ALEXANDER AGASSIZ MAYER. 467 



He saw that the coral reefs which grew around a volcanic mountain 

 remain after the mountain has washed away, and thus an atoll is 

 formed without the agency of subsidence. In other cases, as at 

 Fulangia, there was once an elevated coral limestone island lifted 

 above sea level. Th,en rain water and atmospheric erosion leached 

 out depressions in its central parts and finally the sea entered, form- 

 ing a lagoon surrounded by a ring of detached islets of elevated lime- 

 stone. In other cases the crater rims have washed away a^id a ring 

 of coral reefs now marks the site of the old volcanic ridge. 



According to Agassiz the coral reefs of to-day in the Fiji Islands 

 form only a crust of moderate thickness upon a- base of old limestone 

 or volcanic rock. The present corals form only fringing reefs along 

 the shores, and the contours of the atolls and barrier reefs are thus 

 due to causes which acted at the time when the islands were elevated 

 late in the Tertiary period. 



In so great an archipelago as that of the Fijis with more than 270 

 islands there must be m.any details of reef formation, the elucidation 

 of which requires more prolonged study than Agassiz was enabled to 

 devote to them in his visit of less than three months; for example, he 

 Avas puzzled to explain the great thickness — 1,000 feet and more — of 

 the elevated limestones; for reef corals do not grow at depths greater 

 than about 120 feet. Could these enormous accumulations have been 

 formed by coral reefs during a period of slow subsidence, as Darwin 

 had assumed, or were they merely the talus of broken fragments 

 which had rolled down the seaward sides from the outer edges of the 

 reef, or were they formed by a slow accumulation of limestone frag- 

 ments and shells of marine creatures other than corals which had 

 lived upon the bottom more than 1,000 feet beneath the surface and 

 gradually built up a vast mass of limestone, as was the case with the 

 great submerged Pourtales Plateau off the Florida coast ? He had in 

 mind the fact that even in the richest coral-reef regions the masses 

 of broken shells and fragments of calcareous plants are commonly 

 vastly greater than the bulk of the corals, for the corals grow only 

 here and there over the limestone flats, and flourish luxuriantly only 

 on their seaward slopes. 



AYere such a reef to form during a long period of slow subsidence, 

 and then become elevated above the sea, we should find only an occa- 

 sional coral here and there imbedded in a great mass of limestone. 

 This is the appearance presented by some of these elevated limestone 

 cliffs of the Pacific islands, while others appear to be walls of non- 

 coral-bearing limestone capped above with a crust of corals. In 

 many cases, however, the corals they once contained have disappeared 

 and been replaced by calcite or dolomite. These elevated limestones 

 soon become very hard when exposed to the atmosphere, for they 

 become coated by a dense veneer which rings with a clinkerlike sound 



