THE LUMBWA AND ELGON CAVES 
291 
1 . Pleistocene Period . — Elgon series in eruption : this is 
doubtless correct, but what may be called the birth of Elgon 
will probably be found to date back to late Miocene times. 
2. Cretaceous Period. — Kaptian. Referring to Professor 
Gregory’s ‘ Great Rift Valley,’ p. 230, it will be noted that he 
places the formation of the lava sheets of the Kapiti Plains 
as far back as the Cretaceous era, and considers these plateau 
eruptions to be chronologically coincident with certain denuded 
volcanic cores which protrude through the Jurassic rocks 
near the coast north of Vanga at Jombo and Kiruku. He 
unfortunately does not give the data which led him to the 
conclusion that the Kapiti lava sheets are as old as he states ; 
personally I am inclined to essay the opinion that they are 
not older than the Pliocene era. The insignificant hills of 
Jombo and Kiruku are, however, I consider, the oldest traces 
of volcanic activity in British East Africa (excluding, of course, 
granitic dykes in the old metamorphic series), and quite dis- 
tinct from that of the high plateaux of the interior. Between 
these two hills, for instance, lies another hill called Mrima, 
and this is composed of Triassic sandstones, and an unaltered 
exposure of these beds was observed close to Kiruku peak, 
which shows that at that period Kiruku at all events had ceased 
its active life, and had been eroded down to a mere core. The 
rocks from both Jombo and Kiruku are of an older type than 
anything I have seen up country, but I am unable to accurately 
describe them until a section can be made for microscopic 
examination. At any rate these old volcanoes should, pending 
a more detailed survey, be tentatively considered to date back 
to Jurassic times. 
The full consideration of these problems has, I fear, brought 
under review a number of questions which at first sight may 
have appeared somewhat irrelevant, but if any of the caves are 
of natural origin the causes which produced them are inextri- 
cably bound up with the geological history of the Lake Victoria 
basin. It is to be regretted that fuller information is not 
available, and I would have then ventured to take up 
a more dogmatic view. After considering the evidence, and 
seeing that in Lumbwa the natives of the same race to 
those living on Elgon are actively excavating material 
