SURFACE AND GEOLOGICAL STRUCTURE 17 
in horizontal strata similar to those on the continent of 
Africa. The rainfall being less copious, the soil is far less 
fertile than on the eastern side. It has more the character 
of a steppe, and the land most suitable for cultivation 
is found on the banks of the rivers. The land in the 
south and south-west consists of a wretched plain, barren 
sand or sparse bush predominating. In Menabe it be¬ 
comes somewhat pleasanter and more fertile. In the 
north, volcanic formations crop up, low cones of basalt 
breaking through the horizontal strata of the sedimentary 
rocks. The volcanic rocks can be traced beyond the 
island as far as the Comoro Islands; the island of Nossi-Be, 
for instance, which is of entirely similar character to Mada¬ 
gascar, exhibits many craters filled with water so as to 
form circular lakes, besides the granite masses of Locube. 
More extensive highlands are to be found between 
the rivers St. Vincent and St. Augustine. The most 
northerly of these was visited by Grandidier. It is 
terminated on the east by the picturesque Isalo range 
in which there are precipices 1200 feet high. 
The western rivers have a longer course than those 
in the east, on account of the greater distance of the 
watershed from the coast; not unfrequently they become 
considerable streams. The mightiest of these is the 
O 
Betsiboka with its tributary the Ikopa; it rises in the 
high land of the Inierina Province, and after a course 
of 300 miles falls into the wide bay of Bembatoga; 
river steamers of moderate draught can ascend it to a 
O 
distance of 90 miles from the mouth. To the nqrth 
of this river there is the Majaruba. Further south there 
are a few good-sized streams, as the Manambolo, the 
Mangoka or St. Vincent River, and the River of St. 
Augustine with its broad estuary. The coastline on 
the western side is richly diversified, having numerous 
large and well-protected bays. 
In the north the bay of Passandava, forming a back- 
