J. D. Dana — Origin of Coral Reefs and Islands. 187 



the atoll reef and take up more lime as far as the acid ingredi- 

 ent is present ; and then they pass to the lagoon for work 

 similar to that outside, with probably a diminished amount of 

 free carbonic acid, on account of the loss over the reef-ground 

 previously traversed. 



The lagoon-basin is not, therefore, the part of the atoll 

 that loses most by solution, any more than by abrasion and 

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

 the island is not subsiding at too rapid a rate, they keep ex- 

 tending 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 portion. 



The remark : "It is a common observation in atolls- that the 

 islets on the reefs are situated close to the lagoon shore;" 

 such "facts point out the removal of matter which is going on 

 in the lagoons and lagoon channels,"* I know nothing to sus- 

 tain. 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 out- 

 side as the inner limit of the . platform, and a finer, lower 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 leak- 

 age 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 entrances 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-J 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 



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



