288 Dr. J. D. Dana on the Origin of 



portation. The outer reefs suffer the most ; and yet, if the 

 island is not subsiding at too rapid a rate, they keep extending 

 and encroaching on the ocean, instead of wasting through 

 the drifting into the ocean at large of calcium carbonate in 

 grains and solution ; and the shore-platform also preserves 

 its unvaried level notwithstanding the daily sweep of the 

 tidal floods, and the holes that riddle its outer portions. 



The remarks, " It is a common observation in atolls 

 that the islets on the reefs are situated close to the lagoon 

 shore f and such u facts point out the removal of matter which 

 is going on in the lagoons and lagoon channels," * I know 

 nothing to sustain. The width of the shore-platform on the 

 seaward side is always greater than that on the lagoon side ; 

 but the outside shore-platform has its width determined by 

 tidal and wave action, and this action is powerful on the 

 ocean side, and feeble on the lagoon side ; it produces a high 

 coarse beach on the outside as the inner limit of the platform, 

 and a finer, lower r and much more gently sloping beach on 

 the inside. The amount of erosion is far greater, as it should 

 be, on the side of the powerful agencies. 



/• The loss to the lagoon by abrasion and solution is reduced 

 to a minimum, in the majority of atolls, by the absence of lagoon 

 entrances, which leaves them with only concealed leakage 

 passages for slow discharge. 



Nine tenths of atolls under six miles in length (or in longer 

 diameter), half of those between six and twenty miles, and 

 the majority of all atolls in the Pacific Ocean, have no en- 

 trances to the lagoon a fathom deep ; and the larger part of 

 those included in each of these groups have no open entrances 

 at all. 



For evidence on this subject, I refer to the Wilkes Expedi- 

 tion Hydrographic Atlas. This Atlas contains maps of nearly 

 sixty coral-islands from the surveys of its officers, drawn on 

 a large scale (one or two miles, rarely four, to the inch). 



Out of the number, nine, ranging from 1^ to 3 English 

 miles in the longer diameter of the reef, have no lagoon, but 

 only a small depression in its place ; two of these take in 

 water at high tide, and the rest are dry. 



Of those under six miles in length having lagoons, seventeen 

 in number, sixteen are represented as having no entrances to 

 the lagoon at low tide ; and the one having an entrance is 

 5x4 miles in size. The smallest is about a mile in diameter. 



Of those that are six miles or over in length, twenty-nine 

 in number, seventeen have channels and twelve have none. 



* Mr. Murray, loc. cit. p. 515. 



