CONTINENTAL TERRACES AND SUBMARINE VALLEYS I35 



tidal wave, an earthquake wave in the sea is compounded of 

 alternating, forward and backward motions of the water. The 

 displacement of water in both onset and reflux has a maximum 

 velocity increasing with the height of the wave. Hence a first- 

 class seismic wave, with several times the height of even great 

 storm waves, is characterized by to-and-fro currents of extraor- 

 dinary power. Professor Bucher supposed that the reflux cur- 

 rents so generated run along the continental slopes, with 

 sufficient friction along the sea bottom to tear up the clay and 

 sand and ultimately excavate canyon and furrow. But here too 

 there are troubles. 



That part of the energy that belongs to the reflux current 

 is largely concentrated near the surface of the ocean, the veloc- 

 ity of the motion decreasing with great rapidity as the depth 

 of water increases. The laws of hydraulics demand that even 

 the mightiest seismic wave cannot give a reflux current rapid 

 enough to erode the lower half of the continental slope, down 

 which, nevertheless, the submarine valleys continue for many 

 miles. 



And for another reason the ultimate effectiveness of the 

 reflux may be questioned. The earthquake waves run in packs 

 or so-called trains, but any such train of importance attacks 

 a coast for only an hour or so, and that only once in a 

 stretch of time measurable in decades or centuries. In fact, no 

 major earthquake wave has ever been recorded in the North 

 Atlantic, where submarine valleys are in full development. 

 The possibility that during pre-historic time this ocean was 

 long shocked much more vigorously than at present is empha- 

 sized by Bucher, but only in a purely speculative way and 

 without proof. Similarly there is no direct evidence that the 

 reflux currents have been powerful and numerous enough to 

 account for the valleys sunk in the continental terraces of the 

 Indian Ocean. The western Pacific is the home of seismic 



