MODERN SEISMOLOGY — HODGSON" 357 



recent book by Gutenberg (pp. 32-80 of reference 13) outlines the 

 work of recent writers on this subject. There is no other such well- 

 marked sudden change of velocity with increase in depths as that 

 which occurs at the entrance to the core. The other changes are less 

 abrupt and are, in general, increases in velocity. The other dis- 

 continuities are thus not so well established, in fact or in position, as 

 is that at about half way down the earth's radius. 



Ascending from the 2,900 km level we traverse, in turn, three 

 layers of slightly different properties, the two discontinuites sep- 

 arating them being so ill defined that we are not sure where — or 

 even whether — they are. The discontinuity which surrounds the 

 triple layer is at a depth of 1,200 km (750 miles). The material 

 composing these three layers is supj^osed to be silicon impregnated 

 with iron. The iron content is supposed to increase for points suc- 

 cessively nearer the core, and to be very small at the outer boundary 

 at the 1,200 km discontinuity. The three ill-defined layers, taken 

 together, constitute what is known as the transition layer. The 

 transition layer and the core, taken together, are sometimes known 

 as the " nif e " (ni=nickel: fe=ferrum=iron). 



The next discontinuity is much better marked; its existence is 

 certain; there is some uncertainty as to its position. It may chance 

 to be different in different parts of the world. Much remains to 

 be done in its investigation. The break is usually held to be at a 

 depth of 60 km (37 miles). It marks the upper boundary of the 

 shell of silica and magnesium usually designated as the " sima " (the 

 name indicating the constituents). The increase in velocity with 

 depth within this layer is so uniform that it is not believed to suffer 

 any internal discontinuities. The density inevitably increases some- 

 what with depth due to the superimposed weight ; the elasticity must 

 thus gradually increase downward at a fairly uniform rate in the 

 sima, and at a less regular rate in the transitional layer of the nife 

 until finally we get the great reversal, the fall in velocity, at the 

 central core. Let us come back toward the surface and nearer home. 

 What is the constitution of the upper GO km (37 miles) of the 

 earth's crust? 



Jeffreys ^^ believes that there are three layers, separated by dis- 

 continuities at depths of 12 km (7.5 miles) and 37 km (23 miles). 

 These he terms, in order descending, the granitic layer, the basaltic 

 layer, and the ultrabasic layer, thus indicating the probable nature 

 of their constituent rocks. The three taken together are known as 

 the " sial " (si = silicon; al=aluminum). 



i» Jeffroys, Harold, On noar oarthquakos, Monthly Not. Roy. Astroii. See., Goophys. 

 Suppl., vol. 1, No. 8, pp. 385-402, December, 1926. 



