Groundwork for Study of Megadiastrophism. 269 



going dynamic metamorphism is probably in constant 

 progress in the earth. This, however, does not seem to 

 be adequate to relieve all increases of stress, and so 

 accumulations arise and bring on special stages of dias- 

 trophism of a more vigorous type. 



The state of the earth nucleus. — So far, to avoid com- 

 plication, the state of the earth's nucleus has been neg- 

 lected. As already stated in a quotation from my last 

 paper, 13 it is necessary to correct the velocities of the 

 seismic waves for density to give the full state of rigidity 

 and elasticity. If such correction is applied to the best 

 data now available as to the velocities of the seismic 

 waves, it shows a rise of elastic-rigidity as far toward the 

 center as good records go. The necessity for this correc- 

 tion seems to have been overlooked by those who have 

 inferred the existence of a state of non-rigidity near the 

 center. If question is raised about the trustworthiness 

 of the correction, it may be recalled that the mean density 

 is much higher than the surface density and that the pre- 

 cession of the equinoxes implies higher density in the 

 deep interior. To satisfy these approximately, correc- 

 tion may be made by use of the classic law of density of 

 Laplace, or still better, it may be made by means of the 

 formula of Roche which was devised especially to give a 

 distribution of internal density in as close accord with the 

 astronomical requirements as possible. 



The dying out .of the distortional waves when the 

 chords get so long as to cut the radius about half way 

 from the surface to the center has been much stressed 

 recently as though it were evidence that rigidity ceases at 

 about that depth, but the ways in which both transverse 

 and longitudinal waves become feeble and die out as the 

 chords lengthen, puts another aspect on the matter. 

 They must die out sooner or later in the nature of the 

 case. It is remarkable that they are able to traverse as 

 long chords as they do. The fact that both kinds of 

 waves die out in traversing chords much short of the 

 diameter of the earth seems to point to their natural 

 death by progressive exhaustion. The brachistochronic 

 chord of 140° is about the extreme limit of penetration by 

 the longitudinal waves, while the similar chord of 110°- 



13 Diastrophism and the Formative Processes : XV. The Self -Compression 

 of the Earth as a Problem of Energy, Jour. Geol., vol. 29, pp. 679-700, 1921. 



Am. Jour. Sci.— Fifth Series, Vol. IV. No. 22.— October, 1922. 

 18 



