:}82 MENATtn ANT) T-ADD [f'HAP. 1 .'» 



assumption for tlie Tuaniotu and Nasca Ridge guyots because the dated 

 material was deposited in sliallow water and the side slopes do not indicate a 

 thick reef capping. With regard to the other localities, the coating of pelagic 

 material on a seamount is patchy and extremely thin and material of various 

 ages may be readily available to the dredge or corer. 



The second assumption seems reasonable considering the duration of vul- 

 canism in a given province on land which is roughly 10-50 million years. There 

 can be no doubt that vulcanism of oceanic-island groups persists or is inter- 

 mittent during similar periods because of the mixture of deep guyots and 

 islands or relatively shoal untruncated seamounts in some regions. 



Sediment cores from ocean basins reveal widespread Cretaceous and Tertiary 

 outcrops but nothing older. It is possible that older sediments have been 

 buried by younger ones, however, and no general conclusions can be drawn 

 from the cores alone. Atoll borings to basalt and seamount dredgings are more 

 significant because the oldest sediments are available to the sampling device. 

 The probability of sampling material of any given age, therefore, is proportional 

 to the frequency of such material. 



It may be important that no dredged seamounts, atolls or guyots have 

 sediments older than Cretqjpeous on them. The number of samples is small but 

 so is the number of atoll and guyot groups. All three large guyot groups in the 

 North Atlantic have been sampled. Likewise, in the Pacific, four of the five 

 large guyot groujis and two large atoll groups have been sampled. In addition, 

 perhaps a score of isolated seamounts and guyots has been dredged in widely 

 separated parts of the ocean basins. Granting the stated assumptions, it appears 

 probable that oceanic volcanoes large enough to form islands are almost all of 

 Mesozoic or younger age. The myriad of small seamounts has not been 

 adequately dredged and their age is unknown. However, either all oceanic 

 volcanoes are post-Paleozoic or else the maximum size of the volcanoes increased 

 drastically in Mesozoic times. 



The number and distribution of oceanic islands in geological time may be of 

 interest because of their bearing on faunal and floral migration. Little can be 

 said about the Atlantic or Indian Ocean but the rough outline of faunal migra- 

 tion paths can be sketched for the Pacific. In pre-Mesozoic times islands may 

 not have existed in the Pacific Basin and could, therefore, have no effect on 

 biological migration. However, seamounts may have been present at depths as 

 shallow as a few hundred meters and they might have provided stepping- 

 stones for suitably adapted biota. 



In Mesozoic times a large group of active volcanoes existed in a roughly 

 east-west band along the Marcus-Necker Rise. By Middle Cretaceous times 

 many of the volcanoes in the eastern, or Mid-Pacific Mountains, region of the 

 rise had growTi u]) through water 4 km deep and formed large islands which 

 were being i)laned down to sea-level by stream and wave erosion. Vulcanism 

 and island formation continued for an indefinite time but the region gradually 

 became submerged, and higher and higher submarine volcanoes were unable to 

 reach the sea surface. Under the most favorable ciicumstances some 70 islands 



