1846.] CLARKE ON THE GEOLOGY OF THE ISLAND OF LAFU. 63 



the amount of elevation of the lowest visible coral rock at the level 

 of the sea must have attained at least 500 feet from a former level. 



The general surface of Lafu is that of a table land with such 

 hollows and elevations as now mark the surface of a coral reef; and 

 as the soil upon it is thin and much intermixed with decomposing 

 fragments of calcareous matter^ no great changes have modified its 

 superficial condition since it attained its present elevation. 



Many localities in the Pacific exhibit tiie occurrence of rocks of 

 dead coral at considerable heights above the ocean, as is the case 

 in Java, Timor, &c. In those instances the immediate connection 

 with volcanic action points to a cause of elevation. In the case of 

 Lafu the elevation does not appear to be connected with visible vol- 

 canic force ; but it is highly probable that it is merely the result of 

 the elevation of New Caledonia, which is composed of primary and 

 Silurian rocks, traversed by dikes and intrusive masses of basalt 

 and trap ; and which is everywhere surrounded (for the space of 

 1000 miles in one direction) by fringing and barrier reefs, of which 

 the Loyalty and Britannia groups are elevated portions. 



To connect these reefs with portions of the sea to the south-west- 

 ward, and with the coasts of Australia south of the great barrier 

 reefs, by the discovery of blocks of loose coral, upon the mountains 

 of the continent, is more perhaps than can legitimately be done ; but 

 it is worthy of mention, that fragments of coral, in all the characters 

 of structure, colour and decomposition similar to the Astrcea of 

 Lafu, have been brought to me from the ranges at the head of the 

 river Boyne in the Darling Downs west of Moreton Bay, in lat. 26° 

 40' S. and long. 151° 42' E., full TOO or 800 miles from Lafu, 

 and at the height of not less than 2000 feet above the sea. Whether 

 these fragments are true drift like the pumice, which I have found 

 everywhere dispersed along the shores of Eastern Australia*, and 

 which occurs on the high ranges of Australia Fehx, far to the south ; 

 or whether they have been dropped by the aborigines, who frequently 

 carry strange substances in their bags for many miles, there is no 

 ground to determine ; but whilst the paucity of the fragments op- 

 poses, the size of them favours the idea, that by some physical cause 

 there have been left upon the soil blocks of this species, which in 

 all points resemble the masses on the summit of Lafu ; and upon 

 the supposition that they were taken from the coast and carried in- 

 land, it is clear that the existence of the reef-making corals may be 

 traced further south on the coast of New South Wales than is com- 

 monly imagined. Nay, certain Astraeae and other corals abound in 

 Port Jackson. 



In conclusion, I remark, that the occurrence of the genus AstrcBa 

 in the harbour of Port Jackson proves, that corals of this description 

 can thrive in shallow water of a temperature below that of the reef- 

 studded ocean. The lower limit of sea temperature favourable to 

 the growth of corals has not been ascertained ; but the occurrence 

 of Astrseas on the shores of Port Jackson, where the average tem- 

 perature of the water is between 60° and 70°, varying perhaps occa- 

 * See my remarks in the Tasmanian Journal, vol. i. 



