SYMPOSIUM OX r-\dioactrt:ty. 71 



stress on any large part of the earth is apt to become locally sub- 

 differentiated when solid and liquid parts are intermixed, especially 

 if the liquid and solid states of these parts are partially inter- 

 changeable because their temperatures he so close to the line o1 

 equihbrium between sohdit>- and liquidity-. Tensional strains 

 promote hquef action in bodies constituted as most rocks are ; com- 

 pressive strains resist Uquefaction in such bodies. And so general 

 differential strains co-operate with temperature in promoting or 

 in restraining the passage of matter from the one state to the 

 other, according to the nature of the strain, and thus have some 

 influence in directing and faciUtating movement as well as in 

 forcing it. 



Some of the differential stresses in the earth are essentially 

 fixed and constant, such as the direct pressures that arise from 

 the action of gravit}-. These stresses range from one atmosphere 

 at the surface to about three million atmospheres at the center. 

 Such pressures tend to force Hghter bodies toward the surface 

 while heavier bodies seek the center in ways so famiUar that we 

 need not dwell on them, nor on the fact that, since molten rock is 

 usually hghter than the same rock in a soUd state, this static 

 differential stress of gra\-it\- presents a general condition that 

 favors the ascent of hquid rock. So also the incorporation or gen- 

 eration of gases in hquid rock tends to lessen the specific g^a^^t\' 

 and increase the mobihty and hence the gaseous element adds 

 another general influence that favors ascent. 



In addition to these very general and persistent stresses, more 

 special differential stresses have arisen at various times from 

 inequahties of accession, from transfers of matter, from loss of 

 heat, and from other van.4ng agencies and these have been present, 

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

 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 diastro- 

 phic 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 have not been 

 so generally recognized. These stresses are derived from the 

 differential action of the gravity of neighboring bodies, particu- 



