DIASTROPHISM AND THE FORMATIVE PROCESSES 395 
GENERAL PROPERTIES OF THE EARTH 
1. The solid elastic nature of the earth is accepted as having 
been put beyond serious question by the concurrent testimony 
of seismic waves, the body tides, the polar nutations, and collateral 
evidences. . 
2. The outer and major part of the earth is held to be minutely 
heterogeneous in chemical and physical composition, but yet, in a 
mechanical sense, sufficiently homogeneous to transmit seismic 
vibrations in legible form. 
3. Some questions remain respecting the earth core; it is held 
to be dense and rigid; but earthquake waves traversing it do 
not, as yet, tell an unequivocal story. Possibly it is formed of 
concentric zones rather homogeneous in themselves but varying 
from one another; possibly, also, segregation of metallic matter 
toward the center has gone far enough to give a higher ratio of 
density to elasticity in this inner part than in the accretional zone 
above, and thus introduced seismic anomalies. 
4. So far as now deducible, elasticity and rigidity increase 
toward the center faster than density. Since simple density 
segregation would scarcely carry a relative increase of rigidity and 
elasticity, this seems to imply a dynamic cause. The mean 
rigidity of the earth seems to be distinctly higher than that of 
steel." 
5. The major pulsation-period of the earth seems to be of the 
order of an hour but it is not yet precisely deducible from elastico- 
rigid-density data;? nor is the naturalistic evidence conclusive. 
It may be near enough to commensurability with the semi-diurnal 
tide to strengthen it by resonance, but this is not certain. 
6. The mode of motion of the poles in the earth is an index 
of high elastic rigidity. Schweydar has recently determined the 
t Schweydar’s recent determination is two and one-half times the rigidity of 
steel; he gives the rigidity of the central part of the earth as ten times that of the 
surficial part. ‘‘On the Elasticity of the Earth,” Naturwissenschaften (1917), Potsdam, 
Germany, Part 38. 
2 On the influence of gravity on elastic waves, and in particular on the vibrations 
of an elastic globe, see T. J. A. Bromwitch, Proc. Lond. Math. Soc., Vol. XXX (1899). 
3 Nagaoka tried to deduce it from the eruptions of Krakatoa, Nature (May 26, 
1907), pp. 89-91. 
