GREAT BARRIER REEF OF AUSTRALIA 543 



the coast of Tahiti, where the last important movement was a submer- 

 gence, are now fronted by reef-free beaches of volcanic sand, where the 

 deltas of the larger streams have outgrown the embayments in which their 

 formation began. Eeasons might be adduced, if space permitted, for in- 

 ferring, in advance of observation, that smothered reefs exist under the 

 coastal lowlands of Luzon. 



No examples have been found to illustrate the general retrogradation 

 of a delta-fronted coast of submergence, but a complete search of existing 

 coasts has not yet been made with this object in view. It may well be, 

 however, that the postglacial rise of ocean level has, in combination with 

 movements of subsidence, held the development of most coasts back to an 

 earlier stage than that in which general retrogradation should be ex- 

 pected. There are reasons which I have elsewhere set forth for thinking 

 that the Great Barrier reef of Australia, which today incloses a broad 

 lagoon, had reached a more advanced stage of development before the last 

 strong subsidence of its region, and that its lagoon may then have been 

 converted into a plain, across which the rivers from the rainy highlands 

 of the back country carried their detritus to the reef front and over- 

 whelmed its corals (1917, c, 350). 



REEF8 ON COASTS OF EMERGENCE, AFTERWARD SUBMERGED 



It is evident from the foregoing discussion that a reefless coast of 

 emergence may be converted by submergence into a reef-fronted coast. 

 I have not yet found any examples in which a reef -free coastal plain of 

 emergence has after partial dissection suffered so recent a submergence 

 as to be bordered only by newly established fringing reefs ; but the south- 

 ern coast of Viti Levu, the largest island of the Fiji group, is an example 

 of a somewhat later stage of this sequence, as it is bordered by a dissected 

 and embayed coastal plain, reference to which was briefly made in an 

 earlier paragraph, and fronted by a barrier reef that is on the verge of 

 being overwhelmed where the deltas of two large rivers are advancing 

 toward it. The island is chiefly composed of resistant volcanic rocks, on 

 which the coastal-plain series of marine strata, formed of foraminiferous 

 volcanic muds and locally known as "soapstone," has been unconformably 

 laid down. Since their emergence along the southern coast the strata 

 have been maturely dissected, so that they no longer form a coastal plain 

 but a littoral belt of hills ; and since their dissection they have been partly 

 submerged, whereby their shoreline became well embayed. 



The strong offshore barrier reef of this embayed coast incloses a lagoon, 

 usually a mile or more in width; but the Navua and Eewa rivers have 



