520 THE DISTRIBUTION, ETC., OF ALKALINE ROCKS, 



If planetesimals be formed by the tidal disruption of a pre- 

 existing body and they are similar in composition to meteorites, 

 it is hard to see where the earth's atmosphere and hydrosphere 

 come from, especially the oxygen constituent. Dr. Chamberlin's 

 exposition seems insufficient. A pre-existing body has to be 

 assumed; and even the exposition of Dr. Chamberlin admits of 

 the possibility of a gaseous early stage for his planetesimal 

 earth. 



The backward rotation of the outer planets is as difficult to 

 account for on his hypothesis as on the Laplacian. 



The satellites, instead of moving away from their parent, 

 planets, as they are supposed to do by Professor Darwin, should 

 approach the planets, inasmuch as the latter, being the larger 

 bodies, attract more planetesimals and therefore grow faster and 

 have their power of gravitational attraction increased. 



If the earth is long past its accretion-stage, all heat due to 

 accretion should be gone or nearly gone, and all the earth's heat 

 should be due to contraction. 



We should also ask ourselves if the nebulous surroundings of 

 the young earth would not check radiation to such an extent 

 that any plane tesira ally formed body would be changed to the 

 gaseo-molten stage. 



If these problems can be satisfactorily solved without injury to 

 the planetesimal hypothesis, there still remain two objections on 

 geological grounds, viz., (1) that we have never found any trace 

 of a fragmental zone composed of planetesimals in our oldest 

 Azoic formations; and (2) that the irregularities in igneous 

 succession are difficult to explain on the planetesimal hypothesis. 



For these reasons I propose to review the origin of the earth's 

 crust on the gaseo-molten hypothesis.* 



As the process of cooling of the nebulous mass advanced, the 

 earth became denser and denser, and finally the condensation of 

 all the metallic and silicate material ensued. During this pro- 



* See also Mr. Steel's Presidential Address, these Proceedings, Vol.xxx. 

 p.616, 1906. 



