LUMBWA CAVES 
488 
The Method of Occurrence 
Lumbwa country consists of a gently rolling plateau, 
generally sloping westwards towards the Kisii Ridge and 
southwards towards the Sotik Plains. When one comes to 
march over it, however, it is found to be dissected by numerous 
deep valleys, the majority of which drain away to the Sondu 
River and so into Lake Victoria. The whole area is composed 
of volcanic rocks of no great geological age. The soil is deep 
red and often of great depth, and, generally speaking, the 
rocks next below the soil are beds of soft, yellowish-brown 
ash, fine in grain and uniform in texture. Immediately below 
that, one often finds a rhyolite, the cavities of which contain 
zeolites. Below that again is a sheet of phonolite, which 
appears to be continuous from the north to the south of the 
district in the region of the cart road. Near Bagau Cave it 
is probably over one hundred feet thick, and at Gitoi it 
appears to be about fifty feet. Its character is very uniform 
throughout. 
Below the phonolite lies the bed of ash and tuff, which is 
excavated by the Lumbwa as either a condiment or medicine 
for their live-stock. It varies in thickness from four feet to 
ten feet. Its usual thickness is from four feet to six feet. It 
varies in hardness, and the harder portions are left as pillars 
to support the workings described in the itinerary. Its per- 
sistence appears to be a remarkable fact, for its northernmost 
outcrop occurs at Bagau Cave on the Kipgues River, in lat. 
S. 0° 86', and it was again identified just north of the Kipsonoi 
River in lat. S. 0°38'. It appears to cover, without inter- 
mission, an area of about 160 square miles, and possibly more. 
Below this ash bed another sheet of vesicular rhyolite 
occurs, much harder in character than the upper bed of that 
rock. The thickness of this was not ascertainable during 
our journey. This volcanic product also appears to run in 
an uninterrupted fashion from north to south. 
The question arises as to the origin of this series of volcanic 
rocks, and it is to be regretted that time did not permit of 
detailed inquiry into this problem. It is, however, premised 
that these rocks are the ejecta from a series of volcanic vents 
