FUNDAMENTAL PROBLEMS OF GEOLOGY. 205 



relative rarity of collision also suggests that it should be assigned a 

 secondary place. 



It has been suggested* recently that disruption by differential 

 attraction might satisfy the requirements of the case, though there 

 is perhaps some ground for doubt as to its adequate frequency. 

 According to principles established by Roche, Maxwell, and others, 

 a small body passing within a certain distance (the Roche limit) of 

 a larger dense body will be torn into fragments by differential attrac- 

 tion. The size of this sphere of disruption depends on the densities, 

 cohesion, internal elasticities, and other factors of the two bodies. 

 For incompressible fluids of the same density Roche gives the limit 

 of disruption as 2.44 times the radius of the large body. In most 

 such bodies internal elasticity probably exceeds cohesion, and the 

 sphere of disruption would be larger than this. The moon would 

 probably expand with some violence if its gravity were suddenly 

 removed by differential attraction. In any case fragmentation in this 

 way would be several times more probable than an actual collision. 

 Furthermore, the fragmentation in this case is not minute nor violent, 

 and this fits the meteoritic requirements. 



Relative to their erratic courses, it may be noted that a small 

 body passing near a much larger body is liable to be thrown from 

 its previous orbit into quite a new one. As is well known, this 

 has apparently happened to several comets through the influence of 

 the planet Jupiter, As shown by H. A. Newton, if the orbit of the 

 small body is such that it is caused to pass close in the rear of the 

 large body, say the planet Jupiter, its course will be diverted into a 

 larger orbit. If a small body were to pass in this way sufficiently 

 near to Jupiter, it would be thrown entirely out of the solar system, 

 and its path thence would probably be as unrelated to any stellar 

 system as that of an average meteorite. 



In these two sets of principles there is a combination peculiarly 

 fitted for the results required, for by their joint action a small body 

 passing near a large body is liable to be disrupted into fragments, 

 and tlie.se at the same time to be thrown into erratic courses, which 

 may carry them entirely outside the system to which they belonged 

 and give them independent courses in stellar space. It is obvious 

 that fragmentation and dispersal by the differential attraction of 

 ver\ close approach escapes the adverse contingencies of liquefaction 

 and pulverization incident to explosion or collision. 



* On the Possible Function of Disruptive Approach in the Formation of 

 Meteorites, Comets, and Nebuhe. Jour. Geol., Vol. IX, 1901, p. 369. 



