20 



THE FLOOR OF THE OCEAN 



basalt or gabbro. A glance at some details of the investigation 

 is warranted. 



From a large number of seismograms Dr. B. Gutenberg and 

 Dr. C. F. Richter measured the velocities of Love or Surface- 

 shear waves with periods of 22 to 25 seconds. 5 The average 

 velocities for different kinds of wave-paths are given in the 

 second vertical column of figures in Table III. The same two 

 seismologists made a similar set of measurements for the 

 pseudo-Rayleigh waves, with periods ranging from 17 to 23 

 seconds. The average velocities for these waves, as they rush 

 along differing kinds of wave-paths are given in the last column 

 of the table, as so many kilometers per second. 



Table III 



AVERAGE VELOCITIES OF TYPES OF SURFACE WAVES 



It will be observed : first, that each type of surface wave runs 

 faster across the ocean-covered sectors of the earth than across 

 the continental sectors; second, that each type runs somewhat 

 more slowly across the southwestern Pacific than across the rest 

 of the Pacific region; and, third, that the pseudo-Rayleigh 



