DIASTROPHISM AND THE FORMATIVE PROCESSES 503 



The cumulative evidences of recent investigations on tidal, 

 seismic, nutational, and other phenomena support this view with 

 little less than demonstrative effect. The most of these are now 

 quite familiar. There is space here merely to quote the latest 

 numerical determinations (191 7) that have come to my notice. 

 Schweydar,^ as the result of observational, experimental, and 

 mathematical work on the tides, the polar nutations, and the trans- 

 mission of seismic waves, concludes that the earth conducts itself 

 as though it had a mean rigidity 2| times that of steel, that the 

 constant of rigidity at the surface is about 3X10" dynes, that this 

 increases in depth more rapidly than the density, so that at the 

 center it reaches 30X 10" dynes, or ten times its value at the surface. 

 The transverse seismic waves, as far down as the record permits a 

 confident interpretation, indicate a definite gradation of density, 

 rigidity, and elasticity. To insure that the total rigidity shall 

 reach the mean value of 2J times steel, and at the same time be 

 consistent with the rigidity known to prevail in the outer zone 

 and the gradually rising rigidity implied by seismic waves as far 

 down as their record is good, it seems clear that a high order of 

 rigidity in the remaining central part is imperative. The old 

 hypothesis of an iron core framed, among other reasons, to account 

 for the high mean density of the earth — a purpose which it serves 

 only clumsily — does not help much in meeting the still higher rate 

 of rise of rigidity and elasticity toward the center, for iron is soft 

 and malleable when hot. Nor does any special segregation of 

 inherently heavy material in the earth, however helpful it may 

 be in its place, fully satisfy the phenomena brought out by the 

 comparative studies on the earth, the moon. Mars, and Venus. 

 The whole evidence seems to point clearly to a systematic mass- 

 effect, working on essentially the same material in all cases, aided, 

 to be sure, but aided in only minor degree, by selective segrega- 

 tion. In the heart of the earth very likely the segregation of the 

 metallic from the stony material has gone much farther than in the 

 outer parts, but I see little reason to think the two classes of mate- 

 rial have been wholly separated from one another. A segregation 



' W, Schweydar, "Ueber die Elastizitat der Erde," Sonderabdruck aus Die 

 Naturwissenschaften (191 7), pp. 1-27. 



