^909.] SEE— THE PAST HISTORY OF THE EARTH. 123 



nebula, have a tendency to settle towards the center more rapidly 

 than large planets. In our system the asteroids have been gathered 

 into their present position partly by the effects of resistance, and 

 partly by the disturbing action of Jupiter, which throws them into 

 the stable region within his orbit. When the paths of the asteroids 

 ■cross his orbit, the motion is shown to be unstable, and therefore 

 such overlapping orbits are temporary and not permanent. 



It follows, therefore, that the orbit of the earth was originally 

 much larger and much more eccentric than at present. The earth 

 may have begun to form almost as far away as Jupiter's orbit, or 

 -even beyond it. In time the primordial earth was thrown within 

 that orbit, where the asteroids now revolve. Thus the earth re- 

 volved in safety and continued to grow by gathering up more and 

 more cosmical dust. The history of Mars was similar. The major 

 axis of the orbit was decreased by the effects of resistance, and at 

 the same time the eccentricity steadily diminished, till we have the 

 planets as they are to-day. This is as certain as anything can be, 

 and it throws an interesting light on the past history of our earth. 

 While the information thus given us is meager, it is, so far as I 

 "know, our only means of fathoming the mystery which has always 

 surrounded the origin of our planet. 



We may therefore say that in the beginning the earth was a 

 small body like one of the asteroids; it then revolved in a much 

 larger and more eccentric orbit than at present, and was augmented 

 gradually by the sweeping up of cosmical dust in its ceaseless motion 

 around the sun. In general, this process of building up the earth 

 was excessively slow, though at times the motion through streams 

 may have given larger additions of matter ; but the full process may 

 have occupied a billion years. Of course, geological history began 

 only after the earth had attained about its present dimensions. And 

 the study of the crust of the globe shows that no large additions 

 to the matter of our planet have been made since geological history 

 began. The sedimentary rocks are not filled with any considerable 

 amount of meteoric matter precipitated from the heavenly spaces. 



From these considerations it follows that the earth was built 

 up very gradually by accretion ; and that this growth took place 

 because our globe was revolving in a resisting medium made up of 



