CHAPTER III 
SURFACE AND GEOLOGICAL STRUCTURE 
In the earlier accounts of Geog-raphers the physical 
condition of the island was for the most part described 
erroneously; it was accepted that a central range of 
mountains ran from north to south. According to 
Grandidier, this sierra-like ridge of the older maps is 
non-existent, in spite of the fanciful details of Leguevel 
de Lacombe. On the contrary, the middle of the island 
is occupied by a plateau, stretching from the north to 
about the 22nd parallel and built up of primitive rocks, 
granite and gneiss. The breadth of this elevated zone 
varies. In the province of Imerina it extends to 80 
miles, but becomes narrower towards the south, where 
it is terminated by a tranverse chain of mountains on 
the border of Betsileo-land. 
The edges are higher on every side than the land 
lying between them, so that we may speak of a central 
depression. The descent towards the east and the west 
is rugged, especially towards Sakalava-land. The mean 
height of the interior plateau is roughly 5000 feet, but it 
rises to 8000 feet to the S. W. of the town of Anta¬ 
nanarivo. The watershed does not lie in the middle, but 
near the eastern coast. The descent on the east is by 
three successive steps, the furthest inland of which is the 
most important. At the precipitous edge occur mighty table 
mountains, like natural fortresses, such as Isahazavona in 
the Tanala region, and Ikongo, which is 5 miles long 
and has a precipitous descent of 1500 feet. 
In the north, in the neighbourhood of Lake Alaotra, 
