64 GEOLOGY. 



during the growth of the latter, and this may be a not unimportant 

 factor in their evolutionary history. 



The concurrent bearings of these two considerations are quite in 

 harmony with what might be gathered independently from a com- 

 parison of the apparent amounts of matter in the nebular knots with 

 the amounts in the nebular haze. It will, therefore, be assumed, in 

 our further study, that the nuclei constituted only a small portion of 

 the mass of the grown planets. The fraction was probably larger 

 proportionately for the small planets than for the large ones, for 

 the power of growth probably rose with increased mass in geometrical 

 ratio. In the case of the asteroids and satellites the growth may not 

 have been large. 



The mode of accretion. 



The nature of the planetesimal motions. — The gathering of the 

 planetesimals to the nuclei to form the planets is assigned essentially 

 to conjunctions in the course of their orbital motions, not to simple 

 gravitation, except as gravitation was the fundamental cause of the 

 orbital motions. The nature of the primary motions is, therefore, a 

 first consideration. The combination of outward and rotatory motion 

 to which the formation of the nebula is assigned, while giving a spiral 

 form to the whole, at the outset, is believed to have given to each 

 individual planetesimal an elliptical orbit about the common center. If, 

 recurring to our hypothesis of the origin of the spiral, the outward and 

 the tangential impulses had been duly balanced, circular orbits would 

 have resulted; but neither theory nor observation make it probable that 

 this was often the case. The inevitable inequalities of the two com- 

 ponents should give ellipses varying in eccentricity with every varia- 

 tion in their relations. As, however, both the outward and the rotatory 

 components sprang from the same source — the gravitative disturbance 

 induced by the approach to a massive star — there is reason to think 

 that they would not be extremely unequal, and that the resulting eccen- 

 tricities, though large, would not be excessive. This view is in accord 

 with the forms of the spiral nebulae. These do not present spirally 

 symmetrical configurations of the strictly circuloid type, but broadly 

 elliptical ones, with irregular elements. The development of the 

 present almost circular configuration of the solar system out of such 

 a broadly elliptical, somewhat irregular, spiral configuration, involves 



