416 GEOLOGY 



place that it came to be known as "The Nebular Hypothesis " ; 

 but the advance of inquiry makes it necessary now to consider other 

 hypotheses which maintain that the solar system arose from a 

 nebula, but a nebula whose constitution and mode of evolution 

 differed from that postulated by Laplace. The leading hypotheses 

 of the earth's origin may be grouped in three classes: 



I. The gaseous hypotheses, in which the parent nebula is 

 assumed to have been formed of gas collected into a spheroid by 

 gravity in accordance with the laws of gases, and to have been 

 evolved into the present solar system by loss of heat, and the separa- 

 tion of the outer parts into planets. The type of the class is the 

 Laplacian hypothesis. 



II. The meteoritic hypotheses, in which the parent nebula is 

 assumed to have been a swarm of meteorites, the members of which 

 moved in diverse directions, and suffered frequent collisions giving 

 rise to heat, light, and vaporization. The swarm of meteorites is 

 thought to have behaved essentially as a coarse gas, and the evo- 

 lution of the system to have been dynamically like that of the 

 gaseous system. 



III. The planetesimal hypothesis, in which the original constit- 

 uents are assumed to have been small bodies, molecules, or aggre- 

 gates, moving in orbits about a common center and forming a disk- 

 like system. The bodies are supposed to have been controlled by 

 revolution about the center of the nebula, and not by impact on 

 one another. The evolution consisted in the gathering of these 

 small bodies, planetesimals, into planets and satellites. Dynam- 

 ically, this hypothesis differs more from the other two than they 

 do from one another. 



I. The Laplacian or "Nebular" Hypothesis 

 During the last century the Laplacian hypothesis was very 

 generally accepted, and geological theories as to the early states of 

 the earth, and as to many later events in its history, were built upon 

 it, and these views are still prevalent. The hypothesis is so well 

 known that a few sentences will recall its essential features. It 

 holds that the sun, the planets, and the satellites were once parts of 

 a glowing, rotating, spheroidal, gaseous nebula, which was ex- 



