SURFACE AND GEOLOGICAL STRUCTURE 13 
these three steps unite. The watercourses on the east¬ 
ern side are rapid and often form picturesque waterfalls 
in the mountain region. 
Towards the west of the island the descent is divided 
into four steps, it is thus less rugged and easier to cross; 
the streams attain greater length and have a more gentle 
current, yet even here there occur isolated precipices. 
One of these on the i8th parallel has a fall of 2500 feet. 
This formation of the ground produces a natural division 
into three districts, which we denote as the Eastern, 
Central, and Western regions. Each has its peculiar 
character as regards topography, botany, and zoology. 
The Eastern Region is very mountainous in the interior. 
It stretches, with an average breadth of 60 miles, from 
Diego Suarez to Eort Dauphin. 
Here and there, as, for example, some 6 miles behind 
Tamatave, quartz, gneiss, and mica slate come to the 
surface, forming little valleys with a covering of black 
mould, the slopes being generally thickly covered with 
red losz * which in the rainy season makes progress very 
difficult. As the deeper valleys between the undulating 
hills are generally marshy, the villages have to be built 
on the slopes. The frequent and copious rains bring 
about a great wealth of rivers and streams, which at 
times attain a considerable breadth, as, for example, the 
Ivondrona near Tamatave. The course of the rivers is 
always a short one and their navigability is consequently 
very limited. Their total length also is of no great amount, 
because the watershed approaches very near to the 
east coast. Among the numerous watercourses of the 
Eastern Region we may here mention: The Mahanarabe 
in the north, the Maningoro which drains the great lake 
basin of the Alaotra, the Ivondrona with its extensive 
lagoons which falls into the sea south of Tamatave, the 
* This is a red marl, found along the Elbe and the Yang-t5e-Kiang, 
but not in England. 
