406 



HKKZKN ASl) K\\ 1N(.; 



(MAI- 



lt> 



along the traverse, form a thin and markedly discontinuous deposit. The 

 pockety character of this de]iosit has been taken to indicate very free gravity 

 transport of newly deposited sediments. It has been noted that the rough 

 surface on which these sediments rest appears to form a continuous unit from 

 the ridge crest to a point well into the basins. 



The deeper interfaces depend upon seismic-refraction measurements at 

 scattered points. Each such measurement gives average layer thickness over 

 a segment tens of miles in length. Hence, the shapes of the deeper interfaces 

 are highly generalized. The shape of the boundary between the 5.0 and the 7.3- 

 7.5 km/sec layers is particularly uncertain, owing to the difficulty of making 

 accurate refraction measurements in mountainous regions. The lack of relief on 

 it really indicates a lack of information. 



100 nautical miles 



H H 



Velocities in km/sec 



Fig. 14. A profile across the Mid- Atlantic Ridge based on seismic-refraction and reflection 

 measurements. The unconsolidated sediments of the intermontane basins and abyssal 

 plains rest on the rugged 5.0 km/sec layer. The 8.1 and 6.7 km/sec layers are standard 

 upper mantle and "oceanic" crustal layers. The 7.3-7.5 km/sec is characteristic of 

 mid-oceanic ridges throughout the world. Ewing, Ewing and Talwani have con- 

 cluded that the 5.0 km/sec layer is "primitive" igneous, largely because of the 

 intensely rough surface. (After Ewing and Ewing, 1959; Ewing, Ewing and Talwani, 

 196.3.) 



Identification of the rock types in Fig. 14 depends principally on the seismic- 

 refraction velocities shown and on rocks dredged from a few places nearby. 

 Shand (1949) described the dredge hauls of Atlantis Cruise 150 as olivine gabbro, 

 serpentine, basalt and diabase. Some of them have been dated by the K-Ar 

 method (Erickson, in litt.) at a few tens of million years. It is possible that the 

 basalts consist largely of young volcanics, not identifiable with any of the 

 seismic layers. 



The 8.1 and 6.7 km/sec layers are standard upper mantle and "oceanic" 

 crustal layers. The identity (and even the continuity) of the 5.0 km/sec layer 

 are still debated, owing to the fact that many types of rock are characterized 

 by this velocity; but Ewing, Ewing and Talwani {loc. cit.) have concluded that 

 it is "primitive" igneous, largely on the basis of the rough upper surface. 



The 7.3-7.5 km/sec layer is apparently characteristic of mid-oceanic ridges 

 throughout the world (Raitt, 1956; Shor, 1959; Menard, 1960). 



