FUNDAMENTAL, PROBI^EMS OF GEOLOGY. 239 



mass if the heat were all generated at the same instant ; but if it 

 were generated in successive moieties spread over a long period and 

 generated at the surface, where readilj- radiated away, no large 

 amount might be retained, and high internal heat, such as required 

 for vulcauism, might not be assignable to this source. In the pres- 

 ent state of knowledge the hypothesis may not unreasonably be 

 given such a form as to make this source partially available by 

 assuming that in the early stages of accretion, while the nebular 

 planetesimals were still relatively numerous, the collisions between 

 them and the nucleus were so frequent as to make the latter hot. 

 It is possible that mathematical inquiries contemplated, but not yet 

 carried out, will show that this was probable, and that a rate of 

 accretion so slow as to give a cool exterior would only come later, 

 after the planetesimals of the feeding zone had been thinned out ; 

 but until that can be shown the hypothesis must face the alternative 

 possibility that the collisions did not succeed one another so rapidly 

 as to greatly heat the growing earth body by impact. 



An unknown amount of heat may have been inherited from the 

 nebular knot that constituted the original earth-nucleus. This knot 

 is supposed to have consisted of an assemblage of small aggregates 

 made from the heavy molecules of the nebular material ; in other 

 words, chiefly the metallic and the rock substances. This is held to 

 be so because these substances would condense to the liquid and 

 solid state at high temperatures, and further because, having low 

 molecular velocities and relatively high gravity, they could assemble 

 and remain associated by mutual attraction, while molecules of low 

 weights and high velocities could not. These assemblages were 

 probably rotatory or revolutionary, but perhaps of a very irregular 

 kind, somewhere midway between a well-organized planetesimal 

 system and a heterogenous gaseous or collision-rebound system, and 

 combining some of the qualities of each. The ingathering of planet- 

 esimals from without probably tended to increase the irregularity, 

 and to cause the assemblage to become more and more gas-like in 

 dynamic nature. The matter being rock substance or metallic, and 

 hence partially inelastic, and the collisional velocities generally low, 

 the mode of condensation was probably only in part analogous to 

 that of a gas, but it is possible that an internal temperature not 

 unlike that of a condensing gas might be developed . The young earth 

 may, therefore, have inherited a hot nucleus. 



The chief source of internal heat is, however, assigned to the 

 progressive condensation of the growing body as material was added 

 17 



