64 THE OCEAN FLOOR 



degree of plasticity allows the floating blocks of sial 

 a certain freedom of movement in both vertical and 

 horizontal directions (see page 17 above) . Where rocks 

 of deep origin have been thrust above the surface, as in 

 most oceanic islands, the material is generally of the 

 composition ascribed to sima. The very rare fragments 

 of rocks obtained from great ocean depths by means 

 of dredges and trawls have also, in general, this basaltic 

 composition. Such samples, however, are very scarce 

 and therefore vast stretches of the solid ocean floor are 

 not accessible to direct observation. 



Fortunately there are other methods. As a skilled 

 doctor can investigate the health of certain internal 

 organs by the percussion method, listening to the sound 

 reflected from cavities, so the seismologist can utilize 

 the tremors set up in the interior of the earth by earth- 

 quakes, and by means of seismographs study the prop- 

 agation of the seismic waves, even down to the innermost 

 core of the earth. The character of these vibrations from 

 distant epicenters and their rate of propagation in dif- 

 ferent layers of the earth yield information on the 

 structural status and elastic properties of these layers. 



However, as already mentioned, science has not been 

 content with utilizing the natural tremors of earth- 

 quakes. It has also produced tremors artificially through 

 exploding charges, as described above (p. 34). Studies 

 of this kind have been a valuable aid in attempts to 

 locate on the continents underground mineral ores, 

 rock salt, and especially oil — an application recently 



