402 PRINCIPLES OF STRATIGRAPHY 



served. Thus on the south side of Hogoleu this is no less than 20 

 miles, 6 miles on the east and 14 miles on the south. 



The central mountains in the Society and other groups are gen- 

 erally bordered by a fringe of flat and often marshy, alluvial land, 

 from one to four miles in width, and consisting of coral sand and 

 detritus thrown up from the lagoon channel, and of soil washed 

 down from the hills. These island hills vary greatly in height and 

 character. At Tahiti the height is 7,000 feet, though the maximum 

 diameter of the reef is only thirty-six miles. At Alaurua the cen- 

 tral peak is about 800 feet high, and the maximum diameter only 

 a little more than two miles. In other cases it is much lower; at 

 JManouai only 50 feet. In most cases the rock is that of an old 

 volcano, while in some cases it is old crystalline rock and in others 

 raised coral reef rock. The number of peaks varies from one to 

 nearly a dozen (Hogoleu). 



What has been said of barrier reefs around oceanic islands also 

 applies to the fringing reefs of such islands. These differ from the 

 encircling barriers only in the narrowness of the dividing water 

 channel or moat. Thus on the western side of Mauritius the reef 

 generally lies at a distance of about half a mile from the shore, 

 though in some parts it is distant from one to two and even three 

 miles. In general, the fringing reef rests on the gently sloping sub- 

 merged portion of the island mass, which in most cases does not de- 

 scend, within the limits of the reef, to depths greater than those at 

 which coral polyps flourish. Where such a greater depth exists it is 

 to be accounted for either by progressive subsidence and upbuilding 

 of the reef, or by outward growth upon a basis of coral detritus, 

 which forms a talus in front of the growing reef. Fringing reefs 

 are breached by passages in front of every 'river and streamlet 

 which descends from the enclosed land. Such breaches are found 

 in barrier reefs only in front of the larger streams where the width 

 of the lagoon channel is not too great. The width of the reef varies 

 with the inclination of the substratum. 



Characters of Epicontinental Reefs. Reefs resting for the main 

 ■part upon the continental shelf or its margin are found to-day in 

 the western portions of the three oceans covering the tropical belt, 

 namely, the Pacific, Indian and Atlantic. The largest and most char- 

 acteristic are those of the western Pacific, the Great Barrier Reef 

 of Australia occupying the first place among these. This, as already 

 noted, extends with a few interruptions for about a thousand miles ; 

 its average distance from the land is between twenty and thirty 

 miles, while in some parts this distance increases to between fifty 

 and seventy miles. It encloses a great lagoon strip which varies in 



