268 CARNEGIE INSTITUTION 



logical evidence favors extremely high rigidity and elasticity, in 

 contradistinction to the viscous, and even fluid, implications so long 

 urged on the basis of certain geological phenomena. 



Professor Hoskins has recently computed the effective rigidity of 

 the earth from the periodic variation of latitude, using methods 

 somewhat different from those of Newcomb, Hough, and Woodward, 

 and reaching results in such a form as to be directly applicable to 

 the problem of tidal retardation of the earth's rotation. These 

 results give the basis for a new and promising line of approach to 

 the problem of tidal retardation, for they permit the substitution of 

 a definite rigidity, determined on independent grounds, for the 

 assumed viscosity on which Darwin's classic investigation was 

 founded. Without going into details, it will suffice to indicate the 

 general bearing of the investigation to say that the recomputed rate 

 of retardation is only about one fourth as great as that found by 

 Darwin. In reaching this result the positioyi of the tide was assumed 

 to be that most favorable to retardation. The determination of the 

 actual position that the tide would assume with the given rigidity 

 and elasticity has not yet been attempted, and the amount by which 

 it will modify the above result is unknown. If it modifies it at all 

 it will be in the nature of a reduction and will further tend to bring 

 the results of computation more into harmony with the geological 

 evidences. The same line of inquiry promises other results of value 

 relative to the state of the earth's interior. 



Previous studies had led to the conviction that the stress accumu- 

 lating competency of the earth's body affords a promising line of 

 approach to the physical state of its interior, and Professor Hoskins 

 has cooperated in certain preliminary steps intended to test the 

 validity of this conviction and to develop the problem. The general 

 line of reasoning may be briefly indicated. Innumerable gentle 

 warpings have affected nearly every portion of the surface of the 

 globe at nearly all stages of its history. This implies the perpetual 

 activity of minor forces of deformation and a concurrent yielding of 

 the outer part of the earth to these forces. At the same time, when 

 the master phenomena of movement and deformation are considered, 

 there appear to have been long periods of relative quiescence, fol- 

 lowed by epochs of profound deformation. This general view, long 

 held by leading geologists, is being greatly strengthened by the 

 working out of the great baselevels, which add evidence of the most 

 cogent kind relative to the quiescent stages, while the stratigraphic 

 evidence of the periodicity of the great deformations is regarded as 



