Vol. 51.] PHYSICAL FEATURES AJXD GEOLOGY OF MAURITIUS. 469 



embedded irregularly in a matrix composed of volcanic ash. and earth, 

 caused by the decay of the rocks. Here and there stray lava-flows 

 and sheets occur in the sections exposed in the ravines, and some- 

 times on the surface, to the loss of the sugar-planter. The volcanic 

 ash is sometimes in thick layers without a stone, as at the Tama- 

 rin estate ; sometimes its colour is exceedingly variable, as at 

 Chamarel, where seventeen different shades have been counted in 

 the space of half an acre. Sometimes the ash is burnt, by contact 

 with lava, into a kind of red brick, and among the older lavas it is 

 so indurated as to resemble stone. It occurs in places as a fine 

 clay, in others as a coarse gravel, and even as a crumbling rock, in 

 which concretionary nodules are frequently seen. 



The commonest rock in the island is basalt, of which the variety 

 called dolerite is the most abundant. This is fortunate, for it is a 

 very perishable rock, and the earth derived from its ' weathering ' 

 forms a fertile soil. The doleritic lavas appear to have been mostly 

 ejected from the craters along the central ridge, which were thrown 

 up in the second period of volcanic activity. They form nearly 

 horizontal, terraced plains in the centre of the island, rising to 

 2328 feet above the sea-level. At the period of their ejection the 

 island was covered with some sort of vegetation, for on the edges of 

 some of these lava-flows, in their scoriae, are found traces of vege- 

 table tissues, charred and blackened of course, but still recognizable 

 as wood. 



During the first period of volcanic activity the lavas ejected 

 were, as I believe, principally diorites, of which the greater 

 number of the mountains appear to be composed. These rocks 

 occur most abundantly in the Black River district, where the easily 

 weathered dolerites have nearly all disappeared. The rocks which 

 I call diorites are very dark in colour, owing to their containing a 

 nearly black hornblende, and thus the mud and debris, brought down 

 by this river, are of a sombre hue, and have given it its name. The 

 later lavas abound in a glassy felspar (probably orthoclase), fre- 

 quently causing the rocks in which it occurs to become porphyritic 

 They contain more lime than the older lavas, and in their caves 

 stalactites several inches in length are to be found. Sometimes the 

 lime has crystallized in veins as calcite, but more frequently it 

 occurs in the bubble -holes of masses of scoriae, making them 

 amygdaloids. 



The Mauritius basalts contain a large proportion of lodestone or 

 magnetite, which at one time was worked for the iron it contained. 

 This is a great drawback to the surveyor, for the compass becomes 

 perfectly useless anywhere in the neighbourhood of one of the 

 mountain-masses or extinct craters. 



There is evidence of variations of level and of an elevation of the 

 whole island in recent times, for masses of coral-reef and beach- 

 coral rocks are found at elevations varying from about 40 feet in 

 the south to 12 feet in the north, and to 7 feet in the islands on 

 the bank extending to the north-east. A conglomerate, of rolled 

 basalt-blocks and pebbles embedded in a matrix of coral-limestone, 



