690 T. C. CHAMBERLIN 



Under the planetesimal view the joint effect of these differ- 

 ential stresses and their resulting strains has been at all times 

 to force toward the surface liquefied rock as fast as it gained work- 

 able volume. Much aid in insinuating itself along liquid lines 

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

 is 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, while 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 rela- 

 tively high viscosity for the small mobile masses, naturally arose 

 from the close balance between the liquid and solid states. Such 

 a condition seems equally to be implied by the remarkable mixtures 

 of dense and light matter often seen in the igneous rocks. 1 



The matter forced early to the surface is held to have been 

 buried by further accretions to the growing planet, later to have 

 been subject to a second liquefaction and extrusion, a second 

 burial, and so on. Progressive selection and reselection are postu- 

 lated until the growth essentially ceased. Since then a more 

 complete selection and concentration of the eutectic material at 

 the surface has been in progress as far as further generation of 

 internal heat has furnished the actuating agency. 



Now if this picture in its working details and in its rather sharp 

 antithesis to the older view is clearly in mind, the part which the 

 radioactive substances may be supposed to play in co-operation 

 with this mechanism without changing the general conception 

 is little less than self-evident. The radioactive particles are 

 sources of self-generated heat. Under the planetesimal view the 

 radioactive substances were promiscuously scattered through the 

 mixed mass as it was gathered in heterogeneously from the nebula 



1 Chamberlin and Salisbury, Geology, II, 121-22. 



