100 GEOLOGY. 



(1) Heat from infall. — The total amount of heat produced by 

 the infall of the planetesimals would undoubtedly be more than suffi- 

 cient to melt the whole 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 readily radi- 

 ated away, no large amount might be retained, and high internal heat, 

 such as required for vulcanism, might not be assignable to this source. 

 In the present state of knowledge, the hypothesis may not unrea- 

 sonably be given such a form as to make this source partially avail- 

 able, 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 put, will show that this was probable, and that a rate of accre- 

 tion 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 possi- 

 bility that the collisions did not succeed one another so rapidly as 

 to greatly heat the growing earth-body by impact. 



(2) Heat from quasi-gaseous condensation of the nucleus. — An 

 unknown amount of heat may have been inherited from the nebular 

 knot that constituted the original earth-nucleus. This knot is sup- 

 posed 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 veloci- 

 ties and relatively high gravity, they could assemble and remain asso- 

 ciated by mutual attraction, while molecules of low weight and high 

 velocities could not. These assemblages were probably rotatory or 

 revolutionary, but perhaps rather irregularly so, somewhere mid- 

 way between a well-organized planetesimal system and a heteroge- 

 neous gaseous or collision-rebound system, and combining some of 

 the qualities of each. The ingathering of planetesimals from with- 

 out probably tended to increase the irregularity, and to cause the 

 assemblage to become more and more gas-like in dynamic nature. 

 The 1 matter being rock-substance or metallic, and hence partially 

 inelastic, and the collisional velocities generally low, the mode of con- 



