106 THE FLOOR OF THE OCEAN 



Atlantic; the inbound part of the eddy just to the south tends 

 to drag the sand to the westward. This conflict of currents 

 accounts for the asymmetry of the cusps. Possibly Cape Ro- 

 main may yet be ornamented by a similar cusp. The seaward 

 bends of the ten-fathom line on the map show that, as expected, 

 the outgrowth of the sedimentary terrace is most rapid opposite 

 the cusps. 



Condition of the Terrace Sediments. — The thickness of 

 gravel, sand, and mud composing the continental terraces is 

 important in connection with the origin of the valley systems 

 that fret the outer slopes. It is known that the cover of detritus 

 on the hard-rock floor is highly variable. Wide areas of the 

 shallow shelf off California are practically free from any 

 detrital cover. In general, however, the shelves are surfaced by 

 unconsolidated sediment almost continuously from the beach 

 to the submarine contour at depth of 2000 fathoms. The detrital 

 cover tends regularly to grow thicker in the seaward direction, 

 out to where the flat shelf meets the top of the continental 

 slope. Measurement of thickness for the terrace deposits is now 

 possible through the use of specially designed seismographs 

 and other instruments. 



Pioneer studies of the kind have been made by M. Ewing, 

 A. P. Crary, and H. M. Rutherford, 3 and by E. C. Bullard and 

 T. F. Gaskell. 4 They used the seismic method, which has some 

 analogy with the X-ray technique of surgeon and physician, 

 who also need to know the shapes of structures hidden from 

 human eyes. In our present problem the elastic waves regis- 

 tered by the seismograph are generated by explosive charges 

 placed on the sea floor at known points of latitude and longi- 

 tude. Each explosion sends sound waves through both sedi- 

 ment and hard-rock floor. The times of arrival of these waves 

 at strung-out observing stations are accurately recorded by 

 sensitive instruments called geophones, also placed on the sea 



