SKf'T. 2] OOEANrC ISLANDS, SEAMOUNTS, fiTlYOTS AND ATOLLS 381 



periods the tops of the atolls were exposed to atmospheric erosion. The arago- 

 nite of corals and other invertebrates was removed by solution or recrystallized 

 to calcite. High island vegetation and land shells became established (Ladd, 

 1958; Schlanger, in press). 



6. Submerged Atolls 



Submerged atolls are shallow banks with a relatively continuous elevated 

 rim and a deeper central area. The writers know of 22 in two groups in the 

 Pacific Ocean and one isolated example in the Indian Ocean although many 

 questionable occurrences suggest that they are much more common. One group 

 of submerged atolls in the Pacific is interspersed with the atolls in the western 

 Caroline Islands. Another group lies near the andesite line west of the Samoan 

 Islands and was investigated during the Capricorn Expedition. Alexa Bank 

 has a narrow elevated rim with a depth of about 20 m and a broad flat central 

 "lagoon" at about 40-50 m except where a few pinnacles rise to near the rim 

 depth. A ship-borne seismic-refraction station showed that the calcareous 

 material exposed at the surface is more than 1 km thick (R. W. Raitt, in litt., 

 1953) so that this is definitely a submerged atoll with a long history near sea- 

 level. Diving scientists observed that the bank is covered with calcareous 

 debris except for a few small patches of living lithothamnia and very sparse 

 corals. 



Why these shallow banks are not thriving atolls like those around them is 

 unexplained. 



7. Oceanic Volcanoes in Geological Time 



Pre -Quaternary dated material is available from oceanic volcanoes at Bikini 

 and Eniwetok Atolls in the Marshall Islands (Ladd et al., 1953), the guyots of 

 the Mid-Pacific Mountains (Hamilton, 1956), Erben Guyot of the north- 

 eastern Pacific (Carsola and Dietz, 1952), the Tuamotu guyots and the Nasca 

 Ridge guyots (W. Storrs Cole, in litt., 1959), and in the Atlantic at Caryn Sea- 

 mount, the Horseshoe seamount group, and the Atlantis-Great Meteor 

 seamount group (Heezen et al., 1959). Dredge hauls on all other seamount and 

 guyot groups have obtained Quaternary organisms and undated basalt bed- 

 rock. A geological history based on such a small sample requires the following 

 assumptions : 



1. The oldest fossils in a locality give the approximate date that atolls and 

 guyots were volcanic islands. 



2. Vulcanism in a group of oceanic volcanoes persists for a period that is 

 geologically brief, so that the oldest date in a group is representative of a whole 

 group. 



The first "assumption" is a fact at Eniwetok atoll where the volcanic plat- 

 form was drilled and at the Mid-Pacific Mountains where basalt and coral 

 debris were dredged together from the top of a guyot. It seems a reasonable 



