SEDIMENT CARPET 65 



extended to the bottom of shallow coastal seas, where 

 oil is in fact being found. 



The first definite results regarding the propagation 

 of earthquake waves through the upper parts of the 

 continental crust were given by Mohorovic over forty 

 years ago. He found that the longitudinal waves travel 

 at a velocity of 5.6 km sec through the uppermost, so- 

 called granitic, layer, a velocity which increases to 7.8 

 km sec at a depth of about 50 kilometers. More recent 

 studies of waves propagated by great explosions have 

 revealed the rather startling fact that there exists in the 

 earth's crust a low-velocity layer, within which the 

 velocity decreases instead of increasing, as would be 

 expected with increased depth, a curious analogy to 

 the SOFAR layer of minimum velocity found in the sea. 

 The existence of such a layer within the uppermost 

 crust of the earth would, according to Gutenberg, ex- 

 plain certain apparently contradictory results obtained 

 from the study of seismic wave propagation. This view 

 would also lead to a reduced thickness of the uppermost 

 granitic layer: from over 20 kilometers to only half that 

 amount. 



As for the structure of the ocean bottom, it differs 

 materially from that of the continents. But there are also 

 marked differences between the different oceans. Adopt- 

 ing with Gutenberg the following terminology: sialic 

 rocks with velocities of up to 6^4 km sec, simatic rocks 

 with velocities in excess of this limit but less than 8 

 km/sec, and ultra-simatic rocks with between 8 and 



