18 



The Evolution of the Universe 



ocean; if the earth's surface were completely levelled, it would all 

 be covered by oceans several thousand feet deep. 



The specific gravity of the whole earth is about 5.5 but that of 

 its outer crust is only 2.7, indicating that it becomes more dense with 

 depth. The exact internal composition of the earth is not known, 

 but the study of both shear and compressional waves associated 

 with earthquakes, the density of the earth, and the composition of 

 the crustal rocks has led to several generally accepted interpretations 

 as to the internal structure (Fig. 6), The core is a fluid sphere, pos- 

 sibly composed of nickel-iron (Urey, 1952) and having a diameter 

 of about 2,500 km. Extending about 2,900 km above the core, is the 



-8-40 km —Crust 



2500 km 



2900 km y Montle 



Core, probobly 

 ^ iron-nickel 



Center of the earth 



(a) 



Continents. 



Siol, 

 up to 30 km 



Simo 

 5-10 km I 



Seo level 



yx - ww^ \ Oceo" ^loor (simo) 

 Sliaii^^^^^) 5-IOkm 



Mohorovicic discontinuity 



(b) 



Fig. 6. Diagram of the supposed cross-section of the earth, (a) A quadrant 

 of the earth, diagrammatic, (b) The earth's crust, diagrammatic. 



