DIASTROPHISM AND THE FORMATIVE PROCESSES 137 



on the question of the segregation of inherently heavy from in- 

 herently Hght material, the crux of our problem. Two postulated 

 origins may clearly have such bearings, both of which are now 

 famihar: (i) derivation from a rotating nebular spheroid by 

 centrifugal separation brought into effective action by cooHng and 

 consequent acceleration of rotation, and (2) derivation from solar 

 material ejected either spontaneously or under the stimulus of a 

 passing body. The principles involved in these two types will 

 probably serve to cover any other origin for which good reasons 

 may be assigned. 



While I am unable to see how planets such as form our system 

 can have arisen from a rotating spheroidal nebula by centrifugal 

 action, it yet seems best, out of deference to any who may still 

 think that some view of this general type is tenable, to discuss 

 this postulated mode of genesis in so far as it bears on our problem. 

 It will only be necessary, however, to review the phase of the 

 theory most dependent on the dynamic environment which con- 

 trolled the evolution, for that touches the soul of the subject. 



THE CENTRIFUGAL EVOLUTION OF A GASEOUS SPHEROID UNDER 

 ITS OWN DYNAMIC ENVIRONMENT 



Every organized nebula, like every other organized body, must 

 have an adequate enveloping field of force and sphere of control as 

 a necessity of its organized existence (I and II, above). The pos- 

 tulate that there was once a spheroidal nebula of the mass of the 

 solar system which in contracting shed secondaries at various dis- 

 tances from its center as far out as 2| bilHon miles and yet was able 

 to hold them then and afterward, carries the implicit assumption 

 that it had a distinctly effective sphere of control. The shedding 

 of the four little bodies under study took place only after the 

 postulated nebula had shrunk 'to about one-twentieth of the 

 radius it had when it displayed its ejffective holding power by its 

 control over the material shed for the planet Neptune, while the 

 outermost reach of its holding power must have extended much 

 beyond this. At the relatively concentrated stage when the 

 shedding of the substance for our little group of bodies took place, 

 the inner zone of control must have grown relatively intense; the 



