MAYOR— ROSE ATOLL, AMERICAN SAMOA. 67 



present surface of the ocean, leaving only an occasional mushroom- 

 rock on a pedicel as a vestigial remnant of the old rim. 



Inspection shows that the solid rock of the atoll rim and also 

 the boulders lying upon it consist chiefly of lithothamnium com- 

 pacted into a dense mass of chalky whiteness superficially resembling 

 dolomite, and having a specific gravity of about 2.3, thus being 

 higher than that of pure coral limestone, the specific gravity of 

 which would range from 1.85 to 2. A pure dolomite containing 

 45.65 per cent, of magnesium carbonate should have a specific 

 gravity of about 2.9. 



There are a few fossil corals, chiefly Pocillopora, imbedded in 

 the rock of the atoll rim and the boulders, but the whole visible rock 

 of the atoll consists so largely of lithothamnium that we may call it 

 a "lithothamnium atoll" rather than a "coral atoll." 



The flat upper surface of the atoll rim is in most places planed 

 ofif nearly to low tide level, but it is veneered with a vigorous growth 

 of a beautiful pink lithothamnium which has been provisionally de- 

 termined by Professor W. A. Setchell as Porolithon related to P. 

 craspcdium. In most places this lithothamnium forms irregular, 

 more or less connected, patches growing on the smooth hard floor 

 of the flat. West of the main entrance to the lagoon it grows in 

 long nearly parallel, flat-topped, over-arching ridges all parallel 

 with the line of the wave fronts of the breakers as they surge over 

 the reef. These ridges are about 6 inches high and from 6 inches 

 to several feet in width, and w^ith channels of similar width be- 

 tween them. 



Lithothamnium grows in greater profusion over the reef rim of 

 Rose Atoll than in any other Pacific reef I have seen ; but apart 

 from the single species of pink lithothamnium there are remarkably 

 few organisms growing in the shallows of the reef flat. Occasion- 

 ally we find a pale-olive-green Poriies, allied to P. solida, and there 

 are a few small stocks of Favitcs or Symphyllia; but Acropora and 

 Pocillopora, which are the dominant forms in most breaker-washed 

 reef flats of the Pacific, are practically absent from Rose Atoll, 

 except at the extreme edges of the atoll rim fronting the lagoon on 

 the sea, where a few stunted specimens of these genera occur. 



