OF THE RIFT VALLEY 
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the East African arch exercised heavy pressure on the under- 
lying plastic material, which was forced up the weakest 
points along the adjacent cracks and produced a series of 
volcanic eruptions. 
To determine the dates of these events it is necessary to 
consider the structure of East Africa in relation with that of 
the adjacent parts of the Indian Ocean. We must go back 
to the time when East Africa and India were both included 
in one great tropical continent, which extended from Brazil 
on the west to Australia on the east. This continent is 
known as Gondwanaland — a named based on that of the Gonds, 
a tribe who inhabited a part of India where the deposits of 
this land are well preserved. Gondwanaland existed through 
the Carboniferous Period, during which was formed all the 
world's chief coal-fields. The fresh-water shells which I 
collected on the Sahaki, in 1893, are the only Gondw r ana fossil 
animals yet found in British East Africa, and from their 
evidence it is probable that the sandstones of the Taru Desert 
were deposited on the ancient continent of Gondwanaland. 
This continent continued through the two periods after the 
Carboniferous — the Permian and Trias ; so that no marine 
beds belonging to them were deposited in tropical or southern 
Africa. 
In the next period, 1 the Jurassic, Gondwanaland began 
to break up : the sea invaded the coastlands of India and of 
East Africa around Mombasa, Malindi, and the Juba. 
The Mesozoic marine deposits are less varied in British than 
in German East Africa, where they are famous, as they have 
yielded the remains of Gigantosaurus — so far as is known, 
the biggest animal that has ever walked on land. 
The marine fossils found on the mainland near Mombasa 
1 For convenience of reference, the names of the later geological periods 
and their English representatives may be summarised as follows : 
Pleistocene. — The period from the present to the Great Ice Age. 
Pliocene. — The period including the Crag beds of Suffolk. 
Miocene. — The period of the main elevation of the Alps, and represented 
in East Africa by the Dinotherium Hobleyi beds of Kavirondo. 
Oligocene. — The period including some beds along the Solent and the 
Amber forests of the Baltic and the Nari beds of India. 
Eocene. — The period including the London Clay. 
Cretaceous. — The period of the Chalk. 
Jurassic. — The period of the Oolitic limestones. 
