not formed on primary roclc, tlie probability now aclmitted hy 

 all respectable geologists, tliat tlicsc rocks are of igncous ori- 

 gin, enables us to malce the supposition , without any diHiculty 

 whatever; but, were that not so, Mr. D.navix 1 as not lo our 

 knowledge, found a single instance of a reef encircled island, 

 composed altogether of primary rock, but if he had, ho should 

 have recognised at the Cape of Good Hope (as hereafter to be 

 farfcher noticed), that the inclination of the sides of momitains 

 is often very different from that of the strata of which they 

 are composed, and lastly that these reefs foUow indiflerently 

 the tending of the island itself, or of its submarine prolonga- 

 tion, is precisley what the supposition should lead us to expcct. 

 The northern end of New Caledonia slopes gradually into tho 

 sea, and nothing is more likely than that the extension, which 

 was not raised above its level, would in process of time, be- 

 come covered with the works of the coralline polypi. 



IV. »Tlie Great li.iirier which frouls the N. E. Coasl of Australia, foriiis 

 » a thiril class of reef. Il is described by Flikdeks ;is liaving a leii{;lii of 

 »uearly oiie thousand luiies, and as ruiiniri^^ parallel to the shore at a dis- 

 3» tance of froiu twenty lo tliirly miles froin it, aud in soiue parts even of 

 » fifty lo seveuty. The great arm of ihe sea ihus included, bas a usual 

 »deplh of between ten and twenly i'alhoiiis, but ihis inoreases towards one 

 » end (towards the S. E. projection of New Giiinea) to forty and even sixly. 

 » This probably is both the grandest and most extraordinary reef, now cxis- 

 »ling in any part of the world." 



This does noi form a third class of reef. It evidently 

 belongs to the same class as the northern extension of the New 

 Caledonian, and like that, stands on a submarine prolongation 

 of the easternmost ridge of the Australian supermarine elevations , 

 running also parallel to the submarine isthmus, by which Aus- 

 tralia is connected to New Guinea (under Torres Straits), which 

 is also mostly covered up by coral formatious. The conside- 

 ration of tlie extent of that isthmus and of the continual pas- 

 sage over it, during one season , of the superficially heated waters 

 of the Paciiic Ocean, and during the other, of those of the 

 still warnier Jndonesian seas, both, at all times, bringing abim- 

 dance of food and of matcrials lor the builders to use and eiu- 



