FUNDAMENTAL PROBLEMS OF GEOLOGY. 233 



the early progress of investigation ; (2) to furnish a basis for de- 

 ducing the hypothetical stages of the earth that preceded its known 

 history, and for drawing thence inferences as to the conditions of 

 the interior which the earth inherited from the mode of its birth; 

 and (3) to stimulate inquiry into the elements involved. In short, 

 I have endeavored to give the hypothesis a working form under the 

 conviction that so long as the complicated elements involved remain so 

 imperfectly determined as at present its working value is its chief value. 



To bring out the geological bearings of the planetesimal hj'pothesis, 

 I have given considerable time to a study of the probable stages of 

 growth of the early earth, of the time and mode of introduction of 

 the atmosphere and hydrosphere, and of the initiation of the great 

 topographic features, together with the leading modern processes. 

 While it is clear that there may be a somewhat wide range of sub- 

 hypotheses relative to these stages as to the earlier, it was thought 

 best, as before, to develop a single line definitely. The line selected is 

 in direct sequence to that chosen for the earlier stages, so that there 

 should be no resting back on factors not previously introduced, and so 

 that the whole should be consistent. Of course, the complete scheme 

 contemplates the development of the alternative sub-hypotheses. 



Following the postulates of the previous sketch, a nebular knot is 

 assumed to have been the nucleus of the growing earth. It has 

 not been thought important to consider at much length the special 

 state of organization of the material of this nucleus, since by assump- 

 tion it constituted but a minor part of the grown planet, and its 

 ultimate condition would probably be that of the dominant mass, or, 

 if not, would be so deeply central as to have little geologic impor- 

 tance. Assuming that the nuclear mass was quite small, it is inferred 

 that it was composed chiefly of matter of high molecular weight, since 

 light molecules would be liable to escape because of their velocities. 

 The nucleus is supposed to have been originally an assemblage of 

 planetesimals grouped together by their mutual gravity, and to have 

 passed graduall}- into a solid mass in connection with the capture of 

 outside planetesimals. As the planetesimals were solid aggregates 

 in the main, and only partially elastic, their collisions are assumed to 

 have destroyed their orbital motions in a certain proportion of cases and 

 to have led to their collection at the center. In other cases the orbital 

 motions were doubtless increased, but any planetesimals which were 

 thus temporarily driven away were subject to subsequent capture. 



As the solid nucleus thus formed mav not have been massive 



