310 HISTORICAL GEOLOGY 



condensed into a ball, which became the outermost planet 

 (Neptune). Other rings were produced one by one in the 

 same way, until all the planets had come into existence, and 

 the remainder of the original mass was left as the sun. The 

 satellites of the planets were thought to have originated from 

 equatorial rings left behind by the contracting planets be- 

 fore they solidified. On this theory, then, the earth began as 

 a globe of intensely hot vapor which cooled down to a liquid, 

 and finally crusted over with a solid shell of rock, while the 

 interior remained in the molten state. At that stage the sur- 

 face of the earth was so hot that water could exist on it only 

 in the form of vapor. The atmosphere was then, according 

 to the theory, very heavy, hot, and utterly unfit for living 

 things. In time it cooled sufficiently to allow the water vapor 

 to condense and fall as rain. For a long time the surface was 

 still so hot as to boil off the water as it fell. This process 

 gradually cooled the surface, however, and finally its tem- 

 perature was sufficiently low so that the falling rain remained 

 as surface water. Then the oceans began, and as the tem- 

 perature of the water was reduced and the atmosphere be- 

 came somewhat freed from carbon dioxide and other unwhole- 

 some gases, the surface of the globe became fit for living 

 things. 



For a century or more Laplace's theory seemed to fit the 

 facts then known, and it was regarded as essentially true. 

 But we must remember that, from the very nature of the 

 problem, it is unlikely that any theory of the earth's origin 

 can be proved. The more critical studies of recent years 

 have shown that the Laplacian hypothesis presents many 

 serious difficulties. Efforts have been made to meet these by 

 various changes in the details of the theory, but the more 

 serious objections were not removed by any of these modi- 

 fications, and doubt arose even as to the fundamental ideas 

 of the hypothesis. For example, it now seems improbable 

 that the materials which became the planets could have 

 separated from the equatorial portion of the nebula in the 



