SECT. 2] OCEANIC ISLANDS, SEAMOUNTS, GUYOTS AND ATOLLS 383 



and shallow banks could have existed as closely spaced stepijing-stones, 

 reaching halfway across the equatorial Pacific. 



From the fragmentary evidence available, it appears that late Mesozoic- 

 early Cenozoic times saw a culmination of vulcanism in the basin. In the 

 Eocene, volcanoes had grown up to form islands in the Marshall Islands and 

 Tuamotu Archipelago, and islands probablj^ still existed in the Mid-Pacific 

 Mountains. By inference from the similarity of structural trends, the Gilbert 

 Islands were probably also first formed at this time. With regard to biological 

 migration, stepping-stones became much more common than before. The con- 

 tinuity of the stepping-stones depends on the age of the Line Islands. Inasmuch 

 as the Tuamotu and Line Islands and the Mid-Pacific Mountains and Emperor 

 Seamounts all lie along the median line of the Pacific Basin, it may be argued, 

 by analogy with the Mid- Atlantic Ridge, that aU the volcanoes of the groups 

 were active at about the same time (Menard, 1958). If so, stepping-stones 

 reached from northeast Asia to the southeastern Pacific. One of the puzzles of 

 this time is the close juxtaposition in space and time of atolls and guyots in the 

 Marshall and Tuamotu archipelagoes. Why did atolls develop from some banks 

 and guyots from others in the same locality and of approximately the same 

 age? In particular localities, such as at Bikini Atoll and the adjacent Sylvania 

 Guyot, the atoll apparently is younger than the guyot because the atoll 

 platform is much higher and is unterraced (Emery et ah, 1954). The guyot 

 subsided below the level of reef growth before the atoll developed. How^ever, 

 the oldest fossils on the guyot are pelagic Foraminifera of earliest Eocene age 

 (Hamilton and Rex, 1959), and it is quite improbable that the guyot was an 

 island before Middle Cretaceous times when reef-forming organisms lived in 

 the western central Pacific. The pattern of distribution (Fig. 7) suggests that 

 some general environmental factor may have tended to restrict atoll develop- 

 ment in the Pacific Basin to a narrow latitudinal band. It seems unlikely that 

 temperature was restricting in the warm seas of the time. Current intensity 

 may be a possibility. The mild climate of late Mesozoic-early Cenozoic time may 

 have reduced the intensity of the wind-driven equatorial current system, and 

 the reduced circulation may have decreased the chance that larval forms would 

 be carried to a given island or bank. If the initial development of a reef is 

 precarious, the reef stock may have to be replenished several times, and the in- 

 fluence of current velocity and eddy patterns would be correspondingly 

 important. 



Vulcanism continued through the Tertiary with the development first of 

 Erben Guyot and the guyots of the Nasca Ridge, and later of the existing 

 volcanic archij^elagoes. The number of islands at any given time gradually 

 increased because most islands became atolls and were preserved even though 

 the volcanic platforms continued to subside. Also, as previously noted, there 

 is considerable evidence suggesting that, periodically during the Quaternary, 

 the present-day atolls stood above the sea (some as high islands) for appreciable 

 intervals. The cause of these changes is not known. Correspondingly, the 

 number of guyots formed by subsidence at any time decreased (Fig. 10). 



