CHAPTER XXI 



ORIGINAL CONDITION OF THE EARTH — PRE-CAMBRIAN 

 PERIODS 



As we trace the history of mankind back to very ancient times, 

 we find that the records become more and more scanty and less 

 intelligible, until history fades into myth and tradition. Of a 

 still earlier age we have not even a tradition ; it is prehistoric. 

 Similarly, among the geological records the earliest are in a 

 state of such excessive confusion that they are exceedingly diffi- 

 cult to understand, and between different observers there are 

 radical differences of opinion both as to the facts and as to their 

 interpretation. Furthermore, there must have been an inconceiv- 

 ably long time earlier than the most ancient recorded periods, as 

 to which conjecture and inference are the only resource. In these 

 difficult straits astronomy offers valuable assistance to the baffled 

 geologist. The Nebular Hypothesis is a scheme of the develop- 

 ment of the solar system, which, though not yet demonstrated, nor 

 free from difficulties, so well explains the known facts that it is 

 almost universally accepted as essentially true. 



According to this hypothesis the place of the present solar system 

 was originally occupied by a vast rotating nebula, a mass of in- 

 tensely heated vapour, or possibly clouds of meteorites, extending 

 beyond the orbit of the outermost planet. As the nebula cooled 

 by radiation, it contracted, leaving behind it successive rings, like 

 those of the planet Saturn, but on a vastly larger scale. The rings 

 kept up the rotation imparted by the nebula, and all of them lay 

 in nearly the same plane. Unequal contraction in various parts of 

 each revolving ring caused it to break up and gather by mutual 

 attraction into masses. If these rings were composed of relatively 

 small solid masses, like meteorites, or if thev had solidified by con- 



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