Page:Africa by Élisée Reclus, Volume 4.djvu/525

 RIVERS OF MADAGASCAR. 429 It has bc*en arrested in its flow iu front by the side of the low hills. It is cut through in one part by a stream, which in some places has worn a channel to the depth of 80 or 90 feet. Its surface, which is slightly cellular, is covered by some hundreds of mammiform hillocks, which must have been formed during the cooling of the liquid mass. The hillocks are mostly from 20 to 30 feet high, and apparently are heaped up masses of lava, and not hollow blisters. The lava itself is black, heavy, and compact, being porphyritic, with somewhat large crystals of augite." * The particulars collected by this observer tend on the whole tp show that the Lake Itasy volcanoes have been extinct for a longer period than had hitherto been supposed, and the chaimel 80 or 90 feet deep worn by fluviul action tlirough some of the hard porphyritic eruptive rocks certainly points at very great antiquity. In the northern part of the island volcanoes are also very numerous. North- east of Anton-Gil Bay rises one of these " burnt " mountains, whose crater is flooded by a tarn teeming with fish, and whose outer slopes are formed by refuse, white at the base, and red round about the highest crest. Amber Cape, at the northern- most extremity of Madagascar, is itself a volcano, whose lava sheets still rise above the surrounding waters. The neighbouring insular groups of Nossilie, Mayotte, and Anjuan, are likewise of igneous origin, and in many parts of the mainland occur thermal springs and jets of carbonic acid, fatal to insects and small animals. Earthquakes are also frequent iu several districts. Rivers. Being well exposed to the moist trade winds from the Indian Ocean, Madagascar everywhere abounds in running waters, except towards the extreme south, which is at times swept by the dry atmospheric currents from the South African mainland. The eastern slope of the island receives the heaviest rainfall, although the largest fluvial systems are not developed on this side. Here the prtcipitous slope of the hills, combined with their proximity to the coast, prevents the streams from acquir- ing any great size before reaching the sea. Most of them are in fact mere torrents less than sixty miles in length. One of the most copious is the Tengteng, or* Manompa, which rises in a longitudinal valley between two parallel chains, one of which it pierces through a deep rocky gorge on its course to the sea, which it reaches opposite the island of St. Mary. The Maningori also collects its head-waters on an upland plain between the main range and the barrier formed by the edge of the sustaining plateau. Its waters being hemmed in by this barrier, at first spread out in marshes, and are then collected in the broad but shallow basin of Ijake Alaotra. This flooded depression extends for some lo miles along: the longitudinal vallev before it finds an outlet in a deep rocky channel, through which its overflow finds its way to the outer slope and thence to the coast near Fenoarivo. Formerly Lake Alaotra formed a great inland sea no less than 200 miles long, • nature, Sfarch 4, 18SG.