DIASTROPHISM AND THE FORMATIVE PROCESSES 697 



dynamics as the normal elements. I venture, therefore, to offer 

 three hypothetical, but mutually consistent, ways in which meteors, 

 meteorites, and comets may have arisen naturally and inevitably 

 out of the dynamical system that gave rise to the planets as its 

 normal product. 



The problem of the meteors, meteorites, and comets is regarded 

 as essentially one. Though complete demonstration has perhaps 

 not yet been reached, it is assumed that meteors, meteorites, and 

 comets are not only close of kin dynamically, but in some sense 

 mutual derivatives. The spectacular phenomena which seem to 

 put comets in a class by themselves are here supposed to be mainly 

 the effects of the strongly contrasted conditions to which they are 

 subjected at the extremes of their very elongated orbits. It is a 

 suggestive fact that those comets which are supposed to have 

 been reduced from extremely elongated orbits to shorter ones by 

 the action of the great planets, show a notable tendency to lose 

 their spectacular features and finally to pass by disintegration into 

 meteor swarms. In the case of typical comets of extremely elon- 

 gated orbits, a small loosely organized head — apparently a cluster 

 of still smaller bodies held together by rather feeble gravitative 

 control — swings from a relatively hot perihelion close to the sun 

 to a very cold aphelion far out in space. During its long outer 

 journey, all the constituents must become intensely cold to great 

 depths and be liable to be deeply riven by shrinkage cracks, which, 

 besides leading to coarse fragmentation, should facilitate the 

 adsorption of molecules belonging to the sun's ultra-atmosphere. 

 The action is supposed to be the same as that which gives to 

 meteorites their occluded or combined gases. The outward swing 

 of the comet occupies many years and often centuries, and there is 

 time for even a very attenuated source of supply to furnish the 

 requisite amount of gas-producing material. 



When later the comet head, thus charged, approaches the sun, 

 the gases are supposed to be set free by the solar heat on the sun- 

 ward side, and to be dr>en forth toward the sun. At the same 

 time, this differential heating is supposed to give rise to rapid 

 and rather violent exfoliation, hurling the dissevered chips to 



