OOO W. M. DAVIS SUBSIDENCE OF REEF-ENCIRCLED ISLANDS 



as those above suggested must have since then been in operation in order 

 here to develop marginal depths that so well accord with the marginal 

 depths which are elsewhere believed to be the work of those agencies. 



THE DEPTH OF BARRIER-REEF LAG00X8 



The greater depths than 40 fathoms discovered in certain barrier-reef 

 lagoons appear to be be3^ond explanation by the Glacial-control theor}^, 

 while the}^ are perfecth^ expectable under the subsidence theory. Depths 

 of 80 or 90 fathoms on the eastern part of the great barrier reef that in- 

 closes the Exploring Isles in eastern Fiji have already been mentioned as 

 indicating a recent slanting subsidence of that district, which is confirmed 

 by finding recently elevated reefs to the southwest. The gradual increase 

 in depth to the excei)tional measure of 56 fathoms, with a strong proba- 

 bility of greater depths in the uncharted area of the lagoon northwest of 

 Yiti Levu, the largest island in Fiji, is especially significant because of 

 the gradual submergence of the accompanying barrier reef, as is well 

 shown on a chart in Agassiz' Fiji report (1899, plate 3). The two fea- 

 tures together strongly suggest a slanting subsidence; and for this again 

 confirmation is found by the occurrence of moderately elevated reefs on 

 the southern, but not on the northern, side of this large island. Addi- 

 tional confirmation for the occurrence of slanting subsidence may be 

 found in the southwestward increase of depth, well shown on the charts 

 of a great bank in the Tonga group, above mentioned, to which Daly 

 refers (1916, 208) ; in the visible slant of the uplifted coral island of 

 Salayer, south of Celebes, as described by Weber (1902, 87) ; in the slant 

 of Tinian Island in the Mariana group, as described by Seidel (191-4) ; 

 in the slanting emergence of half of the atoll of Uvea, in the Loyalty 

 Islands, of which I had a good view in 1914, and in the presence of up- 

 lifted reefs in the southern part of the Pelew group and of sealevel reefs 

 in the northern part, which strongly indicate the emergence of one area 

 and the submergence of the other, in spite of Semper's opinion long ago 

 expressed to the contrary. 



The depth of 60 fathoms at mid-length of the imperfectly inclosed bank 

 or platform on the west side of Palawan, in the Philippines, in contrast 

 to smaller depths at its southern end, has been instanced above in evidence 

 of a warping subsidence ; here the movement must have been more rapid 

 and more recent than the slanting subsidence in eastern Fiji, because the 

 bank is bordered only by a very imperfect and discontinuous reef rim, 

 and because the fringing reefs of middle Palawan are unusually narrow 

 or wanting. Depths of from 60 to 70 fathoms are recorded in the lagoon 

 of Yanikoro, in the Santa Cruz group, where many other soundings 



