Groundwork for Study of Megadiastrophism. 265 



higher degree of rigidity and elasticity in the interior, taken as 

 a whole, than in the outer shell. Seismic waves add very specific 

 confirmatory evidence, so far as the outer seven-eighths of the 

 volume of the earth is concerned. The seismic evidence for the 

 remaining central part is as yet obscure, and is differently inter- 

 preted by the special students of the subject. In a general way, 

 the whole of the interior is covered by the tidal and nutational 

 evidences. These favor the interpretation of the central part 

 as highly rigid and elastic, since these qualities fit the general 

 import of the evidence, but for the present it is prudent to leave 

 the question of the state of the center to be settled in the future. 

 It is to be observed that the increasing density of the interior 

 tends to dampen the speed of the seismic waves, and that cor- 

 rection for this effect must be made in deducing the hrward 

 increase of rigidity and elasticity from the seismic records^ 

 "When allowance is made for this, the generalization that rigidity 

 and elasticity are notably higher in the interior than in the outer 

 shell is put beyond serious question. ' ' 



Xow a special significance attaches to nntation in the 

 combination of evidences because much emphasis has 

 been laid on the shortness of the seismic and tidal vibra- 

 tions and the possibility that they might not mean much. 

 The Chandlerian cycle of the nutation has a period of 

 about 14 months (432.8 days according to Sclrvveydar). 11 



Now I think that the reader will agree that, in view of 

 this putting of a part for the whole and of the assignment 

 of ideas of w r edges and rigidity quite foreign to my own, 

 I am entitled to disclaim any responsibility for such other 

 views as my critic may assign me in the rest of the paper, 

 and that it is best that I should use what space remains 

 available in discussing some of the more vital issues 

 raised. Foremost among these is the question, rendered 

 acute by the criticism: 



What is really implied by the properties of rigidity and 

 elasticity disclosed by seismic waves, the body tides, the 

 earth's nutation and associated evidences^ 



The elastic rigidity disclosed by the seismic waves. — 

 Setting aside the earth's nucleus for later consideration, 

 there is agreement among seismologists that the outer 

 portion of the earth to a depth of 3000 kilometers or more 

 transmits transverse or distortional seismic waves and 

 that this implies an elastico-rigid state. So far as the 



11 Op. cit. 



