SECT. 2] ABYSSAIi PLAINS 357 



type which was then interpreted as evidence of thin sediment". Thus, since 

 the sediments were assumed to be very thin, the smoothing of the topography 

 by the shifting of sediment seemed impossible, and even now no one would 

 seriously consider the possibility that deep-ocean currents could effectively 

 erode the crystalline rock to plain flatness. Thus, the theory was based solely 

 on the lack of a leveling process, it being assumed that the smoothness of the 

 topography was simply indicative of the lack of tectonic activity. The early 

 seismic-reflection records from the abyssal plains were satisfactorily reinter- 

 preted (Ericson, Ewing and Heezen, 1952) as reflections from a thinly bedded 

 sequence in which alternate beds diff"er widely in lithology. Recent seismic- 

 profiler records reveal abyssal-hill type topography buried beneath |-2 km 

 of sediment. The fact remains that diastrophism has not been nearly as active 

 in the abyssal floor as in the ridges and rises ; for a mere ^-2 km of sediment 

 (Ewing, Sutton and Oflicer, 1954) has been sufficient to bury all minor topo- 

 graphic features in the abyssal plains. It is clear that a similar small thickness 

 of sediment would be adequate to smooth many of the abyssal-hill areas. 

 Tectonic activity in abyssal-hill areas has probably been no more intense than 

 in the abyssal plains. 



B. Mylonitic Plains 



In direct contrast to the atectonic hypothesis, the suggestion has been made 

 that abyssal plains represent areas of high and complex tectonic activity. 

 According to this hypothesis, abyssal plains represent the nearly level surface 

 of a great bed of mylonite, the rocks having been so completely sheared that 

 nothing remains but a veritable sea of mylonite. This has been suggested 

 apparently through a misunderstanding of the characteristics of abyssal plain 

 deep-sea sands. Some of the first sands discovered in the deep sea were obtained 

 from the Romanche Trench, an exceptionally deep and precipitous fracture 

 zone which offsets the mid-oceanic ridge and rift in the equatorial Atlantic. 

 These cores, obtained from depths of 3000-4000 fm, contained beds of sand. The 

 sand grains were composed of minerals characteristic of basic and ultra-basic 

 rocks (Mellis, 1958). The characteristics of the deep-sea sands occurring on 

 abyssal plains contrast sharply with the sands of the Romanche Trench, and the 

 only thing the two types of sand have in common is that they lie in a great 

 depth of water. The sands of the Romanche Trench are mylonitic in character 

 or, perhaps, are weathering products from the erosion of the Mid-Atlantic 

 Ridge. However, they do not lie in an abyssal plain and are so vastly diff'erent 

 from abyssal-plain sands as to have no real bearing on the origin of the normal 

 abyssal-plain deep-sea sand. 



Pettersson (1954), in an attempt to explain the occurrence on the equatorial 

 Atlantic abyssal plains of graded deep-sea sands that contained "twigs, nuts 

 and the bark of a dicotyledonous tree", offered the following explanation: 

 "In the event that a large island harboring vegetation and with a fairly ex- 

 tensive shelf covered the Mid-Atlantic Ridge northwest of St. Raid's Rocks 



