io8 



THE FLOOR OF THE OCEAN 



upward and have its arrival time clocked at the recording 

 point. Multiple refractions characterize the more complicated 

 paths of the third and fourth layers. With suitable precautions 

 the wave-velocities in all four layers can be computed with 

 considerable accuracy. From these velocities, and from inde- 

 pendent experiments giving the relation of wave-velocity to 

 the nature of the rocky materials of all the common sorts, it 

 becomes possible to name the respective kinds of rock repre- 

 sented in layers one to four. 



The Ewing party used the reflection method and the refrac- 

 tion method along two lines running at right angles to the 

 shore of the eastern United States. One of these sections was 

 lined out from Cape Henry, Virginia; the other, from Woods 

 Hole, Massachusetts. See Figure 58. 



FIGURE 58. MAP SHOWING LOCATION OF EWINg's SOUTHERN SECTION ACROSS 

 THE CONTINENTAL SHELF, EASTERN UNITED STATES. 



The coastal plain of Virginia represents an emerged part of 

 the continental terrace, a part where the results of seismological 

 location of hidden discontinuities can be checked by the logs 

 of deep wells. Partly for this reason the Virginia section was 

 begun at a point 75 miles inland from the shore at Fort Mon- 

 roe. From there the line was run straight out 90 more miles. 



