I04 THE GREAT BARRIER REEF. 



to the height of two or three feet only at low water. The islets, none of which exceeds 

 a mile in its greatest length, agree in their physical character with Lady Elliot Island, pre- 

 viously described. Like that islet, they are of uniformly coral formation, raised some few feet 

 only above high tide level, and are associated, in the majority of instances, on their most 

 weather-exposed, eastern, side, with an irregularly ovate or annular reef, including an enclosed 

 lagoon, that usually occupies three or four times the superficial area of the islet. The same 

 irregularly annular or ovate contour, with an enclosed lagoon, distinguishes the separate 

 reefs. Collectively, therefore, the members of the Capricorn and Bunker groups furnish an 

 appropriate illustration of the atoll-like reefs and islets recognised by Darwin, quoted at page 

 75, as frequentl}- occurring in association with isolated banks in comparatively shallow water. 

 True atolls, as distinctly defined by Mr. Darwin, whilst exhibiting corresponding contours, 

 arise abruptly from very considerable, or, it may be, abyssal, depths. From fifteen to twenty- 

 five or thirty fathoms are the deepest spundings obtainable outside the edges of any of the 

 pseudo-atoll reefs of the Capricorn and the Bunker groups. 



A highly characteristic illustration of the marginal area of one of these atoll-like reefs, 

 together with the distant view of a neighbouring coral-islet of the Capricorn group, is given in 

 Plate XXX. B. The islet on the distant horizon in this picture is known as Heron Island; 

 it is about a mile in diameter, and consists of a bank of white coral-sand and conglomerate, 

 raised but a few feet above the water, and thickly overgrown with trees, whose tops reach to an 

 elevation of sixty feet above high-tide level. An extensive reef, between five and six miles 

 long, and about two in width, subtends in an easterly direction from the island ; but it is 

 necessarily invisible from the point of view at which this photograph was taken. The fore- 

 ground area included in the field of view represents the north-easterly edge of Westari reef, 

 which is separated from Heron Island by a channel about a mile wide, and from eighteen to 

 twenty fathoms deep. Its most conspicuous feature, represented by the huge rock-boulders 

 that bestrew its surface, has already been the subject of remark in the earlier, plate-descrip- 

 tive, chapter ; as there explained, masses of the coral-conglomerate have been torn off the 

 outer edge of the reef and hurled to their present position during storms of abnormal severity. 

 Similar boulder-strewn areas characterise portions of the marginal edges of most of the coral 

 reefs and islets throughout the Barrier system, and are a standing monument to the violence of 

 the storms with which this region is visited. The largest of the rock boulders in this illus- 

 tration is from five to six feet long, and from four to five feet high ; but the dimensions are 

 in many instances three or four times greater. The larger, most weather-exposed, of these 

 conglomerate blocks become blackened with age through encrustment with a species of lichen, • 

 and are then popularly known as " nigger-heads." Although most abundantly cast up on 

 the south-eastern (or normal weather) side of the reefs, these conglomerate boulders may, as 

 in the present case, occur on the northern face, which represents, in point of fact, that aspect 



