SECT. 1] 



THE CRUSTAL ROCKS 



91 



oceanic rises such as the Mid-Atlantic Ridge or Easter Island Rise known to be 

 anomalous regions (Ewing and Ewing, 1959; Raitt, 1956). 



As was indicated above, present results are but a fragment of a continuing 

 investigation. We can exj)ect our conclusions to be modified, or perhaps even 

 changed radically, as new results are unfolded. However, present world-wide 

 results as summarized in the tables support a picture of oceanic structure that 

 has not changed greatly in the last few years. The summary of results presented 

 by Hill in 1957 describes the average oceanic structure about as well as today. 



24 

 20 



'y7//////////W//^^y'WM/'/y' 



Direct wave 



•'^.I6±0.03 



7.0310.06 



..^ 7.I7±0.I4 



4.8I±0.II S. ^5.09 + 0.05 



I I I \ I L 



a. 

 a> 



E 

 o 



o 

 00 



80 60 40 20 20 40 60 80 100 120 

 Distance (km) 



Fig. 8. Travel-time plot of Capricorn Station C2. Layer 2 velocity = 4.98 km/sec; Layer 3 

 velocity = 7.10 km/sec; Layer 4 velocity = 8.16 km/sec. (After Raitt, 1956.) 



2. Layer 2 



Surprisingly, the layer found at the base of the unconsolidated sediments, 

 called Layer 2 for lack of a more descriptive name, has proved to be more 

 difficult to measure than the deeper Layer 3 forming the main constituent of 

 the crust. The reasons lie mostly in the short distance over which it is observed 

 as a first arrival. Furthermore, this distance is sensitive to variation in the 

 thicknesses of Layer 1 and Layer 2. If Layer 1 were thicker than in the example 

 of Fig. 1 or if Layer 2 were thinner, the distance over which Layer 2 brings in 

 a first arrival would be shortened, and it would not require a drastic change to 

 eliminate Layer 2 entirely as a first arrival. It is occasionally observed as a 

 second arrival but interference with the refracted wave of Layer 3 usually 

 makes this observation unreliable. 



