THEORIES OF ORIGIN OF REEFS 411 



growth of corals, and these eventually die out on the interior, while 

 they continue to flourish vigorously on the exterior of the atoll. By 

 the accumulation of coral debris on the outer slope of the reef a 

 foundation is formed for the spreading of the coral ring, as in the 

 case of the Florida reefs, and so the diameter of the atoll increases, 

 while at the same time the solvent action of the water in the lagoon 

 and the scouring action of the tide will enlarge the area of the en- 

 closed water body. It may, however, be questioned if the great 

 depth of some lagoons, 40 to 50 fathoms, can be accounted for in 

 this way, especially as in many cases the tendency is to fill up the 

 lagoon by nullipore growth and accumulation of debris. 



That such an origin must be accepted for certain atoll reefs is 

 shown by the observations of Guppy (41) on certain uplifted 

 islands of the Solomon group. On Ugi, Santa Anna, Treasury and 

 Stirling islands, he found unmistakable evidence of a nucleus of 

 volcanic rock, which was covered by an earthy, stratified deposit of 

 mixed foramini feral and volcanic material similar to the deep-sea 

 muds, several hundred feet thick, and highly fossiliferous, contain- 

 ing the remains of pteropods, pelecypods and echinoderms. On the 

 flanks of these elevated beds are found deposits of coralline lime- 

 stone varying from 16 to 100 feet in thickness, and representing the 

 reef deposits now elevated above the sea-level. Santa Anna has the 

 form of an uplifted lagoon, the coral rock 80 feet in thickness and 

 resting on a friable, somewhat argillaceous rock resembling a deep- 

 sea deposit. Here we have, then, a clear example of reefs growing 

 on a platform, built up of foreign material, and similar features are 

 shown on the Fiji Islands and the Great Barrier Reef, and on the 

 reefs of the Bahamas, Florida and the Bermudas in the West At- 

 lantic. All of these areas show either stationary conditions or 

 actual evidence of recent elevation. 



Multiple Origin of Reefs. On the whole, the subsidence theory 

 seems to be applicable to many, though far from all, oceanic reefs, 

 where at the same time spreading of the coral ring by outward 

 growth may have cooperated. Epicontinental reefs, on the other 

 hand, or those forming on the margins of the continents and around 

 continental islands, are probably in all cases to be explained by 

 growth on suitable foundations, without subsidence, the reef ex- 

 panding as the platform on which it grows expands. Such an ori- 

 gin is almost certain for the Great Barrier Reef of Australia, and 

 for the Florida reefs, and such an origin is to be postulated for 

 most of the older reefs, which all or nearly all belong to the epi- 

 continental type. 



R.\TE OF Growth of Reef Organisms. Comparatively little is 



