110 THE FLOOR OF THE OCEAN 



Fort Monroe to about 12,000 feet at the edge of the flat, sub- 

 merged shelf. 



In principle the reflection method corroborated the con- 

 clusions won by the refraction method, regarding thicknesses 

 for the unconsolidated and semi-consolidated or more com- 

 pacted piles of sediment, and also showed that the lower, 

 thicker pile is interrupted by many thin, presumably more 

 consolidated layers or lenses. These minor discontinuities are 

 represented at their appropriate depths by the short horizontal 

 lines of the diagram. 



Below the composite mass of sediments is "crystalline rock" 

 — hard rock — carrying elastic waves at a speed much greater 

 than that found in the overlying detrital mass. 



The shorter Woods Hole section, shown in the upper cross- 

 section of Figure 59, gave a thickness of about 4000 feet for the 

 terrace sediments, where measured at the outermost recording 

 point, which was 60 miles from the beach and 50 miles inside 

 the top contour of the continental slope. 



With the seismic method, Bullard and Gaskell deduced the 

 thickness of the "relatively unconsolidated" sediments consti- 

 tuting the European continental terrace off the British Isles. 

 Twenty miles and eighty miles off the Lizard they found re- 

 spective thicknesses of 500 feet and 1000 feet; 115 and 175 miles 

 west-southwest of the Lizard, respective thicknesses of at least 

 2000 feet and at least 4000 feet (probably a considerable under- 

 estimate). 



The technical difficulties of a research of this kind are nota- 

 ble, and there is need of further testing of the seismic methods 

 as well as occupation of many other cross-sections of the conti- 

 nental terraces. But already we can be reasonably sure that the 

 soft sediments of the terraces do greatly thicken out to, or 

 nearly to, the seaward limit of the continental shelf. In accord 

 with this conclusion is the proof by actual borings along the 



