l8 THE FLOOR OF THE OCEAN 



properties appropriate to rock denser than granite and less 

 dense than solidified basalt, the most fashionable of the world's 

 lavas. A third layer, beginning at the depth of about 25 miles 

 and, according to one estimate, 20 miles or thereabouts in 

 thickness, has wave-velocities and elastic properties appropriate 

 to basalt itself, when solidified under the conditions of high 

 pressure at the depth described. The technical name for thor- 

 oughly crystallized, deep-seated basalt is "gabbro," and the 

 third layer may be called the "gabbroic" layer. The second 

 layer, more elastic than granite and less elastic than the gab- 

 broic layer, is generally called the "intermediate" layer of the 

 continental crust. 



It is convenient to add to our vocabulary two other technical 

 words. Rocks dominating in the granitic and intermediate 

 layers would yield the chemist relatively large proportions of 

 two oxides, silica and alumina. Hence for such material has 

 come the mnemonic name "sial," with adjectival form "sialic." 

 On the other hand, basalt, whether molten or crystallized, and 

 the gabbroic layer as a whole, and also material still deeper in 

 the earth are all rich in silica and magnesia; all are grouped 

 together as the "sima" of the earth, with adjectival form 

 simatic. 



Proof of the layering under the continents has been made 

 possible by the establishment of seismographic stations at many 

 points on the land. As yet a similar adequate network of sta- 

 tions has not been placed in any oceanic area. Particularly 

 valuable would be chains of stations, located on rows of islands 

 like the Hawaiian group and so strung out for a distance of at 

 least a thousand miles (Figure 8). Without such establishment 

 a satisfying diagnosis of the sea floor through study of the Push 

 and Shake waves is impossible. Yet it is significant that a single 

 station, situated at Apia on Upolu Island of Samoa (Figure 

 24), has already furnished a clue. There Dr. G. Angenheister 



