92 ME. H. N. EIDLET ON" THE 



but the soundings which have been taken round the island are 

 iusufficieut to give us any clear idea of its original size. At 

 a short distance on each side o£ the group, the depth suddenly 

 increases to over 2000 feet. 



Some American petrologists, who have found similar rocks to 

 those o£ Fernando do Noronha in the neighbourhood of Cape 

 San Eoque, seem to consider that the grouj) may have been 

 connected at one time with the mainland at this point. We 

 have not seen tlie specimens, but from the form and arrangement 

 of the rocks here it may be doubted that the evidence is sufficient 

 to prove a connection, while the jjresence and position of the 

 scoria-beds of Pedras Pretas seem most clearly to prove that 

 there was here a large and active-enough crater to supply nearly 

 all the basalt upon the island. 



Now all volcanic activity has long ceased and the last 

 stage in the geological history of the island, that of its breaking 

 up into smaller islets and its slow destruction by denudation, 

 has been reached. 



The Coeal Eeef. 



The reef lying round the group of islands, though of consider- 

 able extent, does not entirely surround them, but occurs in 

 irregular patches of varying extent. As much interest has been 

 lately shown in the structure and origin of reefs, a few notes made 

 here may be of some value. The reef, in structure, is a whitish- 

 brown, friable, calcareous rock, which, when broken, shows, 

 except for a short w^ay below the surface, no identifiable auimal- 

 or plant-remains. It weathers into caves and hollows and rock- 

 pools, so that there are often caverns excavated deep underneath 

 the ledges, into which, at low Avater, the sea rushes with great 

 velocity expelling the air violently. 



The largest of these blow-holes is at Eat Island, where the 

 rush of spray through the hole attains so great a height that it 

 can be seen for some miles. 



Where the reef has been raised above sea-level and subjected 

 to aerial decomposition, it seems to be oolitic in structure and 

 finely granular. In comparing it with the reef-rock of the well- 

 known Eecife at Pernambuco, it seems that the latter is harder 

 and more compact. 



In the rock-pools and crevices of the reef live an immense 

 abundance of marine animals — corals, sponges, echinoderms, 



