CORAL-REEFS. 67 



offers the best example. The central land consists either of 

 one island, or of several : thus, in the Society group, Eimeo 

 stands by itself; while Taha and Raiatea (Fig. 4, Plate II.), 

 both moderately large islands of nearly equal size, are 

 included in one reef. Within the reef of the Gambier group 

 there are four large and some smaller islands (Fig. 5, 

 Plate II.); within that of Hogoleu (Fig. 3, Plate II.) nearly 

 a dozen small islands are scattered over the expanse of one 

 vast lagoon. 



After the details now given, it may be asserted that there 

 is not one point of essential difference between encircling 

 barrier-reefs and atolls : the latter enclose a simple sheet of 

 water, the former encircle an expanse with one or more 

 islands rising from it. I was much struck with this fact, 

 when viewing, from the heights of Tahiti, the distant 

 island of Eimeo standing within smooth water, and encircled 

 by a ring of snow-white breakers. Remove the central 

 land, and an annular reef like that of an atoll in an early 

 stage of its formation is left ; remove it from Bolabola, and 

 there remains a circle of linear coral-islets, crowned with tall 

 cocoa-nut trees, like one of the many atolls scattered over 

 the Pacific and Indian Oceans. 



The barrier-reefs of Australia and of New Caledonia 

 deserve a separate notice from their great dimensions. The 

 reef on the west coast of New Caledonia (Fig. 3, Plate III.) 

 is 400 miles in length ; and for a length of many leagues it 

 seldom approaches within eight miles of the shore; and 

 near the southern end of the island, the space between the 

 reef and the land is sixteen miles in width. The Australian 

 barrier extends, with a few interruptions, for nearly a 

 thousand miles ; its average distance from the land is 

 between twenty and thirty miles, and in some parts from 

 fifty to seventy. The great arm of the sea thus included is 



