'Vol. 51.] PHYSICAL FEATURES AND GEOLOGY OF MAURITIUS. 465 



streams that at some part of their courses have not cut gullies 

 several hundred feet deep. It is often said that these ravines are 

 fissures caused by volcanic action, but in that case the beds of hard 

 lava which stretch across them, causing magnificent waterfalls, 

 would not have been left uncracked. These river-gullies are a great 

 feature of Mauritian scenery ; their sides are so precipitous that 

 one comes upon them quite suddenly, without any warning of their 

 proximity. 



There are but few lakes in Mauritius, although there are many 

 marshes and pools among the lava-beds. The only great sheets 

 of water are the Grand Bassin and the Bassin Blanc, mentioned 

 before, and a few brackish ponds near the coast, in which many 

 fossil bones have been discovered, principally those of the imported 

 deer and of extinct tortoises ; but some dodo-bones also were dug 

 up, and there are many bone-beds which would repay a careful 

 search. 



The Grand Bassin occupies a double crater, the ridge between the 

 two rising as an island ; it is about 650 yards long and 300 wide. 

 The Bassin Blanc is a circular lake in a crater, about 300 yards 

 across. Both are very picturesque, especially the last, which is 

 surrounded by forest-covered cliffs several hundred feet high. 

 There is no visible outflow from either, but the surplus waters find 

 their way through the porous lavas of the sides, and issue on the 

 outer slopes as springs. The Grand Bassin, however, during a 

 hurricane receives so much water (all the rain which falls in the 

 Bassin Sec runs into it) that it rises some 10 or 12 feet, and over- 

 flows down a water-channel, which usually is choked with grass 

 and weeds. The principal marsh is the Mare aux Vacoas, which 

 covers nearly a square mile of area. It is situated in a hollow 

 between the Trou Kanaka, Montagnes Cateau, Peruche, and the 

 unnamed crater, christened by the author Bavenala Crater, from 

 the number of those trees which grow in it. The surface of the 

 marsh is covered with reeds and ' vacoa ' or screw-pine trees 

 (Pandanus palustris) ; the seeds of this screw-pine float od the water 

 and send down roots many feet long to the soil below, and on these 

 roots, as upon stilts, the trunk stands. There are other marshes, 

 all very much smaller than formerly : the Mares de Petrin and 

 Longue near the Brise de Fer mountains, d'Houil in Grand Port, 

 and Lubin in Flacq are the principal. Except during the rainy 

 season, all of them can be crossed on foot. 



The principal surface-tracts of lava-beds are situated in the gaps 

 in the mountains mentioned before. There is also a large area of 

 them at Bipailles, and another at Vacoas, the former being the out- 

 pourings of the Grand Trou North and the Caves Crater, and the latter 

 coming probably from the Trou aux Cerfs. In these tracts lava-flows 

 can be traced sometimes for many miles, as thin rounded masses 

 winding about in snaky courses, with occasional hummocks of broken 

 boulders or cindery scoria?. These districts are all honeycombed with 



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