352 THE POPULAR SCIENCE MONTHLY 



Eeef-making corals can not endure exposure to the air and they can 

 not thrive at a depth of more than 20 fathoms, so that their vertical 

 range is about 115 feet; yet hooks and anchors brought up coral rock 

 and sand from many hundreds of feet below the limit of growth; in a 

 great number of instances, the atolls or ring-like reefs are mere peaks 

 rising with abrupt slopes from " fathomless " abysses. Coral-bearing 

 areas within the Indian and Pacific Oceans are of vast extent, there 

 being chains of archipelagos, 1,000 to 1,500 miles long. The reefs are 

 rudely circular or elliptical in the islands but are linear along the 

 coasts ; in the one case, the reef encloses a lagoon, in the other, a lagoon- 

 like channel separates the reef from the coast. These are fundamental 

 elements of the problem, not one of which may be neglected in the solu- 

 tion. A clue to the explanation was found by this keen observer when 

 he saw an islet of old rock, fringed with coral, rising from the lagoon 

 of an atoll, so that the atoll-ring resembled in many respects the barrier 

 reef of a continent and the lagoon itself resembled the lagoon-like 

 channel seen on the Australian and other coasts. 



Chamisso's suggestion that coral reefs had been formed on banks of 

 sedimentary material seemed wholly incompetent to meet the condi- 

 tions, for the areas are too vast, and Darwin was compelled to believe 

 that the atolls rest on rocky bases; but even on this supposition, it 

 appears incredible that peaks of several great mountain chains should 

 all come to within less than 180 feet of the surface and that not one 

 rose any higher. The long study in South America had prepared him 

 to seek an explanation in mobility of the earth's crust ; but it was clear 

 that elevation could not bring about the conditions, as that would 

 destroy the corals themselves; subsidence alone can account for the 

 phenomena. And thus Darwin presents his case : 



If then the foundations of the many atolls were not uplifted into the 

 requisite position, they must of necessity have subsided into it; and this at 

 once solves every difficulty, for we may safely infer from the facts given in 

 the last chapter, that during a subsidence the corals would be favorably 

 circumstanced for building up their solid framework and reaching the surface, 

 as island after island slowly disappeared. Thus areas of immense extent in 

 the central and most profound parts of the oceans might become interspersed 

 with coral islets, none of which would rise to greater height than that attained 

 by detritus heaped up by the sea, and nevertheless they might all have been 

 formed by corals which absolutely require for their growth a solid foundation 

 within a few fathoms of the surface. . . . The rocky bases slowly and suc- 

 cessively sank beneath the level of the sea, while corals continued to grow 

 upward. 



The origin of the ring as well as that of the barrier reef seemed to 

 be easily explained by this hypothesis. The corals on the outer side of 

 the reef grew with greater rapidity than did those within, as the supply 

 of food is constant ; those on the inner side became starved and eventually 

 the interior growth ceased and the lagoon was shallowed by wind-drifted 

 material from the shores. 



