420 GEOLOGY 



Earth's building. This notion, however simple and natural, may 

 be dismissed without serious consideration, for the diverse direc- 

 tions of motion and the various velocities of meteorites are such 

 as to forbid the belief that a system of such symmetrical discoid al 

 form and of such harmonious motions as the solar system presents, 

 could have been formed in this manner. 



The hypothesis of Lockyer and Darwin. A meteoritic hypo- 

 thesis of more logical character has been based on the conception 

 that meteorites may be aggregated into swarms and constitute 

 nebulae. This hypothesis is, therefore, nebulo-meteoritic. Inquiry 

 into the mechanical possibilities of the case led Sir George Darwin 

 to the conclusion that such a swarm of meteorites would act very 

 much like a gas, and that the laws of gases may be applied in deter- 

 mining its mechanical properties. So far as it applies to the origin 

 of the earth, this form of the meteoritic hypothesis is thus found 

 to be practically identical with the gaseous hypothesis, and so far as 

 it applies to the dynamics of the solar system, it is subject to the 

 criticisms already urged against that hypothesis. It is to be noted 

 that these criticisms apply only to such meteoroidal nebulae as may 

 be formed of swarms of meteorites the members of which move in 

 diverse directions, and frequently collide with one another. If 

 the meteorites were assembled so as to pursue concentric orbits 

 and form a disk-like system, they would be controlled by planetoid al 

 or planetesimal dynamics, and fall into the following class. The 

 term meteoritic hypothesis is here used essentially as employed 1 > y 

 Lockyer and Darwin, and should be carefully distinguished from 

 the planetesimal hypothesis, whose mode of evolution is radically 

 different. The meteoritic hypothesis as thus denned is applied by 

 its authors chiefly to the earlier and more scattered states of the 

 nebulae, and has not been specifically applied to the formation of a 

 planet, perhaps because a meteoritic nebula tends to pass into a 

 gaseous nebula as it condenses. 



III. The Planetesimal Hypothesis 



When the shortcomings of the Laplacian hypothesis came to be 

 so clear and cogent that there was no apparent way of escape from 

 them, an alternative better in accord with the facts was sought. 



