THE ATLANTIC FLOOR 23 



origin of this topography (Heezen, 1960). The continental displacements which 

 may have created the Atlantic in the Mesozoic and Tertiary may be the result 

 of a process which continually added material along the crest of the Mid- 

 Oceanic Ridge. The mechanism of this continual addition of new crust has 

 been variously ascribed to mantle convection currents which rise beneath 

 the ridge, diverge and carry the oceanic crust toward the continents (Dietz, 

 1961). The sinking of these currents below the continents may, in turn, cause 

 compression and uplift in the continents. However, others prefer to explain 

 the displacement of continents through an overall expansion of the interior 

 of the earth (Heezen, 1960; Wilson, 1960). Needless to say, for students 

 of the Atlantic it makes little difference which of the two hypotheses is 

 favored, for in regard to the Atlantic the effects of either mechanism would 

 be identical. 



It might occur to some that the absence of sediments from the crest of the 

 Mid-Atlantic Ridge could be explained in terms of a recent emergence. It 

 might be argued that the sediments were eroded from the Mid- Atlantic 

 Ridge by subaerial erosion and that the ridge has only recently been sub- 

 merged beneath the ocean. However, if this were true one would expect to 

 find exceptionally thick deposits of sediment on the margins of the Mid- 

 Oceanic Ridge near the former shorelines. The recent data indicate that the 

 entire width of the Mid-Atlantic Ridge from deep basin on one side to deep 

 basin on the other is nearly devoid of sediment. This would require that the 

 entire Mid-Atlantic Ridge some 1200 miles wide be raised 3 or 4 km above 

 the adjacent basins in order to affect denudation and that the products of 

 denudation lie in the basins. However, the pattern of distribution of sediment 

 thickness in the basins does not support this view, for there is a gradual 

 increase in thickness toward the continents and no evidence of thickening 

 along the margins of the Mid-Oceanic Ridge. The discovery of a few fresh- 

 water diatoms in a core from the crest of the equatorial Mid-Oceanic Ridge at 

 one time led certain investigators to propose an emergence of a short duration 

 (Kolbe, 1957). However, it need only be mentioned that, (1) the layer of 

 freshwater diatoms is approximately a millimeter thick interbedded with 

 typical deep-sea sediments, and (2) the winds blowing off Africa often carry 

 such large quantities of diatom tests as to lay down layers of appreciable 

 thickness on the decks of ships. 



We may conclude that land connections across the deep basins of the 

 Atlantic have not existed in the form of sunken continents, isthmian links, or 

 closely-spaced insular "stepping stones". But it is now probable that a dis- 

 placement of Europe and North America has occurred and that at some time 

 in the Paleozoic or Early Mesozoic parts of Europe and America lay adjacent 

 to one another without an intervening ocean. It seems unlikely that once the 

 two continents were displaced from one another that any land connections 

 existed where there is now deep sea. 



