66 THE OCEAN FLOOR 



8^/i km/sec, we may locate the Mohorovicic Discon- 

 tinuity at the upper boundary of the last-mentioned 

 rocks. 



One then finds an area in the eastern part of the 

 Pacific Basin where the ultra-simatic layer begins at 

 a depth of only a few kilometers, whereas in the north- 

 west Atlantic Ocean material with a velocity of IV2 

 km/sec seems to exist at the corresponding depth. In 

 the eastern Atlantic Basin the velocities found by Hill, 

 working from a weather ship, may even indicate the 

 presence there of a sialic layer beneath the sediment. 



There are, if we return to the Pacific Ocean, clear 

 indications of a boundary between the deep ocean basin 

 and the continents. On the ocean side of this boundary 

 one finds great deeps, negative gravity anomalies, and 

 shallow earthquake centers. Nearer land comes a 

 narrow belt of earthquake centers, starting from depths 

 of 70 to 150 kilometers, and fines of active volcanoes. 

 Still farther landward is the region from which emanate 

 the shocks of very deep earthquakes, down to 700 

 kilometers. Within the Pacific Ocean proper, with the 

 exception of the Hawaiian area, no earthquakes are 

 known to have occurred. 



No similar earthquake belts surround the whole or 

 part of the Atlantic or Indian Oceans. In both, active 

 belts of shallow earthquakes follow submarine ridges 

 within the oceans, similarly to continental earthquake 

 epicenters along mountain ranges. Gutenberg, from 

 whose recent paper on crustal layers of the continents 



