666 T. C. CHAM BERLIN 



toward high density, rigidity, and elasticity in the central parts. 

 It was further found that the shapes of the planetary cores were 

 influenced from the very outset by the gyratory system of circula- 

 tion that attended their formation, and that they thus failed to 

 take on strictly spherical forms, so that they were likely to yield 

 unsymmetrically to the heavy masses later built upon them by 

 planetesimal growth. Even the primitive circulation thus had 

 its influence on the diastrophism that developed much later. 



Let us now consider the planetesimal growth. This involves 

 (i) a study of the nature of the planetesimals at the start, (2) the 

 conditions that affected the mode and extent of their growth, and 

 (3) the modes and rates of their infall and the effects of these on 

 the molten or solid state of the earth, as also on its content of 

 explosive gases. 



THE NATURE OF THE PLANETESIMALS AT THE START 



The way in which the planetesimals are supposed to have arisen 

 has been made clear in previous articles, but it will facilitate our 

 present study to note that they took their starts from two main 

 sources: (i) solar molecules driven into orbits by the original solar 

 expulsion, and (2) molecules thrown out into orbits from the nuclei 

 later by molecular interaction. There were other sources of 

 planetesimals, but they may be neglected here. In both classes 

 the planetesimals started as molecules chiefly. To some extent 

 they may have been newly formed precipitates from the solar gases, 

 or precipitate aggregates formed by the union of the fresh pre- 

 cipitates. Such precipitates are thought to form in the sun's 

 photosphere now. They would be likely to have been formed by 

 the expansion of the solar gases just after these emerged from solar 

 pressure. The essential point here is that, whether molecules, 

 precipitates, or precipitate aggregates, they were minute. Whether 

 they afterward grew to notable sizes depended on the conditions 

 that controlled their later history. Chief among the controlling 

 influences were the dynamic properties given the planetesimals by 

 their expulsion, and the gravitative stresses that controlled the 

 field into which they were driven. It is to be kept ever in mind 

 that they were bodies projected into swift independent flight, each 



