454 Mr. G. C. Bourne. The Atoll of Diego Garcia [Mar. 22, 



and violent currents if the polyps could be sufficiently protected from 

 the shingle, but the branching Madrepores are soon broken off and 

 swept away, and even the more massive Mseandrina soon follows, for 

 whilst the surface of the colony grows the base is dead, is soon 

 riddled by boring sponges, Serpuhe, &c, and is no longer able to bear 

 the sti^in put upon it. The great mass then breaks off and is rolled 

 along the reef, pounding other corals in its course. I was long 

 puzzled at Diego Garcia when I found that the outer reef was nearly 

 barren of coral, even where it is covered by a foot or two of water at 

 the lowest spring tides. I noticed directly after my arrival that the 

 sea always broke on the reef west of Middle Islet (Spurs' Reef), and 

 believing, from what I had read, that this must be a most favourable 

 spot for coral growth, I took the first opportunity of visiting it at low 

 tide. To my surprise I waded for nearly a mile, nearly waist deep, 

 without finding a single living coral on the seaward flat, although 

 the lagoon just within the reef was filled with knolls of living corals 

 growing at various depths. All around the shores I found the same 

 thing, flat reefs of barren coral rock, sloping very gently down to the 

 ocean, on whose surfaces, even where they were constantly covered 

 with water, no live corals were to be found. Just at the edges of the 

 reef, where the sea breaks at low spring tides, I could detect during 

 the reflux of the waves, solid masses of Millepora, and dead rock 

 covered with Millepores, but no live Madrepores, excepting where 

 narrow channels ran up into the reef, the sides of which generally 

 bore a few colonies of Maiandrina, Porites, or Madrepora aspera. On 

 the other hand, it can readily be seen that the external slopes, just 

 beyond the rim of the reef behind the breakers, are covered with 

 masses of coral. A short experience of wading across the reefs 

 showed me the reason of the want of coral on the flat upper surfaces. 

 Debris, torn from the corals growing on the slopes, is continually 

 washed across the flat reef surfaces, and in strong breezes large 

 masses are rolled along. Even in the calmest weather the coarse 

 fragments can be felt sweeping past one's legs with some force, and one 

 can readily understand that the soft polypes could not withstand the 

 constant wear and tear. The moderate currents, on the other hand, do 

 not carry large fragments, but prevent deposition of sand which would 

 choke the corals, and at the same time supply the conditions of nutri- 

 tion necessary to their growth. In Stillwater, sand is thrown down in 

 large quantities, and this and the absence of a constant supply of 

 food prevents coral growth. The conclusions to which I came were 

 that the external slopes afforded conditions eminently favourable to 

 coral growth, that the upper surfaces of the shore reefs were very 

 unfavourable, and that in certain parts of the lagoon favourable con- 

 dit'ons again occur, though the growth is never so luxuriant there as 

 on the external slopes. The favourable conditions on the last-named 



