GEOLOGY. 377 



energy is mainly converted into heat by atmospheric friction earlier 

 in their courses when their speed is greatest, and this heat is largely 

 consumed in melting a surface film which is mainly dissipated along 

 their paths. The heating of the atmosphere would therefore be the 

 first effect of infalling bodies, and only when it was raised to sufficient 

 temperature would melting of the solid earth follow, an event that 

 seems extremely improbable. 



In support of the foregoing conclusions, which rest in some large meas- 

 ure on theoretical considerations, there are several sources of concrete 

 testimony whose convergent import has no little value. 



THE TESTIMONY OF THE ZODIACAL PLANETESIMALS. 



The position, relations, and lens-like form of the Zodiacal Light 

 imply that the particles which reflect it are maintained by orbital 

 dynamics of the planetarj^ type. If so, these particles are to be regarded 

 as planetesimals. They are interpreted either as remnants of the 

 original planetesimal system or as more recent products due to the 

 projection of solar matter so close to the planets that it is drawn for- 

 ward into elliptical orbits by them. If either intei-pretation is correct, 

 they give explicit testimony as to the sizes of planetesimals, for they 

 are clearly minute. No individual particle has ever been detected, 

 although the lens which they collectively form envelopes the earth and 

 makes it certain that some of them are near us at times. 



THE TESTIMONY OF THE CRATERS OF THE MOON. 



The crater-like pits of the moon have sometimes been attributed 

 to the impacts of large planetesimals or planetoids. The results of 

 the inquiry already cited in respect to the size of the planetesimals, as 

 well as the absence of planetoids in the terrestrial region, are quite 

 adverse to the theory. In addition, the verticality of the pits seems 

 to be an insuperable objection, for bodies pursuing orbits would strike 

 the moon's surface obliquely in many if not most cases. The alterna- 

 tive theory that the pits arose from explosive action of the volcanic 

 type is supported by their vertical axes, their cu'cular outlines, the sym- 

 metrical distribution of debris, and other features. Now, the volcanic 

 interpretation has usually been thought to imply a molten state of the 

 moon, at least formerly. Precisely the opposite inference is here 

 held to be much more logical. These volcanic craters are held to be 

 good evidence that the moon has never been in a holo-molten state, 

 though of course it held local hot spots such as give rise to terrestrial 

 volcanoes. The moon now holds no appreciable atmosphere. It is 

 presumably more massive and colder than in its earlier stages. There 

 seems, therefore, to be no ground for assigning it the power of holding 

 free volcanic gases at any stage of its history, least of all when in a 

 molten state. All gases that could be set free by a liquefjdng tempera- 

 ture should certainly have been boiled out and have escaped because 



