412 Notices of Memoirs — Professor Alexander Agassiz — 



is not bounded by a reef forming a closed wall rising well above tbe 

 level of the sea. The greater part of the reef of many a lagoon of 

 an atoll or barrier reef has from two to three fathoms of water upon 

 it at high tide. The reef is also riddled on all sides with narrow 

 channels or openings with from one to two fathoms or more at low 

 tide, in addition to the wider and deeper passages to leeward, 

 through which access is gained into the lagoon. But for all this the 

 lagoon exists, while it may not have more than a few fathoms in 

 maximum depth. This, however, does not prevent the coral heads 

 on the inner slope of the reef from gradually becoming connected 

 with the reef, and from encroaching little by little, but very slowly, 

 upon the outer margin of the lagoon to a depth of seven or eight 

 fathoms, at which the growth is checked either from the sediment 

 accumulating on the floor, or from the strength of the currents 

 scouring the bottom of the lagoon. The amount of dead coral which 

 is ground up upon a reef flat is considerable. Much of it is 

 cemented together and forms a breccia in the cavities of the coral 

 heads, or in the ojDen spaces between them. Still more of it is 

 changed into sand and mud, which cover the floor of the lagoons of 

 barrier reefs and of atolls, and finally a quantity is carried off in 

 solution after the dead coral has become thoroughly rotten and 

 •crumbling. 



" Darwin also visited the western side of Mauritius, where, he 

 fiays : ' It is probable that a reef on a shelving shore, like that of 

 'Mauritius, would at first grow up not attached to the actual beach, 

 but at some little distance from it ; and the corals on the outer 

 margin would be the most vigorous. A shallow channel would 

 thus be formed within the reef, and this channel could be filled up 

 only very slowly with sediment, for the breakers cannot cut on the 

 shores of the island, and they do not often tear up and cast inside 

 fragments from the outer edge of the reef, while every streamlet 

 carries away its mud through breaches in the reef. .... 

 A fringing reef, if elevated in a perfect condition'above the level of 

 ■the sea, would present the singular appearance of a broad dry moat 

 bounded by a low wall or mound.' 



'• Darwin, when meeting Semper's objection that the existence of 

 atolls or barrier reefs in a region of elevation was a fatal argument 

 against his (Darwin's) views, is obliged to say that therein ' seems 

 to me no improbability in their having originally subsided, then 

 having been upraised .... and again having subsided.' He 

 further says, ' The existence of atolls and of barrier reefs in close 

 proximity is manifestly not opposed to my views.' Certainly not, 

 but their existence in an area of elevation as claimed by Semper 

 is. Darwin also says that, ' When the land is px'olonged beneath the 

 sea in an extremely steep slope, reefs formed there during subsidence 

 will remain closely attached to the shore, and will be undistinguish- 

 able from fringing reefs.' This seems to me impossible. The 

 disintegration of the inner edge of the fringing reef, the action of 

 the sea upon this disintegrated material, the solvent action of sea 

 water, all will tend to form a channel between the outer parts of the 



