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Ch. xlix.] size of atolls and baeeier eeefs 



GOl 



one. The fact observed universally, that the princij)al opening 

 fronts a considerable valley in the encircled island, between 

 the shores of which and the onter reef there is often deep 

 water, scarcely leaves any doubt as to the real origin of the 

 channel in all those countless atolls where the nucleus of 

 land has vanished. 



Size of atolls and harrier reefs.— 



—In regard to the dimen- 

 sions of atolls, it was stated that some of the smallest ob- 

 served by Beechey in the Pacific were only a mile in diameter. 

 If their external slope under water equals upon an average 

 an angle of 45°, then would such an atoll at the depth of 

 half a mile, or 2,640 feet, have a diameter of two miles. 

 Hence it would appear that there must be a tendency in 

 every atoll to grow smaller, except in those cases where 

 oscillations of level enlarge the base on which the coral 

 grows by throwing down a talus of detrital matter all round 

 the original cone of limestone. 



Bow Island is described by Captain Beechey as 70 miles in 

 circumference, and 30 in its greatest diameter, but we have 

 seen that some of the Maldives are much larger. 



As the shore of an island or continent which is subsiding 

 will recede from a coral reef at a slow or rapid rate according 

 as the surface of the land has a steep or gentle slope, we 

 cannot measure the thickness of the coral by its distance 

 from the coast ; yet, as a general rule, those reefs which are 

 farthest from the land imply the greatest amount of sub- 

 sidence. We learn from Tlinders, that the barrier reef of 

 north-eastern Australia is in some places 70 miles from the 

 mainland, and it should seem that a calcareous formation is 

 there in progress 1,000 miles long from north to south, with 

 a breadth varying from 20 to 70 miles. It may not, indeed, 

 be continuous over this vast area, for doubtless innumerable 

 islands have been submerged one after another between the 

 reef and mainland, like some which still remain, as, for 

 example, Murray's Islands, lat. 9° 54' S. We are told that 

 some parts of the gulf enclosed within the barrier are 400 



feet deep, so that the efficient rock-building corals cannot be 

 growing there, and in other parts of it islands appear en- 

 circled by reefs. 



