54 KARIANDUSS DEPOSITS OF RIFT VALLEY 
of the silica in these lavas and ashes is amorphous, that is 
to say, not in a crystalline form, and any crystalline silica 
there may be is probably in what is known as the tridymite 
form, a form which usually occurs in tiny crystals in rocks 
of the character now being discussed. Now certain forms 
of amorphous silica, and also the crystalline form known as 
tridymite, are soluble to a varying extent in carbonate of soda 
solution. Professor Gregory has called attention to the high 
percentage of soda-carrying minerals in many of the lavas he 
collected in his journey through this area, and although proper 
analyses of the rocks have rarely been made, we know that the 
Rift Valley region abounds in soda deposits, vide the Magadi 
Lake and the similar deposits north of Baringo. Lakes Hanning- 
ton, Nakuru, and Elmentaita are so heavily impregnated with 
soda salts as to be undrinkable. The water of Lake Naivasha 
too is impregnated with soda in a lesser degree. We also 
know that at one period of the history of the Rift Valley, 
Lake Naivasha stretched from near Gil-gil to the slopes of Lon- 
genot, and south of that again formerly occurred what Gregory 
calls Lake Suess ; Nakuru and Elmentaita also about that 
time coalesced into one huge lake covering the Elmentaita 
Plains. We thus have all the conditions and materials at 
hand necessary for the formation of these great beds of diato- 
mite ; picture Suswa, Longenot and Eburu all periodically 
in active eruption, and in addition to lava flows ejecting great 
clouds of volcanic dust and streams of mud mainly composed 
of siliceous fragments. This is almost certain to have been, 
as is the case in all volcanoes of this kind ; the steam tearing 
its way through the magma which formed the flows of obsidian 
and trachytic tuffs would naturally blow large quantities into 
a state of very fine division, and this would be spread far and 
wide by the wind and also carried into the lakes by the torrential 
downpours which always accompany volcanic activity. The 
soda-laden water would dissolve the silica and place it ready 
for the diatoms to work upon, and with such rich material to 
build with one can quite see that this form of life could flourish 
with great luxuriance. All these lakes have greatly decreased 
in size since those days, and the walls of the Rift Valley have 
slowly risen owing to faulting, and consequently the beds of 
