OF THE RIFT VALLEY 
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practically to its present form by the time of the Great Ice 
Age and at the end of the Pliocene. And Kenya is not 
the earliest of the East African volcanoes. Hence the whole 
volcanic history of the country, from the beginning of the 
volcanic eruptions to the wearing down of Kenya into its 
present condition, cannot be compressed into only one geological 
period — the Pliocene. 
The fact, then, that the crater of Kenya had been destroyed, 
and the plug of the old volcano worn down to essentially 
its present form and dimensions by glacial times, is evidence 
that Kenya is a very old volcano, and that the volcanic history 
of British East Africa must be carried back into pre-Pliocene 
times. 
As the phonolites of Lumbwa are post-Miocene, and those 
of the Kapiti Plains not later than the Miocene, the simple 
classification which would attribute them both to the same 
volcanic period appears inadmissible, and we must be prepared 
for a longer and more complex classification of the volcanic 
rocks of British East Africa. 
The main stages seem to be as follows : The oldest lavas 
are the phonolites of the Kapiti Plains, and the earth- 
movements which occasioned their widespread eruption were 
probably connected with the foundering of the Indian Ocean. 
The lavas may be co-related with the Deccan Traps and be 
of Upper Cretaceous age. 
The eruptions of the soda-rich phonolites culminated in 
the eruption of the soda-rich kenyte of Kenya. Then followed 
a series of eruptions which produced the basaltic agglomerates 
of northern Kikuyu, vast basalt sheets around Lake Magadi, 
and the porphyritic basalts of Kijabe and the Aberdares. 
These eruptions were probably accompanied by the first 
of the Rift Valley faults, and were followed by a long stage 
of denudation ; and then renewed volcanic disturbances dis- 
charged the rhyolites and trachytes, which are especially well 
developed in the Kikuyu country. Then, after further faults, 
ensued a long, quiet period, during which the level of the Vic- 
toria Nyanza was higher than it is to-day, and the occurrence 
of Dinotherium Hobleyi in the lake deposits proves their Miocene 
age. Then followed the post-Miocene eruptions, including 
