406 PROCEEDINGS OF SECTION C 



being a usual form of island, this type is rare or absent. Indeed marine 

 erosion seems to us to play a paltry part in the moulding of the Queens- 

 land coast, the main features of which we ascribe to subsidence. Though 

 Jukes probably exaggerated the mass of coral in the Great Barrier, yet 

 Agassiz certainly went to the opposite extreme and underestimated 

 it as a thin veneer " of at most 20 fathoms " (p. 139). 



It may be allowed — though Darwin deprecated the idea^ — that 

 the continental shelf was ready prepared with numerous banks repre- 

 senting eroded islands, just reaching to within the required distance 

 of the surface, when the first coral builders came. 



V. THE BUILDING OF A CAY. 



This hypothetical history of a Barrier Reef islet, as deduced from 

 the Hope Islands and Cairns Reef, commenced when the coast of 

 Queensland stood at a higher level than it does now. Or it would 

 equally suit our purpose to consider that the sea lay then at a lower 

 level. Queensland had not remained at that height sufficiently long 

 for the sea-board to have been reduced to a peneplain. 



The high islands, then a part of the main, commenced to sink. 

 No evidence is available, but it is as likely that the descending cycle 

 was advanced, that the movement had already made much progress, 

 and that it had swallowed a coastal plain, as that it began with the 

 attack on the high islands. 



Invasion of the sea has parted the high islands from the main by 

 channels 15 to 20 fathoms deep. No sediment from the Queensland 

 rivers escapes to the open sea beyond the Barrier. All is deposited in 

 the channel along the coast. Therefore the present sea-floor as shown 

 by the soundings on the chart, would not represent the original bed of 

 the sunken valleys. And 200ft. may be assumed as a moderate estimate 

 of the depth to which the islands have been submerged. 



The subsidence that isolated the high islands has also left its mark 

 on the mainland. Mourilyan Harbor is a good example of a drowned 

 land valley. As instanced later, all the rivers of north-east Queensland 

 debouch through estuaries filled with sediment and lying in troughs 

 too deep for excavation at present level. 



An exchange of flora and fauna between Australia and New Guinea 

 points to a time when an isthmus occupied the place of Torres Straits. (e) 

 The alteration in level needed to restore this isthmus corresponds to the 

 alteration which would restore the high islands to the main. Since the 

 time when communication by land was held across Torres Strait, the 

 fauna has undergone some specific change. This affords us a time 

 index, and we date the isthmus as Pleiocene. 



When the sea had so far overrun the land as to submerge the 

 position of the Cairns Reef, the corals would not at once occupy it. 

 The mud brought down by the streams renders the immediate vicinity 

 of the land too turbid for reef growth. Not till the coast had retreated 

 further would colonists from the seaward reefs settle there. 



(e) Hedley.— Proc. Linn. Soc, N.S. Wales, xxiv., 1899, p. 396. 



