732 PROCEEDINGS OF THE AMERICAN ACADEMY. 



distance around it. This seems to have occurred with such craters as 

 Tycho and Copernicus. If we go farther back in time and think of the 

 possible building up of the sun and planets of our system by gradual 

 accretions such as are now taking place when a meteor enters our air, 

 but which at some time, as many millions of years ago, occurred on a 

 far grander scale, we can understand the craters on the moon as the last 

 records of such a process, and the Meteor Crater in Arizona as the larg- 

 est known record on the earth, all others belonging to the earliest ge- 

 ological periods having been obliterated. 



In the solar system the survival of the fittest rules. The numerous 

 comets with highly elliptical orbits must succumb and be gathered up by 

 those bodies like the sun, and by the planets which perform their rounds 

 in a less erratic way with orbits nearly circular. If the conclusion be 

 correct that the comets actually belong to and form part of our system, 

 then the thought is permissible that at an early period they were innu- 

 merable and that they have been reduced and obliterated in feeding 

 those masses, like themselves originally, which, however, possessed more 

 conservative orbits, to use that term. 



That the meteoric masses which come to earth are solid and have a 

 structure strongly suggests that they are fragments of a much larger 

 body. 



The solidity suggests either a high temperature or a high pressure 

 due to gravitational forces, or both. Particles floating in free space 

 could never themselves form such dense and solid aggregations. When 

 as in the Arizona crater irons we find diamonds, we are fairly sure of 

 high pressure having existed. When we find also a coarse crystalline 

 structure and segregation we think of a long time of slow cooling. 

 When we see that the solid mass is not only coarse but that the parts 

 are divided by fine crevices or small cracks we infer the sudden relief 

 from high compression, when the material was cold and solid. In short, 

 we infer that we have before us a fragment of a large body which in the 

 long lapse of time had consolidated and cooled, and which later has 

 been torn to pieces, disrupted into fragments large and small. Was it 

 a collision of two large bodies in space, such as may have given rise to 

 a great, irregular nebula like that in Orion 1 Doubtless collisions can 

 occur but they must be very rare. 



The order of nature, however, may easily involve the more or less 

 close passage of large orbs, either still hot or cooled off, past each other 

 at a distance of a few millions of miles. In such a case the stupendous 

 tidal and centrifugal actions attendant on the neutralization of gravity 

 on a line joining the centers of the two bodies must inevitably result 

 in crushing them, as if an irresistible force were exerted in all other di- 



