58 THE FLOOR OF THE OCEAN 



material has been poured out on a scale not rivalled by any 

 other of the hundreds of known chemical species of lava. Its 

 true liquidity during actual eruption is illustrated by the 

 photograph of Figure 29. 



Moreover, the eruption of the non-basaltic kinds of lava has 

 been preceded, in many regions, by flows of basalt; and, as 

 already remarked, it appears from observations in field and 

 laboratory that the non-basaltic species are themselves deriva- 

 tives of liquid basalt or of liquid basalt which had been con- 

 taminated with material from the solid rocks through which 

 the molten basalt was forced during its upward journey. 



The great individual volumes and enormous total bulk of 

 the visible basaltic piles, the high temperature of their flows, 

 the world-wide distribution of basaltic eruptives, and the ge- 

 netic relation of basalt to other types of volcanic rock — all these 

 are interrelated problems. All seem to find solution if we 

 assume that both the material and the elevated temperature of 

 the basaltic flows have come, in general, from a subcrustal layer 

 which is too hot to crystallize. 



Until a comparatively recent geological epoch this basaltic 

 layer seems to have been continuous all around the globe, thus 

 forming a complete earth-shell. See Figure 30. Judging from 

 new observations on the elasticity of basaltic glass at high 

 pressures and temperatures, it appears probable that glassy 

 basalt cannot now be world-wide and continuous: that here 

 and there, within broad segments, the ancient, energy-charged 

 layer has crystallized because of the slow cooling of our planet. 



The velocities of the earthquake waves at depth suggest that 

 at most only a few miles below the crystallized basalt of the 

 crust there now exists a thick, world-circling layer of vitreous, 

 non-crystallized, material which is rich in the constituents of 

 the mineral olivine or peridot, and is therefore named peri- 

 dotite. 



