6 
THE MIOCENE BEDS OF 
and sometimes fantastic shapes to the action of gentle currents 
disturbing the uniform deposition of the travertine. 
Finally the Upper Series (Nos. 1-12), although equal to the 
combined thickness of the Middle and Lower Series, consists 
mainly of grey and brown clays and shales with scarcely a 
trace of fossils. It is only in the lowest bed (No. 12) that 
fossils are still present to any extent, e.g. a river-crab 
(Thel'phusa), bones and scutes of crocodile, &c. At Kachuku 
I found crocodiles’ teeth with Ampullaria ovata in the grey 
clay of No. 5, but this was the highest level at which vertebrate 
remains occurred. 
These clays were evidently deposited at a time when the 
rivers had nearly reached their base-level, and were normally 
only able to deposit fine mud which was probably derived 
mainly from the much-weathered and decomposed gneiss of 
the Kamagambo peneplain. Thin seams of travertine are 
frequently intercalated with the clays. 
It needed some exceptionally wet season to bring down 
coarse sandy material in order to form the grey, current-bedded 
sandstones, which occur at rare intervals and often pass 
laterally into grey clay. The only one of these bands (No. 8) 
that persists throughout the area is about 6 feet thick ; it 
forms a noticeable ledge in the upper part of the main gully 
at Nira (Fig. 1) and is composed of quartz-grains with plates 
of biotite and small crystals of augite. At Kikongo I found 
it to contain a few land-shells ( Tro , pido / p}iora nyasana, Limi- 
colaria, and Cerastus). 
In the topmost bed of grey clay (No. 1) the petrified stems 
of extinct species of trees occur, allied to Bornbax, laurels, &c., 
and are particularly well preserved at Kikongo. They were 
the result of quite unusual circumstances by which water- 
logged trunks were calcified by the agency of calcareous springs, 
the wood being replaced by lime, particle by particle, so that 
when thin slices of the fossil stems are prepared and placed under 
the microscope the most delicate cell- structures are revealed 
as clearly as if the sections had been made from living plants. 
It is somewhat surprising that the fossil shells consist 
entirely of gasteropods to the complete exclusion of bivalves. 
This would seem to indicate that the strata were laid down so 
