SUBMARINE MOUNTAINS 



59 



FIGURE 30. DIAGRAMMATIC SECTION ILLUSTRATING INFERRED CHARACTER OF 

 THE EARTH'S OUTER SHELLS AT AN OLDER GEOLOGICAL EPOCH, WHEN THE 

 CRUST WAS THINNER THAN NOW AND THE VITREOUS BASALT WAS WORLD- 

 CIRCLING. 



Because the temperature of the vitreous rock approaches or 

 actually reaches that of white heat, the lowest of the earth- 

 shells described is extremely weak; the great hydrostatic pres- 

 sure on it keeps it highly viscous, but its material will flow if 

 subjected to small unbalanced pressure. 



In summarizing this story of the underground we recall 

 once more the two technical nouns, "sial" and "sima," with 

 corresponding adjectives, "sialic" and "simatic." The oceanic 

 sectors of the earth are characteristically simatic; that is, their 

 accessible rocks are characteristically rich in the metals silicon 

 and magnesium. Under the ooze and mud of the sea bottom 

 there begins a true crust of frozen, crystallized, basalt. Also 

 simatic is any vitreous basalt which may form "pockets" in the 

 crust. Simatic, too, is the still deeper peridotite. The conti- 

 nental sectors differ by including two sub-layers, both of which 

 are "sialic," that is, relatively rich in the metals silicon and 



