i8 
MADAGASCAR 
ground to the island of Nossi-Be, cuts deep into the land. 
Next, to the south, comes Bafala Bay studded with the 
Radama islands, then Narinda Bay, the broad Bemba- 
toka Bay, the Gulfs of Baly and Bueni, the Bay of 
Tullear (Tolia) and lastly the Bay of St. Augustine, girt 
with mangroves. 
The Geologic Structure of the island of Madagascar 
displays to us in the first place a nucleus of granite 
and gneiss in the interior; this rests upon a submarine 
base and comes to light in the central mountain region 
wherever it is not overlaid with alluvial soil. 
We have apparently to do with an old mountain sum¬ 
mit which remained standing when the mighty basin of 
the Indian Ocean was formed. Certain fragments have 
remained standing in the region of disruption, as, for 
example, the island of Nossi-Be which appears to be 
closely connected with the west coast and exhibits a 
primitive rock formation. The Radama islands and Juan 
de Nova are similar fragments. The long and sharply 
marked parallel terraces stretch in a north-easterly direc¬ 
tion and appear to be continued, with interruptions, as 
far as the Seychelles. 
With the exception of isolated patches of soft sand¬ 
stone the later sedimentary rocks are entirely wanting 
in the interior. Near Sirabe there is a local layer of 
limestone which is made use of for building purposes, 
but its geological character requires closer investigation 
than it has yet received. In the south of Belsileo-land 
a slate is found which is turned to account in the arts. 
In the south-west the land of Bara has a desolate chaotic 
character, but appears to be chiefly granitic, while the 
Isalo-mountain, between 22° 10' and 22° 25' S. lat., 
forms a sandstone plateau deeply furrowed by water¬ 
courses. 
There are many kinds of volcanic rocks overlying 
