KARIANDUSS DEPOSITS OF RIFT VALLEY 58 
of the water and slowly form a siliceous deposit, and this is 
the basis of our Diatomite. 
Diatoms form an enormous family, one that is said to 
number several thousand species. Myriads live in the sea as 
well as in fresh water, and in the great depths of the ocean 
there are enormous deposits of ooze formed of the skeletons 
of diatoms, globigerina and radiolaria, the last two, however, 
being classed among the animal kingdom. 
Diatoms are known to belong to the vegetable kingdom 
by the presence of chlorophyll, a tiny fragment of which 
colours each cell green or brown, and it is upon this chlorophyll 
that their life depends, for it has the power of absorbing carbon 
dioxide from the air, picking out the carbon and with it building 
up the ceil substance of the diatom, rejecting the oxygen. 
Any other nutriment the cell requires is obtained by absorbing 
various salts which happen to be in a state of solution in the 
water in which its habitat lies. On the other hand, diatoms 
possess a faculty which is usually looked upon as a peculiarity 
of the animal kingdom, namely, that of motion; its method 
of progression is a mystery, but is possibly due to the move- 
ments of tiny cilia microscopically invisible. The pace is so 
slow compared with that with which it will naturally drift 
by wind or currents that it is not easy to see what the organism 
gains by such feeble motive powers. It has, however, been 
suggested that its power of motion may prevent it being 
covered with mud when it sinks to the bottom of a pool, and 
it also may enable it to retire below the surface where the 
upper layers of water are disturbed by a strong breeze. 
The next question one probably asks is why such enormous 
deposits of diatomaceous clay should be found in the Rift 
Valley, and this can, I think, be explained to a great extent. 
From Tertiary times onwards the Rift Valley has been the 
scene of tremendous volcanic activity, and the eruptive matter 
has uniformly been of what geologists call an acidic type, that 
is to say, the lavas and ashes ejected from the volcanoes 
contain a preponderance of silica (Si0 2 ). For convenience 
of classification all volcanic materials containing over 60 per 
cent, of Si0 2 are classed as acidic rocks, and all containing 
below that percentage are classed as basic. Now the bulk 
