362 A NATURALIST ON THE " CHALLENGEE 



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about two miles long, it is made up of quartz porphyry, forming 

 hill masses, a couple of hundred feet or so in height, with sandy 

 flats at their bases. 



In places, the hill slopes come right down to the sea, 

 forming small headlands, and here the beach is composed of 

 boulders with small stretches of quartz sand derived from the 

 rocks between them. Along a wider bay to the north, the 

 whole beach is made up of calcareous sand and broken and 

 dead shells. A shore platform reef extends all along this side 

 of the island; in some places it is made up of consolidated 

 coral rock, full of large masses of dead corals cemented together 

 with coral mud, or seen projecting here and there between 

 muddy pools of water. 



In other places the coral rock passes gradually into regular 

 mud flats. There were very few living corals indeed about the 

 shore platform ; it required careful searching to find them. I 

 found only the species of Uuphyllia, which was at Somerset, and 

 a small Astrcea, One large mass of Astrcea thrown up by the 

 waves and embedded in the mud, had a small patch on one 

 side of it still alive, the rest was quite dead. 



Though stony corals were so scarce, soft Alcyonarians were 

 in great abundance. The rock was full everywhere of the Giant 

 Clam (Tridacna), the largest bivalve shell which has ever existed, 

 a familar adornment of fountains and oyster-shops in England- 

 This mollusk lives sunk in a cavity of its own in the rock, with 

 only its brilliant blue or green mantle fringes showing and 

 betraying its retreat. These protruded mantle lobes have the 

 appearance of huge expanded elongate sea anemones, and at 

 first sicdit one takes them for such. The shells must be quarried 

 out of the rock with a hammer and chisel if they are wanted. 



The main peculiarity of these coral flats, as at Somerset, is 

 their extreme muddiness and the small quantity of life about 

 them. A Sargassum grows abundantly on the rock masses, with 

 several other algas. No doubt the decomposition of these and 

 the soft Alcyonarians is that which renders the coral mud so 

 dark and slimy. The occurrence of beaches of calcareous and 

 siliceous sand close together, both rising from the same coral 

 Hat, is an interesting fact, as showing how easily beds of such 



