2 28 Cruise of the ''Alert r 



was composed of upraised coral sandstone, standing in situ, and 

 exhibiting excavated grottoes and jagged pinnacles, resulting from 

 old marine degradation. It may therefore be inferred that these 

 two islands have been subjected to a movement of elevation to 

 the extent of at least a few feet at some period subsequent to 

 the formation of their present reef-coral surfaces. Again, at Isle 

 des Roches, which, however, it should be remembered lies on a 

 separate, although adjacent bank, there were along its south- 

 eastern margin stratified beds of hard coral sandstone occupying 

 a position above high-water mark, and presenting to seaward an 

 abrupt eroded face of hard rock which was undergoing degrada- 

 tion, and was being undermined by the action of the waves on a soft 

 subjacent stratum. As regards the other islands of the group, I 

 have seen no evidence of elevation beyond the fact that they are 

 higher than either African or Eagle Island ; one of the most 

 southerly being as much as fifteen feet above high-water mark. 

 I may add that the absence of " barrier reefs " throughout the 

 group militates against the probability of subsidence having taken 

 place. There is, therefore, reason to believe that the entire group 

 have undergone elevation rather than subsidence ; and if the 

 forces which produced this condition be still in operation, and 

 continue so until a further elevation of fourteen fathoms has been 

 effected, there will result an atoll over eighty miles long by twenty 

 in width, and studded with lofty coral islands, somewhat resem- 

 bling the high islands of the south-eastern Paumotus, such as 

 Elizabeth Island, which Dana describes as being eighty feet in 

 height. 



The Amirante Group furnishes an illustration of the generally 

 accepted position that corals grow more luxuriantly on the 

 weather than on the lee side of banks and reefs. In this region, 

 a wind, varying in direction between east and south-east, prevails 

 throughout ten months of the year, and consequently gives rise 

 to a proportionately constant surface current ; and, on looking 

 at the grouping of the islets, we find that of the eight which rest 



