MUSEUM OF COMPARATIVE ZOOLOGY. 129 



of larger fragments of coral and reef limestone, that the constituent 

 parts of the beaches were, as could easily be seen by the eye, invari- 

 ably corroded far beyond the condition to which the sand or breccia, or 

 larger fragments, could be reduced if merely subjected to the triturat- 

 ing agency of the rollers. An analysis made by Prof. F. W. Clark, the 

 chemist of the United States Geological Survey, of such fragments, 

 either as sand or in all intermediate stages up to fragments of coral, or 

 coral limestone, showed very clearly that the chemical composition of 

 the pieces was practically the same. 



The only analysis known of the chemical constituents of the sea-water 

 of the lagoon of an atoll is given as determined by Messrs. Stillwell and 

 Gladding,* from which it would appear that the amount of chlorine was 

 considerably larger than the amount given in the latest analysis of 

 Dittmar, and that the water of the lagoon is fresher than that of ocean 

 water. 



It may not be out of place to mention here, that there is a most excel- 

 lent figure and plan of an atoll in an account of Caroline Island,'^ by 

 Prof. E. S. Holden, the director of the American. Eclipse Expedition of 

 1883. Not only is the description of the atoll admirable, but the illus- 

 trations of the various parts of the island are most characteristic, in- 

 cluding one of the best figures perhaps of a bird's-eye view of an atoll. 

 A map also accompanies the description, but unfortunately no sound- 

 ings are given. 



Lagoons without openings are perhaps older lagoons, in which the 

 openings have from local causes been gradually closing, and from the 

 porous nature of the surface coral rock there still remains a chance for 

 the exchange of waters from the interior to the exterior of a reef. 



Jukes,* who in 1845 surveyed the Great Barrier Reef of Australia, 

 came to the conclusion that the " northeast coast of Australia has either 

 been slightly elevated, or that it has at lea^t not suffered any depression 

 during a long period of time." From this he has satisfied himself that, 

 wherever we find coral reefs rising abruptly from unfathomable depths, 

 they must necessarily have been produced by the depression of the sea 

 bottom, the corals building on upwards as the bottom was slowly sink- 

 ing, so as to keep the upper portion of the reef always within the 

 required depth. The depression of the bottom, according to this view, 



1 Mem. Nat. Acad, of Science, Vol. III. p. 96. 



* Report of the Eclipse Expedition to Caroline Island, May, 1883, Mem. Nat. 

 Acad, of Science. 



8 Jukes, J. B., Narrative of the Surveying Voyage of H. M. S. Flv, Vol. I. p. 311. 



VOL. XVII. — NO. 3. 9 



