432 
THE GEOLOGICAL HISTORY 
Gomorrah. Sudden movements have no doubt taken place 
in East Africa — such as that which is reported to have made 
the Simbi Crater, to the south of Kavirondo Gulf, in a single 
night, and to have destroyed the inhabitants, with one righteous 
exception, in punishment for their inhospitality. There have 
probably been volcanic explosions in the Rift Valley of world- 
shaking violence. The formation of the Rift Valley itself 
was, however, not one swift catastrophe, but a long, slow 
process which lasted for millions of years and may not yet 
be complete. The process was probably not continuous : 
there were doubtless long breaks, during which the move- 
ments ceased ; and we may now be in one of the quiet 
intervals. 
The history of the Rift Valley may be compiled either 
with the interpretation that it has had a short history (all 
post-Miocene) or that its history is long and complex and 
extends to pre-Miocene times. The latter is the conclusion 
to which I was led, twenty-five years ago, and to which the 
evidence as a whole seems to point. 
As in British East Africa the rocks along the Rift Valley 
are mainly volcanic, and there is a complete absence of rocks 
formed in the sea, its history depends on the history of volcanic 
action in the adjacent country. A volcano, it should be re- 
membered, is not a burning mountain, but a vent through which 
subterranean plastic material can escape to the surface, when 
squeezed out by earth pressure. If you press in the sides 
of a kerosene-tin the oil escapes from any weak point along 
the seams. And if a block of the earth’s crust sinks, the 
plastic rock below w T ill tend to rise along the fractures around 
the sinking block and may be discharged at the surface in a 
volcanic eruption. 
The first stage in the formation of the Rift Valley was 
the elevation of the country along it into a low, broad arch, 
which would have formed a belt of down-like highlands, 
ranging across British East Africa from north to south. In 
the second stage the lateral pressure was reduced, the sides of 
the arch cracked, and the blocks along the highest line sank 
as the key stone of a bridge sinks if the buttresses give way. 
The sinking of the great block which formed the keystone of 
