes a date which otherwise must be left uncertain, 

 it is convenient to admit it. 



Accepting, then, that the fossil in question came from East 

 Africa, we know that a new stage in the history of the country 

 began after the close of the Jurassic. Up to this point, ever 

 since the Archean era, Eastern Equatorial Africa had been 

 stable and restful ; but then the old volcanic fires broke out 

 anew, and inaugurated a series of events which have given the 

 region its especial interest. In summarising these it is con- 

 venient to give names to the principal divisions, and the native 

 terms for the geographical features determined in each of them 

 may be appropriately adopted. 



The first of these we may call the Kaptian, as in it were 

 formed the Kapte plains. That this period began later than the 

 Jurassic is probable, for Hobley has shown that the rocks of 

 this age on the coast are cut by dykes. The composition of 

 the rocks suggest that these dykes belong to the first series of 

 eruptions. The volcanic action near the coast was not very 

 powerful, but in the interior innumerable small volcanic cones 

 buried an enormous tract of country under a flood of molten 

 rock. 



The earliest lavas ejected by these " plateau eruptions " 

 were of the kind known as " trachytes," and these were fol- 

 lowed by others containing less silica, which are named 

 " andesites " owing to their importance in the Andes. 



The duration of this period of intense volcanic activity 

 cannot be precisely determined, but it must have been pro- 

 longed. When the pressure on the lava sources was relaxed, 

 the volcanoes dwindled, became dormant, and then extinct. 

 They had, however, destroyed the structural stability of the 

 country ; for the subterranean reservoirs were empty, while 

 masses of volcanic material had been piled upon the surface. 

 The upper layers of the earth's crust were therefore over- 

 weighted above and weakened below, and earth -movements 

 were necessary to restore equilibrium. The first change was 

 probably a subsidence of the country to the east and west of 

 the East African lake-chain, leaving this line as a ridge or 

 arch running north and south from Basso Narok, past Baringo 



