72 ILLINOIS ACADEMY OF SCIENCE. 



larly that of the moon and of the sun. Tidal and tide-like stresses 

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

 cycle bringing to bear on each part a perpetual succession of com- 

 pressive and tensional stresses and strains alternating with one 

 another. The effect may be pictured as that of a minute knead- 

 ing of the earth-body. There is not only a superposition of pul- 

 sating 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 part 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 co-existence 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 application 

 they are all transformed into sub-differentiational effects on the 

 liquid and solid parts. 



Under the planetesimal view the joint effect of these differential 

 stresses and their resulting strains has been at all times to force 

 toward the surface liquefied rock as fast as it gained workable 

 volume. Much aid in insinuating itself along liquid lines and in 

 fluxing a more open path until the fracture zone was reached, was 

 assigned to the mixed nature of the material and to the local 

 strains imposed by the stress agencies. The whole picture centers 

 on the fundamental dynamic proposition that energy in mobile 

 and expansive embodiments seeks the surface, ivhile its fixed 

 embodiments are forced more firmly together toward the center. 



The extrusion is held to have begun as soon as the susceptible 

 matter took the mobile form. Possible exception is admitted in 

 the case of matter that may have been too dense to be forced to 

 the surface. However, a high density of small masses enmeshed 

 in masses of less density could only contribute to an average effect 

 so long as a high state of viscosity was retained, and a relatively 



