326 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
nearly 3,200 miles, and the inner core occupying the remaining 800 
miles to the earth’s center. As will be shown later it is probable that 
the inner core is solid. In between these two parts of the central 
core there may exist a transition region about 100 miles thick. Jeffreys 
has found evidence of such a region, though not all seismologists as 
yet support the finding. It is difficult at the present stage to draw 
sharp conclusions on this part of the earth, and more evidence is 
desirable. 
The following table summarizes in broad terms the division of the 
earth into regions according to the evidence from seismology. 
TABLE 1.—Division of the earth into regions according to the evidence from seismology 
Region Range of depth Description 
A 0-10 miles (oceanic) _____ The earth’s ocean and “outer layers.’’ 
Tpke Tal 0-25 miles (continental) -_- Very variable in composition. 
By Ge2t 28 25-700 miles+ Us! 2232 The upper mantle, in which there is 
some variation of composition not yet 
precisely located. 
The lower mantle; D’ appears to be 
|) Liaise aN OO—t 00 imilegias hee ae nearly uniform in composition; in 
Oe Ss 2/01; F001 S00mmiles se Fat D’’ there is probably some accumula- 
tion of somewhat denser material. 
BAe See 1,800-3,100 miles_____--- The outer core. Fluid, and probably 
uniform in composition. 
12 pees eee 3,100-3,200 miles_______- Somewhat uncertain transition region 
between outer and inner core. 
(Ga kee bs Sea 3,200-3,960 miles____-_-- The inner core, probably solid. 
DENSITY, PRESSURE, GRAVITY, AND ELASTICITY IN THE EARTH’S 
DEEP INTERIOR 
Toa good approximation, the velocities of seismic waves at any point 
inside the earth depend on just three properties, namely the incom- 
pressibility %, the rigidity », and the density p. The previous section 
shows that values of the P and S velocities are known down to the base 
of the layer E. (Inside EH, the S velocity is taken to be zero because 
of the evidence for fluidity.) This knowledge supplies two equations 
connecting the three items /, », and p. If only a third independent 
source of information were available, it would be a matter of simple 
algebra to calculate the values of all three of /, », and p down to a 
depth of more than 3,000 miles, and so obtain good knowledge of the 
variation of density and elasticity in the earth. 
In practice, no such third source of information is directly avail- 
able, and the problem has to be attacked indirectly. Use is made first 
of the fact that in parts of the earth where the composition is uniform, 
