422 CARNEGIE INSTITUTION OF WASHINGTON. 



considerations is much strengthened when it is considered that the 

 rate of rise of gravitative pressure is much slower in the outer zone 

 than in the mid-depths of the earth. An excess of density would be 

 indicated, if we did not assume that the curve of density found in the 

 outer shell, like the curve of temperature, falls off, in proportion to 

 pressure, as the depth increases. 



The next step in the inquiry was a consideration of the specific 

 phases assumed by the compressional energy and the paths the energy 

 took in passing from one phase to another. 



SPECIFIC PHASES AND PATHS OF THE COMPRESSIONAL ENERGY. 



For the sake of brevity, little more than conclusions can be given 

 here.^ It is to be understood that the earth is assumed to be a solid 

 elastic body of heterogeneous constitution, that it was subjected to a 

 slowly growing gravitative pressure, occupying in its rise a period of the 

 order of two or three billion years, and that the smallness of the incre- 

 ments of stress in any epoch and the long time available for adjust- 

 ment are important factors in the case. 



The first step in the compressional process was the passage of a 

 part of the stress into the form of strain, while another part took on 

 the thermal form. Within elastic limits the energy of strain was 

 stored or latent. Not a little energy has probably been thus stored 

 all thi-ough the geologic ages. It appears from stratigraphic evidence 

 that the strain-limit within the earth-body may become high enough 

 under pressure to permit the accumulation of stored energy sufficient 

 to actuate deformative ''revolutions," in spite of such partial easement 

 as may have been realized in the meantime from the milder forms of 

 idiomolecular action about to be described. 



The second step was the cooperative action of this stored energy 

 of strain and the agitative thermal energy. The latter aided change 

 by loosening the fixed elastico-rigid attachments of the molecules. 

 The hold of crystals and clastic fragments upon their constituent 

 molecules is unequal, because a portion of these lie at the angles, or 

 on edges, or in sharp curves of the little masses, where fewer other 

 molecules support them. The strains arising from pressure upon 

 the interlocking crystals or fragments are also unequal for analogous 

 reasons. It is obvious, therefore, that the particular molecules least 

 securely held, or else those most severely strained, would yield first, 

 easing those particular molecular strains and permitting a new ad- 

 justment of crystals or fragments, giving rise to new differences of 

 molecular strain. The detached molecules were naturally forced to 

 take the lines of least resistance, or else those of greatest crj^stalline 

 attraction, until they reached points where new attachments were 

 made in response to the crystalline force of some crystal so situated 



^A Bomewhat fuller statement will be made in the Journal of Geology, probably No. 8, 1921. 



