1888.] and the Cored Formations of the Indian Ocean. 



445 



said, its upper surface is 4 feet above the level of high spring tides. 

 On one occasion when the tide rose to an abnormal height and invaded 

 several parts of the main island, I saw that the water reached to 

 within 3 feet of the top of the shore, but even then the whole of the 

 upper stratum of coral rock was well above the waves. It is scarcely 

 credible that an even layer of shingle rock could have been formed 

 above the highest high water mark. 



Owing to the dense growth of bushes it was not easy to explore 

 the surface of Bast Islet ; but in one spot where the undergrowth 

 was less thick I observed a very shallow, basin-like depression, of 

 which the edges were surrounded by a miniature beach of coral debris, 

 giving evidence that the sea had formerly occupied this spot, and yet 

 it is now 4 feet above high water mark. Near this spot were lying 

 great blocks of Mceandrina and Astrcea, which could not possibly have 

 been thrown by the waves to their present position if the surface on 

 which they lie had not formerly occupied a lower level than it does 

 now. Similar blocks were noticed by Semper in the atoll of Kriangle 

 in the Pelew Islands, but in that case the blocks appear to have been 

 of much larger size. 



These facts may not be convincing testimony in favour of a recent 

 elevation of a few feet, but my belief in such an elevation is further 

 strengthened by the following facts communicated to me by M. Spurs, 

 a resident for twenty-five years at Diego, an ardent naturalist, and 

 much interested in coral formations. 



A small shore crab of the genas Ocypus is always to be found on the 

 sandy flats between high and low water mark. These crabs, as is 

 well known, form numerous galleries in the fine muddy sand, which 

 they line with seaweed, &c, to prevent their falling in. These 

 galleries open to the surface by short passages placed perpendicularly, 

 the mouths of which open only a few inches above the level of low 

 tide. This crab is only found on the shore between tide marks ; on 

 the dry land its place is taken by Gecarcinus, another genus of crab, 

 which forms different burrows. In the west part of East Islet there 

 is an aggregate of friable, scarcely compacted sand, which has some- 

 what the appearance of half-dried clay. It lies 5 feet above high 

 water mark, and was found by M. Spurs, daring some excavations 

 which he had to make for the purpose of constructing a slip for boats, 

 to be riddled with the seaweed-lined galleries of Ocypus, evidently 

 long since disused and empty. 



Having made this observation on East Island, M. Spurs made a 

 search in similar formations on the main island, and found, he tells me, 

 precisely the same facts in several instances, aggregates of sand lying 

 at some distance above high water mark, riddled with the abandoned 

 burrows of Ocypus. Now since the burrows of Ocypus are qnito 

 characteristic, and could not have been mistaken by so good an 



