346 W. M. Davis — The Great Barrier Reef of Australia. 



movement of the coastal region seems to be of the same nature as, 

 but of smaller amount than the movements inferred to have 

 taken place at much earlier periods. A slight emergence of 

 very recent date need not be considered here ; it may be the 

 result of the latest upflexing, or it may, following the principle 

 of shore line changes demonstrated by D. W. Johnson, be the 

 result of diminished tidal range in consequence of the enclosure 

 of the lagoon by the present barrier reef. 



The problem before us is to determine what opportunity for 

 reef formation may have been offered during the successive 

 down-flexures of the sea floor and during the long intervening 

 periods of rest as indicated by the successive peneplanations of 

 the coastal belt; but as this inquiry involves the conception of 

 young barrier reefs, mature reef-plains, and abraded reef- 

 plains, two paragraphs will be given to that aspect of the sub- 

 ject, following and extending a view of reef development^ first 

 clearly enunciated by Hedley in 1907. 



Imagine a recently uplifted and reef-free coast, or for con- 

 venience of graphic illustration imagine a recently constructed, 

 i-eef-free volcanic cone, of which an undissected sector is shown 

 at D, fig. 5. The shore is attacked by waves and a cliff and 

 platform are developed, sector E, on which no reefs can be 

 formed* until submergence embays the radial valleys, sector F, 

 when a narrow young reef will make its appearance, enclosing 

 a lagoon where aggradation takes place. Tahiti, I believe, 

 exemplifies this sequence of development. Let it be supposed 

 that the reef thus initiated is maintained in a narrow or youth- 

 ful stage with a widening lagoon by continued subsidence, until 

 in sector G a long enduring still-stand period sets in. The 

 youthful reef will thereupon widen to more mature form by 

 outgrowth and overwash; the embayed valleys will be filled 

 by deltas, which will advance farther and farther into the 

 lagoon, as in sectors H, J, and K; and the lagoon will be at 

 last converted into a mature reef-plain, as in sector L. 



The mature reef-plain is not, however, the limit of orderly 

 change if the island suffers no variation of level. The detritus 

 from the central island will be delivered by the streams to the 

 outer face of the mature reef, and the growing corals may thus 

 be smothered. When that eventuality is reached the waves will 

 cut the dead reef farther and farther back, reducing it to a 

 shallow submarine platform, as in sectors M and N. Finally 

 the central island itself will be again attacked, unless before 

 that goal is reached subsidence sets in anew, whereupon a 

 rejuvenated reef will make its appearance on the outer edge of 

 the widening platform, or on the outer edge of the narrowed 



* See Clift Island sin the Coral Seas, Proceedings Nat. Acad. Sci,, ii, 1916, 



284-288. 



