150 BULLETIN OF THE 



when the inlet still gave free entrance for the sea-water driven through 

 it by the trade winds. The reef finally choked up this passage, flourish- 

 ing thereafter only at the northern edge, where it is still active. Little 

 by little the old reef has been completely hidden by the mass of drift 

 sand derived from the beach of coral limestone sand to the east of 

 Spreckelsville, which at one time may have been much farther inland. 

 The coral sand on the beach is finely triturated, and the finer fragments 

 form regular dunes of all possible sizes, from small horseshoe-shaped 

 heaps, driven slowly along by the trades and growing constantly, so that 

 we find some of these dunes of no less than twenty feet in height, which 

 have travelled two to three miles towards the foot of the Western Maui 

 slope, where they are comparatively sheltered and become cemented to- 

 gether by the rain. The Spreckelsville beach thus supplies the drifting 

 coral sand, afterward hardened into the rock mentioned above, as well as 

 the remarkable sand dunes which travel inland, obstructing the roads 

 and the trails. They resemble the huge travelling sand dunes found on 

 the desert back of Mollendo, which frequently cross the railroad tracks 

 leading to Arequipa, and impede the progress of trains as much as snow 

 drifts do in a northern region. 



In estimating the thickness of coral reefs, 1 it has been usual to take 

 the declivity of the land, and to calculate from the estimated slope and 

 distance from shore the thickness at any given point. This must be a 

 very defective method, at least in volcanic countries, where the fringing 

 coral reefs have frequently been entirely covered over by volcanic out- 

 bursts, such as ashes, lava, or perhaps torrential rains, bringing down 

 from the mountain-sides an unusual amount of detrital matter. The 

 drilling for artesian wells near Honolulu has most plainly shown this 

 alternation of growth of reef corals and of either lava outflows or water- 



1 Darwin and Dana both argue that the subsidence of the land is the only pos- 

 sible cause for the thickness of a fringing or barrier reef, which may be as much 

 as one or two thousand feet. The evidence brought forward by Mr. W. O. Crosby 

 (On the Elevated Coral Reefs of Cuba, Proc. Bost. Soc. Nat. Hist., 1882, p. 124) does 

 not throw any additional light on Darwin's theory of subsidence ; it is of the same 

 character as all the statements which prove the subsidence by the existence of coral 

 reefs, and while there may have been coral reefs formed during subsidence, it 

 does not prove that their growth is due to subsidence any more than the presence of 

 elevated reefs proves them to be due to elevation. They grow and must have 

 flourished continuously in periods of both elevation and subsidence, as long as 

 neither the elevation nor the subsidence was more rapid than the rate of growth 

 of corals, and as long as the area in which they were found as elevated reefs was 

 inside of the limits of depth within which we know corals to grow. 



