SECT. 1] 



THE MANTLE ROCKS 



earthquake-wave observations. As mentioned earlier, these tend to average 

 the velocities over long paths and would most likely give the result as 8.1 

 km/sec. 



The velocities plotted in Fig. 1 are from profiles which were selected only 

 insofar as none was used unless a crustal velocity was measured in the same 

 profile. Fig. 2 shows similar plots for Atlantic and Pacific profiles which were 

 selected as probably unaffected by proximity to prominent topographic 

 features such as oceanic ridges, trenches, large seamounts and continental 

 margins. The princi]3al difference between the two oceans shown by these 

 plots is that none of the Pacific profiles shows a velocity below 8.0 km/sec. 

 These data are from Raitt (1956), and of 31 determinations of sub-crustal 

 velocity he found only four to be below 8 km/sec. These four were from profiles 



10 



7.5 8.0 8.5 7.5 8.0 8.5 



COMPRESSIONAL WAVE VELOCITY (KM/SEC) 



Fig. 2. Comparision of sub-crustal velocities versus number of observations for Atlantic 

 and Pacific Oceans. 



near Hawaii, Tonga and on the Easter Island Rise. These have been omitted 

 in the graph. In the Atlantic, there are several ocean-basin profiles in which 

 the sub-crustal velocity is lower than 8 km/sec. In addition to those plotted, 

 there are a few low values in the eastern basin which were omitted because of 

 obvious complication by large-scale topography. With the exception of two 

 values wliich came from weakly shot profiles in the Nares Basin, the low 

 velocities plotted were measured either on the Bermuda Rise, within 200 miles 

 of the islands, or near the continental rise of eastern North America. Thus it is 

 a fair statement that the open basin areas of the Atlantic are not different 

 from those in the Pacific. Being much smaller, the Atlantic obviously has a 

 noticeably higher percentage of its area infiuenced by large-scale topographic 

 features. In these areas where low sub-crustal velocities are found, it is most 

 probable that longer profiles would show an increase of velocity with depth, 

 reaching a value of 8 km/sec or higher at a depth not far below the bottom of 

 the crustal layer. Structure of this type has been measured by profiles in the 

 Venezuelan Basin of the Caribbean Sea (Officer et al., 1959). 



3. Shear-Wave Measurements 



Measurements of shear-wave velocity in the mantle by seismic-refraction 

 profiles have been reported for profile 26 (1956 cruise) in the Caribbean Sea by 



