SECT. 1] THE CRUSTAL, ROCKS 101 



its nataire is purely speculative. Its velocity puts it at the upper end of the 

 range for rocks of sedimentary origin ; but if it is sediment, it would probably 

 be a very hard limestone, which seems hardly likely in view of the extraordinary 

 supply of CaCOa required. 



Comparing it with laboratory measurements by Birch (1960), it falls in the 

 range expected for a rock of composition intermediate between acidic granite 

 and ultrabasic dunite. Velocities of 6.7 km/sec are commonly observed in 

 gabbro and rocks of similar composition. Whatever it is, its uniformity through- 

 out the world indicates a similar nature. The variability of the measured 

 velocities seems larger than experimental error and indicates a small-scale 

 variability. It is signiiicant that averages of large regions show such little 

 systematic differences. 



4. Average Oceanic Crust 



In summary, the results of seismic-refraction measurements of the velocity 

 and thickness of the crustal layers beneath the unconsolidated sediment show 

 little significant difference between major oceanic areas. The average picture 

 of the oceanic crust can be represented as an average of all available oceanic 

 stations. The current averages and standard deviations of published values, 

 including all of the data tabulated above, are : 



Velocity, km/sec Thickness, km 



Andreyeva, I. B. and G. B. Udintsev, 1958. The structui-e of the bottom of the Japan Sea 



according to data from investigations by the expedition on the "Vityaz\ Izvest. 



Akad. Nauk S.S.S.K., Ser. GeoL, 10, 3-2o". 

 Birch, F., 1960. The velocity of compressional waves in rocks to 10 kilobars, Part I. 



J. Geophys. Res., 65, 1083-1102. 

 Birch, F., J. F. Schairer and H. C. Spicer, Editors, 1942. Handbook of physical constants. 



Geol. Soc. Amer. Spec. Paper 36, 325 pp. 

 Ewing, J. and M. Ewing, 1959. Seismic refraction measurements in the Atlantic Ocean 



basins in the Mediterranean, on the Mid-Atlantic Ridge, and in the Norwegian Sea. 



Bull. Geol. Soc. Amer., 70, 291-318. 

 Ewing, J. I., C. B. Officer, N. R. Johnson and R. S. Edwards, 1957. Geophysical investi- 

 gations in the eastern Caribbean: Trinidad Shelf, Tobago Trough, Barbados Ridge, 



Atlantic Ocean. Bull. Geol. Soc. Amer., 68, 897-912. 

 Ewing, M. and F. Press, 1955. Geophysical contrasts between continents and ocean basins. 



Geol. Soc. Amer. Spec. Paper 62, 1-6. 

 Ewing, M., G. H. Sutton and C. B. Officer, 1954. Seismic refraction measurements in the 



Atlantic Ocean, Part VI : Typical deep stations, N. America Basin. Bull. Seism. Soc. 



Amer., 44, 21-38. 



