SECT. 2] OCEANIC ISLANDS, SEAMOUNTS, GUYOTS AND ATOLLS 385 



floor, as proposed by Revelle (1955), Clearly more points are needed on the 

 graph to resolve the important question of the cause of subsidence of oceanic 

 volcanoes. 



8. Importance of Islands and Seamounts 



Oceanic islands have played important roles in the exploration of the world. 

 In the Atlantic the Azores were an outpost for ships of all kinds and still 

 function as a convenient stopping place for air traffic. In the Pacific the hundreds 

 of islands and atolls now in existence were stepping stones in the migrations of 

 man from Indonesia eastward and northward to Hawaii. Atolls with their thin 

 soils and meager water supply are not ideal dwelling-places yet many of them 

 support a population of several hundred each. In the days of sailing ships, 

 many islands, including the (Jalapagos, were used to replenish supplies of water 

 and other necessities. When steamships replaced sailing vessels, islands with 

 good harbors were valuable as coaling stations and naval bases. The importance 

 of islands and atolls in the Second World War can liardly be overemphasized 

 and many are used as air stops today. 



In the geological past Pacific islands and reefs were a controlling factor in the 

 distribution of marine and terrestrial life over vast areas (Ladd, 1960). In the 

 1 9th centiuy studies of the distribution of life on islands gave valuable support 

 to the theory of evolution as developed by Darwin and Wallace. Islands of all 

 sorts are still of value in surface navigation and as world-wide undersea naviga- 

 tion is now being developed, seamounts and guyots are of great value. 



Less obvious, but perhaps of equal importance, is the part played by islands 

 and atolls in oceanic circulation and their effect on the distribution of sea 

 temperatures. The Marshall and Gilbert chains, for example — small and 

 widely spaced as the units are — form a topographic barrier across the Equa- 

 torial Current system and give rise to large-scale eddies that affect the North 

 Equatorial Currents, the Equatorial Counter Current and possibly the South 

 Equatorial Current (Robinson, 1954). 



References 



Arx, W. S. von, 1954. Circulation systems of Bikini and Rongelap lagoons. U.S. Geol. 



Surv. Prof. Paper, 260B. 265-273. 

 Burnside, R. J., 1959. Geology of part of the Horshoe Atoll in Borden and Howard 



Counties, Texas. U.S. Geol. Surr. Prof. Paper, 315B, 21-34. 

 Carsola, A. J. and R. S. Dietz, 1952. Submarine geology of two flat -topped northeast 



Pacific seamounts. Amer. J. Sci., 250, 481-497. 

 Cloud, P. E., Jr., 1958. Nature and origin of atolls. Proc. 8th Pacific Sci. Cong., 111-A, 



1009-1023. 

 Cloud, P. E., R. G. Schmidt and H. W. Burke, 1956. Geology of Saipan, Mariana Islands. 



U.S. Geol. Surv. Prof. Paper, 280A, 1 126. 

 Cole, W. S., Ruth Todd and C. G. Johnson, 1960. Conflicting age determinations suggested 



by Foraminifera on Yap, Caroline Islands. Bull. Amer. Paleont., 186, 73-112. 

 Daly, R. A., 1925. The geology of Ascension Island. Proc. Amer. Acad. Arts Sci., 60, 1-80. 

 Dana, J. D., 1849. Geol. U.S. Expl. Exped. Wilkes, Philadelphia, 10, 1-756. 



