INTRODUCTION. xili 



the formation of barrier reefs and atolls has been subjected to prolonged 

 criticism. Following Rein and Semper, recent investigators have, with 

 few exceptions, regarded it as untenable. Semper's investigations in the 

 Pelew Archipelago have hardlj received the recognition they deserve. As 

 far back as 1861 he spent nearly ten months in the Pelews,^ and called 

 attention to the fact that all kinds of reefs (atolls, barrier and fringing 

 reefs) are found in the Pelews, within a comparatively short distance, 

 in a region considered by Darwin as one of subsidence, and that at the 

 southern extremity of the group an elevated coralliferous limestone island 

 (Ngaur) rose to a height of over 300 feet, containing, according to Wich- 

 man, fossils of late tertiary age. Semper lays great stress on the effects of 

 currents between the land rim and the barrier reef in widening barrier reef 

 lagoons ; reef platform lagoons such as I have described illustrate this well. 

 He further suggests that marine animals may build up a foundation for the 

 growth of reef corals, from far greater depths than those at which corals 

 can thrive. 



Semper fii'st called attention to the effect of solution in removing mate- 

 rial from an atoll, but it is to Sir John Murray ^ that we owe careful experi- 

 ments to determine the amount of lime removed by solution and the sugges- 

 tion of its importance as a factor in the formation of lagoons. Semper 

 considers the effect of solution in removing lime from the lagoon of an 

 atoll as balancing the constructive agencies proved by the growth of 

 corals in it. 



To Professor Dana was undoubtedly due the general acceptance of 

 Darwin's theory of the formation of coral reefs. Dana's views on coral 

 reefs were based upon the study of the coral reefs of Samoa, the Hawaiian 

 Islands, Tahiti, the larger Fiji Islands, seven of the islands in the Pau- 

 motus, of Tongatabu, of Tapeteuea and Apaiang in the Gilbert Islands, 

 of Gente Hermosa, Fakaofu, Oatafu, Hull, and Enderbury among the Line 

 Islands. The publication of his results in 1849 ^ supporting in the main 

 Darwin's theory could not fail to supply a mass of evidence wanting in the 

 Report of Darwin's researches, based as they were upon the survey of the 



' Die Naturlichen Existenzbedingungen d. Thiere, Leipzig, II., 1880, p. 39. 



^ Proceedings of the Royal Society of Edinburgh, XVII., 1889-90, p. 79. 



^ Geology of the United States Exploring Expedition under Commodore Wilkes. 



