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



and this has been taken to show that the platform is an inor- 

 ganic continental shelf and not an antecedent reef plain. But 

 no reason has been adduced to show why a reef should grow 

 up at a moderate distance back from the margin of a smooth 

 continental shelf, and why it should not grow up at a similar 

 distance back from the margin of submerged and almost equally 

 smooth mature reef -plain. On the other hand, reasons for the 

 upgrowth of a young barrier reef at a certain distance back 

 from the margin of a submerged reef-plain are suggested in 

 sectors M and N of fig. 5: — a partly abraded reef-plain might, 

 after rapid submergence, be too deep for coral growth at the 

 outer margin of its abraded platform, but not too deep at the 

 margin of the un consumed part of the plain. Hence the con- 

 trol of the location of the present barrier reef may well have 

 been the result of other conditions and factors than those which 

 formed the platform that serves as its foundation. 



There is one way, to which attention has not hitherto been 

 called, in which the inorganic processes now at work in devel- 

 oping the continental shelf of New South Wales may be in the 

 future, and may have been in the past, unfavorable to the 

 development of a mature reef-plain along the Queensland 

 coast. In that part of the coast where the Great Barrier reef 

 of Queensland and the great continental shelf of J^ew South 

 Wales adjoin, the 'long-shore currents are at present engaged 

 in forming extensive sand reefs, which appear to be extending 

 northward. The sand reefs have already, in the relatively 

 short interval since the latest flexure by which the coast was 

 embayed, gained lengths of scores of miles. Hence in an 

 earlier and much longer interval, sufficient for the partial pene- 

 planation of an upflexed coastal belt, similar sand reefs might 

 have extended much farther northward along the coast than 

 they now reach; as fast as they advanced they would certainly 

 kill all the reef-b.uilding organisms, and thereafter the waves 

 might be able to cut away the reef. It is, therefore, conceivable 

 that, toward the close of the several still-stand periods in which 

 the lowlands, LU, QQ^ UH', fig. 4, were developed, the 

 coral reefs (W^, ) X^, Y', of the Queensland coast may have 

 been encroached upon for a few hundred miles by sand reefs 

 from the south; but inasmuch as the present immature reef has 

 its southern end determined chiefly by temperature, however far 

 its mature predecessors were encroached upon by shore sands, it 

 is quite possible that earlier young reefs ( W,) X, Y, may also 

 have been able in their youth to extend as far south as their 

 successor of today. But this transcendental aspect of the prob- 

 lem need not be pursued further. 



