TOPOGRAPHY AND GEOLOGY. Madagascar may be divided into two distinct regions. The central and the eastern parts of the island, or more than half of it, are very mountainous and are formed of primitive rocks of igneous origin, except for a narrow fringe of sedimentary rocks along the coast. Gneiss. and particularly granitoid gneiss, predominates; there are also many granite dykes, and, here and there, basalts, the cones of ancient volcanoes, magnetite. and other iron ores, and numerous hot springs. The gneiss, decomposed to great depths, forms the reddish, clayey soil that covers most of these highlands. The vol canic centres (now extinct) of this region are Mount Ankaratra in the central region, the cul minating summit of the island (nearly 9000 feet high), Mount Amber near the north end of the island, and I-ohitsomhe near the southeast coast, north of Fort Dauphin. These highlands, the primitive land mass, rise from 3000 to 6000 feet above the sea.
The western and southern regions and a con siderable district in the north are comparatively low, not much over 600 feet above sea-level, and are built up of sedimentary rocks, of Permo carhonifcrous. Secondary and Eocene strata. Here also are many volcanic cones. that have pierced through the sedimentary fossiliferous beds. The lowlands, broken by sonic ranges of hills extend ing north and south, rise gently to the central highlands, and the rivers have excavated numer ous valleys, so that the west of the island is, for the most part, a deeply dissected plain. The coasts are in the main comparatively straight with no deep indentations. The northwest coast, however, is broken up by great inlets, smile of them landlocked, so that Mojanga and a few other bays are among the largest and finest of natliral harbors. Elsewhere the island is poorly supplied with good ports for shipping.