208 CARNEGIE INSTITUTION OF WASHINGTON. 



and planets. The exceedingly small amount of meteoritic material 

 picked up by the earth seems to be consistent with this interpretation. 

 In conclusion, it may be remarked that, so far as my studies have 

 gone, the meteoritic condition seems most probably to be an inci- 

 dental result of cosmic mechanics of trivial importance, and to be a 

 source of merely incidental accretion to existing bodies. Meteoritic 

 aggregation of the type defined does not seem to represent a great 

 generative method whereby stellar systems are evolved. On the 

 contrary, the meteoritic condition seems to be inherently moribund, 

 passing into the gaseous state on the one hand, or into the planet- 

 esimal on the other, or, in the absence of assemblage, losing its 

 constituents to existing suns and planets by capture one by one. 



A much larger portion of my study during the past year has been 

 devoted to a development of the planetesimal hypothesis into greater 

 precision and detail, to the applying of such tests as I could devise, 

 and to the working out of its concrete relations to the many geolog- 

 ical problems whose solution is vitally dependent on the mode of the 

 earth's origin. From the geological point of view the ultimate test 

 of this hypothesis and of all other hypotheses of the earth's origin lies 

 in their working qualities. As a complete statement of the planet- 

 esimal hypothesis has not yet appeared in print, it will doubtless be 

 best that I should outline with some detail the form the hypothesis 

 has assumed as the result of the work upon it, particularly as this 

 will best indicate the work that has been done. 



Under the typical form of the planetesimal hypothesis it is assumed 

 that the parent nebula of the solar system consisted of innumerable 

 small bodies, planetesimals, revolving about a central gaseous mass, 

 somewhat as do the planets to-day. The hypothesis, therefore, postu- 

 lates no fundamental change in the system of dynamics after the 

 nebula was once formed, but only an assemblage of the scattered 

 material. The state of dispersion of the material at the outset and 

 throughout, as now, was maintained by orbital revolution, or, more 

 closely speaking, by the tangential component of the energy of rev- 

 olution. The planetesimal hypothesis by no means excludes gases 

 from playing a part in the parent nebula or in its evolution, any 

 more than it denies their presence in the sun or the atmosphere 

 to-day, but it assigns to gaseous action a subordinate place in the 

 evolution of the planetary system after the planetesimal condition 

 had become established. 



An inquiry into the possible modes by which the planetesimal 



