480 
THE GEOLOGICAL HISTOKY 
Some doubt was expressed as to this conclusion : I was 
therefore glad in 1892 to accept an invitation to visit East 
Africa. When the collapse of the large expedition with 
which I came out left me to make fresh plans, I resolved 
to examine the country round Lake Naivasha at the highest 
part of this trench, and also around Baringo, its next fresh- 
water lake to the north. 
The journey along the valley from the Kedong to Baringo, 
and examination of its walls convinced me that it had not 
been carved out by rivers and rain and wind, but was directly 
due to earth-movements — that is to say, it is a tectonic valley 
and not a valley of excavation. 
The existence of tectonic valleys was distrusted by the 
then orthodox school of British geology, according to which, 
valleys are scooped out by the geographical agents that act 
on the earth’s surface. The discussion of a recent paper by Mr. 
Parkinson on Lake Magadi shows that some authorities doubt 
whether earth-movements along faults and fractures ever give 
rise to cliffs and valleys. According to that view, the earth is 
an inert mass, on which the surface features are incised by such 
agents as rivers, glaciers, rain, wind, and the surf along the 
coasts, or are built up by piles of volcanic materials. The 
earth’s crust is regarded as having no more share in producing 
the ups and downs on its surface — other than those produced 
by wave-like foldings of the crust — than a block of marble 
shares in the development of the features which are being carved 
on it by a sculptor. 
According to the alternative view, the earth is not a mere 
impassive mass : its crust constantly pulsates under move- 
ments of its own ; its surface heaves and falls in response 
to internal influences ; and it is owing to these crustal move- 
ments that the earth remains suitable for the home of man. 
The adoption of the latter view has been largely aided 
by the evidence of the Great Bift Yalley, which is now 
generally accepted as due to a long strip of the earth’s crust, 
extending from northern Palestine to southern Portuguese 
East Africa, having foundered between a series of parallel 
fractures. 
The majority of valleys are no doubt valleys of excavation. 
