GEOLOGY. 381 



ous as are those of meteorites and comets. Certain comets are known 

 to have orbits definitely related to the giant planets. This relation is 

 commonly regarded as the result of reduction from larger and more 

 eccentric orbits by the planet's influence. Without questioning the 

 validity of this, it may be held that in some cases the comet arose de 

 novo from the planetary action as here suggested. 



These three hypotheses are consistent with one another and may 

 all be true. They have the merit of being made to rest on the same 

 genetic basis as the planetary system itself. The scheme thus gives 

 unity to the whole solar system. 



If this be the true line of interpretation, the masses of meteorites and 

 their methods of infall throw a flood of light on the sizes and modes of infall 

 of the planetesimals, for, hy this interpretation, they are bodies of like ori- 

 gin and like general conditions. Now, on a conservative estimate, there 

 are 100,000,000 minute meteorites ("shooting stars"), that are so 

 small as to be wholly dissipated in the upper an*, for every one that is 

 massive enough to reach the earth as a visible body. Even of the latter 

 none is known to exceed a dozen feet in mean diameter; none of them 

 has been seen to produce melted soil or rock where they strike. The 

 supposed meteorite responsible for the crater of Coon Butte requires 

 special consideration which cannot be given here. Even if given the 

 full benefit of all doubts, this meteorite does not call for any modifica- 

 tion of the general statement that meteorites are competent to produce 

 only very local and hmited liquefaction. So far as their evidence 

 bears on the size, rate of infall, and hquefying power of their relatives, 

 the planetesimals, it is wholly in accord with the deductions herein- 

 before drawn from theoretical considerations. 



CONCLUSIONS. 



1 . The density of the earth compared with that of Mars, Venus, and 

 the moon — all formed under similar genetic conditions — implies a 

 shrinkage more than sufficient to justify the exceptionally large esti- 

 mate of deformation which prompted the inquiry. 



2. Since this large deformation is recorded in solid matter, the re- 

 quisite shrinkage must have taken place after the solidification of the 

 earth was sufficiently advanced to register it. This favors the view 

 that solidification of the requisite type began early. 



3. The amount of this shrinkage is so large as to imply that the po- 

 tential resources of shrinkage were conserved in a high degree until 

 the solidification requisite for a deformative record had been reahzed. 

 As the accretion of solid clastic material retains the possibilities of 

 shrinkage in a higher degree than formation in a molten state, the 

 former is favored. 



