286 
THE LUMWBA AND ELGON CAVES 
probability to the effect that the lake once had a great ex- 
tension to the north-east, and it is quite likely that this Nandi 
fault line, extending in a W.N.W. direction along the southern 
face of the Maragoli Hills and on past the Samia Hills, slightly 
raised the northern shore of the lake between Samia and the 
Nile, and cut off the ancient north-east arm, thus forming 
the present lake shore. The upthrow of the fault would 
naturally decrease to the westward. A section and map 
showing the faulting of this region is given. 
To come back to the Elgon caves and the possible modes 
of formation. Caves can be classified under various heads. 
There are the caves of limestone countries due to the excava- 
tion of the rock by a process of solution by water charged 
with carbonic acid gas, which finds its way down the joint 
planes and along the bedding. There are the caves formed 
by the action of waves beating against a rocky shore and 
detaching a bed of softer rock. Examples of both these 
classes are common in Europe, and examples of the latter class 
are to be found along the shores of East Africa. Now what 
do we find in the way of caves in volcanic countries ? We find 
caves due to the occurrence of huge bubbles in lava, which are 
caused by accumulations of water met with during the course 
of the lava stream and which generate steam ; the lava cools 
rapidly on the surface and forms a crust over such pockets 
of water -vapour. Ages afterwards the lava weathers away 
and exposes such cavities. Other caves in lava occur owing 
to the rapid cooling of the crust and the molten mass under- 
neath breaking out in a weak place and leaving behind it a 
cavity covered by a thin crust. These caves are more common 
than those caused by steam, and many examples are to be 
found in Hawaii, Iceland, &c. 
Caves are also sometimes formed upon lines of faults ; 
the space between the two rock walls is often filled with 
broken debris, and this may be removed by the agency of 
water, leaving curious caverns. 
Since examination of the Lumbwa caves, I must confess 
to being prejudiced against my former view — that the 
Elgon caves were caused by the action of water, especially as 
it necessitate the foundations of a satisfactory theory explain- 
