SECT. 1] 



THE CRUSTAL KOCKS 



Table IV 

 Western North Pacific 



97 



** Anomalous value omitted from averages and standard deviations. 



5.09 km/sec observed elsewhere on the "Mid Pacific" and "Capricorn" expedi- 

 tions (Raitt, 1956). Similar observations were made by Gaskell, Hill and 

 Swallow (1958) in the Philippine Sea. 



The observed range of velocities belongs to a wide variety of rock types, 

 and is of little help in identifying the nature of the material (Birch, Schairer 

 and Spicer, 1942 ; Birch, 1960). Rocks of nearly all types have velocities in this 

 range. There are many lithified sedimentary types and metamorphic rocks of 

 sedimentary origin. Also these velocities are characteristic of basaltic lavas, 

 as indicated by laboratory experiments (Hill, 1957) and seismic-refraction 

 measurements of the volcanic core of Eniwetok and Bikini Atolls (Raitt, 1954, 

 1957). Most phitonic rocks, especially firm specimens of dense basic rocks such 

 as gabbro, diabase, norite and dunite, have velocities well above the range of 

 Layer 2. However, many granites and granodiorites have compressional wave 

 velocities of about 5 km/sec (Birch, 1960), a characteristic Layer 2 velocity. 



