62 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 4, 



instances may be found in the city of Sydney, on walls but a few 

 years old. Similar incrustations face the cliffs of the carboniferous 

 formation, north of Bullai on the coast of the Illawarra, derived 

 from the evaporation of the moist particles of shells and sand blown 

 up by the wind far out of the reach of the waves. 



The resemblance of the calcareous conglomerates of some of the 

 coral islands to cretaceous rocks is so great, that it is easy to ima- 

 gine by what processes some of the latter may have been produced ; 

 nor is it unlikely, that in some future geological epoch, repre- 

 sentatives of them may be recognised in these recent formations 

 when subjected to the transmuting influence to which the secondary 

 chalk has been exposed. 



Above these lower calcareous masses, at the height of from fifty 

 to 100 feet above the sea, the rocks are chiefly composed of a dead 

 coral. In the hollowed cells of this species are found calcareous 

 mud, vegetable fibres, small decomposing shells and fragments of 

 shells, together with attached Serpulae, the latter proving that some 

 of these extraneous substances have not been carried upwards by 

 the sea or wind, but were elevated with the rock itself. 



The upper part of the island consists of a dead species, apparently 

 an Astrcea, In this species the diverging lamellae are connected by 

 transverse radiating septa, which produce rectangular cellules with 

 vertical walls and inclined floors. The stars are very irregular, some 

 being concavely quadrilateral and others much elongated, as if 

 from crowding or pressure of the animals in growth. 



Masses of this occur full 250 feet above the sea, and at that 

 height the hollows and cells are filled with minute fragments of 

 shells, vegetable fibres and portions of fuci, coral sand and other 

 marine detritus. The aspect of these upper corals is that of great 

 age, and the more exposed are extremely crumbling, but the lowest 

 corals have an appearance of great freshness. 



Some of the appearances presented by the surface of Lafu, such 

 as the parallelism of structure in the headlands, and the overhanging 

 character of the cliffs, lead to the idea of a unity of action in their 

 formation. The existence of beds of coral at the height of 250 feet 

 above the sea, together with the adventitious circumstances of en- 

 tangled marine substances, the whole island being free from cracks 

 or fissures, prove that its present conditions are due to simple ele- 

 vation ; that this has been interrupted by a period of repose may be 

 inferred from another interesting fact. At the height of from seventy 

 to eighty feet above the sea, a ledge or shelf of the same character 

 as that now surrounding the base of the island runs all round the 

 cliffs ; and where along the bays and hollows near the coast this 

 water-mark is cut off by the lower level of the neighbouring shores, 

 on pushing into the interior, the real continuation of the water-mark 

 may be traced so soon as the country rises to a suflicient height. 

 Nothing can be clearer than this evidence to show, that in the island 

 of Lafu there have been two distinct elevations of the land, the latter 

 amounting to nearly eighty feet, the former to at least 170 feet. 

 Supposing that, in forming, coral islands must of necessity subside, 



