FUNDAMENTAL PROBLEMS OF GEOLOGY. 223 



In postulating a mode of growth I have departed radically from 

 the older hypotheses and assigned the gathering of the planetesimals 

 into the nuclei to conjunctions in the course of their orbital move- 

 ments — not simply or chiefly to the attraction of the nuclei. The 

 nature of the original motions of the planetesimals is therefore a 

 point of vital importance. 



I have assumed that the combined outward and rotatory motions to 

 which the formation of the nebula is assigned gave to each individual 

 planetesimal an elliptical orbit about the common center, while their 

 distribution was such as to give a spiral form to the whole. In this 

 I have departed from the common assumption that the arms of the 

 nebulae marked the courses of the individual constituents. 



If the outward and the tangential impulses had been duly bal- 

 anced, it is believed that circular orbits must have resulted ; but 

 neither theory nor observation make it probable that this was often 

 the case. The inevitable inequalities of the two components should 

 give ellipses varying in eccentricity with every variation in their 

 relations. As, however, both the outward and the rotatorj^ com- 

 ponents sprang from the same source — the gravitative disturbance 

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

 that they would be measurably subequal, and that the resulting 

 eccentricities, though large, would not be excessive. This view is 

 in accord with the forms of the spiral nebulce. These do not pre- 

 sent spirally symmetrical configurations of the strictly circuloid 

 type, but broadly elliptical ones, with irregular elements. The de- 

 velopment of the present almost circular configuration of the solar 

 system out of such a broadly elliptical, somewhat irregular, spiral 

 configuration involves an evolution in the direction of circularity 

 and symmetry in the course of the aggregation of the scattered 

 matter. How this might have come about I have endeavored to 

 determine. 



In the initial stages the orbital ellipses of the nuclei and of the 

 innumerable planetesimals were, by reason of their common origin, 

 rudely concentric. They were, to be sure, more or less discordant 

 in form and in attitude from the effects of unequal projection, of 

 differential expansion of the solar matter when set free by projection, 

 and of the collisions of the constituent planetesimals ; but all of this 

 was subordinate to a general concentric arrangement of the ellip- 

 tical paths. Under the laws of celestial mechanics, these paths 

 must have been constantly modified by the different attractions of 

 the different portions of the nebula. The axes of the orbits must 

 16 



