INTRODUCTION. 173 



an impact do not travel far from their point of origin before being practi- 

 cally wiped out by friction, thus exhibiting a kind of mechanical radiation 

 of the energy brought in kinetic form by the impinging mass, but in such a 

 way that frictional absorption confines the distribution of that energy to the 

 neighborhood of its source. This sort of process would be closely imitated 

 if the surface structure were that of a loose aggregation of small masses, 

 even if the latter were perfectly solid. It is clear, therefore, that after the 

 transformation the impinging mass retains only a small part of its own orig- 

 inal energy, but it secures a certain compensation from the masses whose 

 deposition occurs in time and place near its own. Now, when the average 

 or normal energy of impact per unit-mass does not vary sensibly during the 

 time required for depositing a layer whose thickness is somewhat in excess 

 of the radius of influence implied above, then the compensation may be 

 regarded as practically exact, except as affected by direct radiation into 

 space during the time that the mass remains exposed. 



With this interpretation it seems a fair equivalent to assume that each 

 planetesimal mass retains its own primitive kinetic energy after impact in 

 thermal form, but immediately loses a portion by ordinary radiation before 

 it is covered up. This setting of the matter will be accepted hereafter, and 

 it will be further assumed that the process of accretion, though slow enough 

 to permit the loss of a large portion of the heat of impact by immediate 

 radiation, is yet sufl^iciently rapid so that internal conduction has not time 

 to modify sensibly the distribution of heat arising from compression before 

 the growth is complete. 



It may also be supposed that in connection with high viscosity the mass 

 would possess sufficient plasticity to enable its own gravitation to keep it in 

 a condition approaching hydrostatic equilibrium, with an approximately 

 spherical form, aside from the secondary effects of rotation and consequent 

 polar flattening; for it is supposed that in the long run the accretion would 

 be practically equable over the whole surface and that the effects of tem- 

 porary inequalities of serious magnitude would be quickly obliterated. 



It is well understood, by analogy with the behavior of such materials as 

 wax and pitch, that the combination of plasticity and viscosity, such as 

 here contemplated as appearing under slow changes, is in no way inconsis- 

 tent with the appearance of extreme rigidity under the action of sudden 

 or rapidly varying forces. It should be noted, however, that a satisfactory 

 theory as to the history of the earth's dominant surface features seems to 

 require that to the earth-substance be attributed a rigidity sufficient to allow 

 the alternate accumulation and subsidence of shearing strains, deep in the 

 body of the earth, to such an extent that the periods involved, though short 

 perhaps in comparison with the durations implied in phenomena of thermal 

 conduction in bodies of cosmic size, are nevertheless of higher order than 

 the periods of precession, nutation, and tidal phenomena, which have 

 hitherto furnished the chief data pointing to the practically perfect extreme 

 rigidity of the rotating earth. The hypothesis of practical fluidity under 

 slow deformation must therefore be understood only as a crude first approx- 

 imation from a geological point of view. But on account of its simplicity, 

 and because of its occurrence in previous theories of the earth's constitution. 



