66 MAYOR— ROSE ATOLL, AMERICAN SAMOA. 



bases. The largest boulder we observed lay loosely upon the reef 

 flat east of Rose Islet and was somewhat tilted by being jammed 

 against another rock. It was 12 feet 5 inches long, 8 feet wide, and 

 7 feet 6 inches high, and as its specific gravity was 2.3, it apparently 

 weighs 46 tons. 



The appearance of these boulders supports the view that the atoll 

 rim was once about 6 to 8 feet higher than at present, and has been 

 cut down to present sea level in recent times ; most of the mushroom- 

 rocks having been completely undercut so that they now lie loosely 

 upon the floor of the flat. 



It can be seen that the surface of the present reef flat consists 

 chiefly of lithothamnium, a beautiful bright pink variety of which 

 forms a veritable veneer over its surface. Professor Alexander H. 

 Phillips made an analysis of this lithothamnium and found it to con- 

 tain 74.4 per cent, of calcium carbonate and 19.47 per cent, mag- 

 nesium carbonate. Also rock from the solid floor of the atoll rim 

 west of the main entrance to the lagoon gave 83.86 per cent, of 

 calcium carbonate and 14.36 per cent, of magnesium carbonate ; 

 while a large loose boulder from the same region consisted of 77.28 

 per cent, of calcium carbonate and 18.3 per cent, of magnesium 

 carbonate. Also, Professor C. B. Lipman found that the largest 

 loose boulder on the reef flat east of Rose Islet contained 79.5 per 

 cent, calcium carbonate and 14.54 per cent, magnesium carbonate. 

 It will be recalled that Hogbom found the magnesium carbonate in 

 various species of lithothamnium to range from 3.76 to 13.19 per 

 cent., and Clarke and Wheeler'- found from 10.93 to 25.17 in 15 

 species, and thus the Rose Island species seems to be peculiar in 

 possessing a fairly high magnesium content." 



It thus appears that the loose boulders lying upon the atoll rim 

 have the same general chemical composition as the solid rock of the 

 rim itself and are remarkable in that they contain a large amount 

 of magnesium. In fact, these boulders are only remnants of the 

 old rim which was once about 6 or 8 feet higher than at present, but 

 has been almost entirely planed down to the lowered level of the 



1 U. S. Geol. Survey, Prof. Paper No. 102, 1917. 



- See J. W. Judd, Funefuti Report, 1904, 1904, p. 377. 



