BEARINGS OF RADIOACTIVITY ON GEOLOGY 689 



in one form or another, at nearly all times in the earth's history. 

 They have often been cumulative until they reached diastrophic 

 intensity and manifested themselves in impressive deformations. 

 That these have been effective agencies in forcing the movement 

 of liquid parts within the earth in the lines of least resistance and 

 of best accommodation to existent conditions is scarcely debatable. 



In addition to the simple stresses of gravity and to the dia- 

 strophic stresses, there have been superposed at all times a series 

 of stresses of a rhythmical pulsatory nature acting throughout the 

 body of the earth. The nature and function of these has not 

 been so generally recognized. These stresses are derived from the 

 differential action of the gravity of neighboring bodies, particularly 

 that of the moon and of the sun. Tidal and tidelike stresses and 

 strains have swept through the earth's body in a constant cycle 

 bringing to bear on each part a perpetual succession of compres- 

 sive and tensional stresses and strains alternating with one another. 

 The effect may be pictured as that of a minute kneading of the 

 earth-body. There is not only a superposition of pulsating strains 

 on the more static strains but a superposition of pulsating strains 

 on pulsating strains. The pulses of the twelve-hour body tides 

 are overrun by tides of longer periods and these are attended by 

 shifts of direction of strain, all of which tend to knead the mixed 

 matter to and fro and promote insinuation of the liquid parts 

 along the lines of escape. 



Underlying all these rhythmical strains there has been ever 

 present a variation in intensity from center to surface. Sir George 

 Darwin has shown that the tidal stresses generated by the moon at 

 the earth's center are eight times as great as those at its surface. 

 Each compressive strain squeezes the lower part of each liquid 

 vesicle or thread more than the upper part. 



The coexistence of these pulsatory and periodic strains with 

 the simple static stresses of gravity and the less constant dias- 

 trophic stresses sufficiently implies their co-operative nature. 

 All these three classes are either differential stresses or have 

 factors or phases that are differential, and so, in specific local appli- 

 cation, they are all transformed into sub-differentiational effects 

 on the liquid and solid parts. 



