22 BRUCE C. HEEZEN AND MARIE THARP 



of the Mid-Oceanic Ridge. The same velocity material has been found 

 beneath the crest of the Mid-Atlantic Ridge and over much of the floor of the 

 Norwegian Sea and the Labrador Sea (Ewing and Ewing, 1959). 



If a continent subsided to oceanic depths (greater than 4000 m), one 

 would expect the crustal structure to be markedly different in the area formerly 

 occupied by the continent. In addition, in order to maintain isostatic balance, 

 a yet unknown process, whereby a thick section of continental-type crustal 

 rocks is transfornied into thin section oceanic crustal rocks, is required. Thus, 

 in view of these difficulties, it seems necessary to limit any consideration of land 

 connections across the deep sea to "island stepping stones" and to continental 

 displacements. 



Flat-topped seamounts of the central Pacific were islands in the Cretaceous 

 (Hamilton, 1956). Thus numerous "stepping stones" did exist across many 

 parts of the Pacific in the Cretaceous. These flat-topped seamounts (guyots) 

 are less plentiful in the Atlantic. Botanists in particular have serious reserva- 

 tions about a "stepping stone" type of land bridge, for they maintain that 

 small islands would not develop continental-type vegetation and therefore 

 would not act as a bridge. 



The continental displacement hypothesis, once rejected for lack of a 

 mechanism, has received new support in the last decade from the results of 

 the studies of paleomagnetism. The gradual opening of the Atlantic by 2000 

 miles in the Mesozoic and Early Tertiary is supported by a consistent dis- 

 crepancy between paleomagnetic measurements made in North America and 

 Europe (Runcorn, 1959). The fact that no sediments older than Cretaceous 

 have been found on the Atlantic deep-sea floor is not in opposition to this 

 view, although it might be argued that the few hundred outcrops of Tertiary 

 and Cretaceous sediment constitute too small a sample upon which to base 

 such an important generalization (Ericson et aL, 1961). Recently, detailed 

 studies of the thickness of the sedimentary layer which lies above the oceanic 

 crust have been made by the seismic-reflection technique (Ewing, 1961). In 

 general it is found that there is an exceptionally homogeneous 1-2 km layer 

 which overlies a layer of similar thickness which in turn rests on an intensely 

 irregular sub-bottom topography. The material lying beneath the second 

 sedimentary layer is presumably one which refraction measurements have 

 indicated to have a crustal velocity of 4.5 to 5.5 km/sec. Both sedimentary 

 layers thicken markedly upon approaching the continents, apparently in 

 response to greater rates of sedimentation. 



When approaching the Mid-Oceanic Ridge, the thickness of sediment thins 

 markedly. On the flanks of the Mid-Atlantic Ridge the irregular deeper layer 

 apparently reaches the surface in many areas, sediment being restricted to the 

 bottoms of occasional intermontane basins. Near the crest of the Mid- 

 Oceanic Ridge sediment is virtually absent. The absence of sediment from the 

 crest of the Mid-Oceanic Ridge has been taken as evidence of the recent 



