1 8 INDEX TO THE PACIFIC ISLANDS. 



and by the renewed explorations of Alexander Agassiz, but at the present writing the 

 evidence is not conclusive on either side. 



Without adopting either theory we may state that coral islands have a fringing 

 reef more or less interrupted, sometimes a barrier reef, while the island in many cases 

 becomes simply a ring of circular or irregular form, and the enclosed space is called a 

 lagoon in the atoll. An opening into this lagoon may convert it into a good boat 

 harbor, or the continuity of the ring and the growth of coral or the wash of sand and 

 debris may fill the lagoon converting it into a simple coral island with a fringing reef. 

 Many islands have simply a depression in the centre marking the former lagoon. 

 Atolls have often man}- islets inhabited on the ring, while other islets rise from the 

 shallow lagoon. 



From the organic nature of the reefs they are constantly changing, and the 

 change is generally a growth : hence channels become shallower and. unless kept open 

 by some fresh water stream finally close ; lagoons which have served for harbor to ves- 

 sels of light draft become dry land. Coral rock is easily cut and artificial channels 

 can often be cut to good harbors, and the apparent scarcity of such havens in the cen- 

 tral Pacific may be remedied. The growth of coral patches off harbors and in channels 

 is a serious danger to navigation and requires frequent surveys. The rate of growth 

 of coral reefs is not yet satisfactorily determined. Darwin's Voyage of the Beagle, and 

 Dana's Corals and Coral Islands may be consulted for farther information as to the 

 growth of corals. In regard to the geographical distribution of reefs it may be briefly 

 stated that there are no reefs on the South American coast, and only detached corals in 

 the Panama region. Easter Island is without reefs, so is Pitcairn, although there are 

 some growing corals about the latter, while the neighboring Paumotus consist of eighty 

 coral islands, nearly all with lagoons; the Marquesas have little coral about them; 

 the Society Islands and Fiji abound in reefs. The Samoan and Tongan are well pro- 

 vided with reefs, although in the former group Tutuila has less coral than Upolu. Of 

 the Hawaiian Islands Kauai, Oahu and Molokai have extensive reefs, while Maui and 

 Hawaii have very little except detached corals ; Necker and Nihoa have none, but 

 farther toward the west are many reefs. The Gilbert, Marshal and Caroline Islands 

 are almost entirely coral. The Marianas are actively volcanic in the northern por- 

 tion where there are no reefs, but the southernmost Guam has extensive reefs ; so have 

 Yap and the Pelews. The New Hebrides again are actively volcanic and have few 

 reefs, while New Caledonia abounds in them. The Coral Sea and Great Barrier reef 

 continue the line southward. The Louisiade Group and the Admiralty Islands have 

 barrier and fringing reefs, while the north coast of New Guinea which is fringed with 

 volcanic islands has no reefs. Of the Solomon Group only the western portion has 

 extensive fringing reefs. As to the extent of all the reefs in the western Pacific there 



is great lack of trustworthy information. 



[102] 



