188 J. D. Dana — Origin of Coital Beefs and Islands. 



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.. 

 Those having channels are mostly over ten miles in length. A 

 list of them is here given with their sizes, and also the propor- 

 tion of the reef around the lagoon which is under water above 

 third tide, and bare at low tide, a feature of much interest in this 

 connection. 



Ellice Group. — Depeyster's: 6X6 m. ; three-fourths of the 

 encircling reef bare. Ellice's: 9X5 m. ; three-fourths bare. 



Gilbert Group. — Apia: 17X7 m. ; half bare. Tarawa: 

 21X9 m. ; half bare. Taritari: 18X11 m. ; two-thirds bare. 

 Apamama: 12X5 m. ; half bai'e. Taputeuea; west side mostly 

 submerged. 



Marshall Islands (northern). — Pescadores: 10X8 m. ; four- 

 fifths bare. Korsakoff: 26m. ; four-fifths bare. 



Paumotus. — Peacock : ] 5 x V m. ; nearly all wooded. Manhii : 

 13X5 m. ; nearly all wooded. Raraka: 6X9 m. ; three-fourths 

 wooded. Vincennes : 13x9 m. ; mostly wooded. Aratica: 

 18X11 m. ; three-fifths bare. Tiokea : 18X4 m. ; two-thirds 

 wooded. Kruesenstern's : 16X10 m. ; mostly wooded. Dean's 

 (or Nairsa) : 53 X 18 m. ; half or more bare. 



g. The absence of open channels in so large a proportion of 

 lagoons, and especially in lagoons of the smaller atolls, appears 

 to be fatal to the abrasion-solution theory. The method of 

 enlarging atolls through currents and solution can act only 

 feebly if at all 'where waters have no free outlet; and this is 

 eminently so with the smaller atolls which have been assumed 

 by the theory to be most favorable in purity of water and in 

 abundant life for progress; if the small cannot grow, the large 

 lagoons cannot be made from them by the proposed method. 



Eeverse the method, letting the large precede the small (as 

 under the subsidence theory), and then we have a consistent 

 order of events. We have large atoll reefs with several large 

 entrances (like the great barrier reef about a high island in this 

 and other respects) gradually contracting, and the entrances 

 concurrently narrowing through the growing corals and the 

 consolidating debris, in spite of the efforts of abrasion and solu- 

 tion to keep them open and make them deeper ; and, after- 

 wards, the atoll becoming still smaller until the entrances close 

 up ; and finally the lagoon-basin is reduced to a dry depression 

 with nothing of the old sea-water remaining except, perhaps, 

 some of its gypsum. 



h. Instead of small lagoons having the purest waters, the re- 

 verse is most decidedly and manifestly the fact, and this 

 accords with the reversal in the history just suggested. Since 



