PROCEEDINGS OF SECTION C. 409 



the coral clusters. WTien deposition has advanced a short way, a map 

 {vide i late li., Cairn's Keef Lagoon) will show patches towards the 

 centre, where the original floor is still bare. Thence to the reef wall 

 stretches a talus slope. Given a sufficient period of quiescence, the 

 lagoon will fill up solid and be obliterated. Masthead Reef offers a 

 case where the lagoon is in the last phase of disappearance. 



Late in the development of a reef occurs the inception of the islet. 

 Some of the sand and shingle that perpetually drift across the reef is 

 caught by a chance obstruction (as in Bee Reef, Plate I ). Round 

 this nucleus more and more sand is packed. Saville Kent has well 

 figured this stage as " the birthplace of a coral island. "(t) 



When the sand has piled up enough to be dry at high water, it 

 is soon possessed by drift seeds, and grows green with shrubs and 

 herbage. The vegetation binds and protects the sandbank, so that 

 dunes rarely, if ever form on these cays. However far the periphery may 

 advance, the land does not gain in height. Torres Straits pilots have 

 informed us that certain islets have considerably enlarged their borders 

 within their recollection. Percolation of water through the sand of 

 the cays forms coral sand rock by solution and deposition of lime. 

 Such rock is generally visible where any temporary denudation of the 

 beach has occurred. In the phrase of physiographers describing the 

 history of rivers, the cay is now " mature.' The " senile " stage 

 might be represented by a reef entirely overspread by a vegetated 

 sandbank. 



VI. EVIDENCE OF THE MAINLAND. 



The present section is an attempt to correlate the broad geological 

 features of Eastern Australia with movements affecting the origin 

 of the Great Barrier Reef. 



In New South Wales the present drowned coastline is of com- 

 paratively recent date. The old river courses ran north and south 

 {vide Shoalhaven, WoUondilly, and Clarence), and hence more or less 

 parallel to the present coastline. Rivers flowing directly to the coast — 

 whose mouths are in many cases in the form of drowned valleys — are 

 most probably of late Tertiary age. Hence one may refer an im- 

 portant portion of the subsidence of the New South Wales coast to 

 late Tertiary times. It is significant that the western portion of the 

 State (the lower Darling and Murray, see Plate III.) exhibits an eleva- 

 tion of somewhat the same period. This gave rise to the engrafting 

 of the Murray tributaries and to the exposure of wonderfully rich 

 Tertiary fossil beds. 



It seems probable, therefore, that a " tectonic rocking " about 

 the main mountain axis (north and south) of the State obtained at the 

 close of the Tertiary, leading to depression in the east and elevation 

 in the west. 



(i) Saville Kent: "Naturalist in Australia," 1897, p. 147, text figure. 



