484 
LUMBWA CAVES 
which occur on the southern flank of the Nyando Valley, and 
commence with the mountain known as Cheblil. There are 
other hills, which bear the appearance of weathered volcanoes, 
in the vicinity of what is known as Tugenon Camp ; detailed 
investigation is, however, necessary before a definite state- 
ment can be made, and our visit was mainly concerned 
with the economic possibilities of the deposit worked by the 
Lumbwa. 
As regards the origin of the phosphate, this may be due 
to the presence of minute crystals of apatite in the tuff. The 
Analyst’s report shows that neither the phonolite above, 
nor the rhyolite below, contains any appreciable amount of 
phosphate. No microscopic examination of the tuff has been 
possible up to date. 
The great thickness of the red earth in parts of Lumbwa 
has been referred to, and it has been suggested that this is 
due to the eruption of vast quantities of volcanic mud from 
the same volcanoes which at an earlier date produced the 
other rocks. No evidence to support this theory was, however, 
observed, and it is believed to be due to the oxidation of the 
upper bed of fine-grained yellowish-brown ash, the variability 
in thickness being dependent on its accessibility to denudation. 
The whole of the Lumbwa was, it is believed, clothed at 
one time by dense forest, and this was probably almost entirely 
cleared off by a former race of aboriginals, the sites of whose 
huts can now be discerned, for they had a curious practice 
of building their dwellings in pits in the surface of the ground. 
Once this forest was removed, a period of rapid denudation 
set in, and continued until other growth asserted itself. 
Whether the ancestors of the Lumbwa drove these people 
out is not clear ; but since the advent of the Lumbwa occupa- 
tion considerable areas of secondary forest have sprung up, 
composed of thorny acacias and such like, and the typical 
forest-trees such as the olive, podocarpus, &c., which still grow 
a few miles farther up the slope, are absent in Lumbwa 
proper : in fact, the boundary marking the extent of the ancient 
forest clearing can still be seen, and tongues of it still run 
down into, and have been spared in, some of the valleys, as at 
Kerieho Station, thus showing that the general absence of 
