THE INTERIOR OF THE EARTH. 229 



Not to detain you with other considerations, the foregoing seem 

 best to comport with an essentially solid state of the earth's interior, 

 if they do not point rather definitely to such a state. Even if they 

 stood alone, they would seem to make a prevailing solid state the 

 most tenable working hypothesis. 



But they are far from standing alone ; the geological evidences are 

 strongly supported by considerations that spring from several 

 kindred lines of inquiry. The testimony of astronomic evidence 

 is given below by Dr. Schlesinger. The import of seismic studies, 

 the subject of Dr. Eeid's contribution, lends very special support to 

 the view that the interior of the earth is elastico-rigid at least to the 

 extent that distortional waves pass through its interior. It seems 

 certain already that this condition prevails throughout much more 

 than half the volume of the earth; concerning the rest, the deep 

 interior, the seismic evidence is perhaps still to be regarded as indeter- 

 minate. But on the seismic evidence it does not fall to me to dAvell. 



The tidal studies of Hecker, Orloff, and others lend support to 

 the tenet of a rigid earth but they fall somewhat short of con- 

 clusiveness. The brilliant experimental determinations of Michelson 

 and Gale, correlated with the computations of Moulton, have carried 

 the evidence to the point of preliminary demonstration. They need 

 only to be adequately repeated and verified to become final, so far 

 at least as elastic rigidity can be indicated by the response of the 

 earth body to solar and lunar attractions. The special feature of 

 most critical value in the demonstrations of Michelson and his col- 

 leagues is the high degree of elasticity shown by the almost instan- 

 taneous response of the earth to the distorting pull of the tide- 

 producing bodies. This cuts at the very base of concepts founded on 

 the supposed properties of a viscous earth. These tidal determi- 

 nations of elasticity are in close accord with the seismic evidences. 

 The tw^o are happily complementary to one another. The one 

 deals with the earth as a whole under a rhytlmiical series of in- 

 creasing and diminishing stress differences springing from exter- 

 nal attractions; the other deals in an intensive vibratory way with 

 earth substance by sharp short stresses that call into action its most 

 intimate structural qualities. While it is wise, no doubt, to refrain 

 from resting too much on these early results of relatively new and 

 radical lines of inquiry, until their results shall be more mature, 

 their prospective import is radical and decisive in favor of a solid 

 earth not only, but of an elastico-rigid earth. Assuming that the 

 present import of these inquiries will be amply justified by more 

 mature research, it is pertinent to bring into consideration the 

 corollary they so distinctly imply, viz, that the molten and viscous 

 material in the earth, or at least in its outer half, if not throughout 

 its deep interior, is a negligible factor in general studies, and enters 

 73839°— SM 1916 16 



