236 PROGRESS OF SCIENCE IN THE CENTURY. 



great steps in the early history of the Earth before 

 it became fit to be a home of life, and also to reach 

 from physical and astronomical data some secure 

 conclusion as to the present physical state of the 

 Earth's interior. 



Chapters in the Ancient History of the Earth. 

 The Earth probably had its beginning as one of the 

 many rings swirled off from the great nebular mass 

 which gradually condensed into our sun ; but other 

 origins are conceivable. In any case, it had a be- 

 ginning as a rapidly rotating molten planet It solid- 

 ified about the centre into a metallic nucleus, which 

 was probably composed in great part of iron ; it was 

 surrounded by a deep atmosphere, the larger part of 

 which has been condensed into the waters of our 

 present seas. Its molten ocean was profoundly dis- 

 turbed by solar tides, for there was as yet no moon, 

 and it was perhaps a particularly high tide which 

 made the earth give birth to its satellite. 



" This event may be regarded as marking the first 

 critical period, or catastrophe if we please, in the 

 history of our planet. The career of our satellite, 

 after its escape from the earth, is not known till it 

 attained a distance of nine terrestrial radii; after this 

 its progress can be clearly followed. At the eventful 

 time of parturition the earth was rotating, with a 

 period of from two to four hours, about an axis in- 

 clined at some 11 or 12 to the ecliptic. The time 

 which has elapsed since the moon occupied a position 

 nine terrestrial radii distant from the earth is at least 

 fifty-six to fifty-seven millions of years, but may have 

 been much more." * 



" The outer envelope of the earth drawn off to 

 form the moon was charged with steam and other 



*W. J. Sollas, Pres. Address, Sec. C, Hep. Brit. Ass., 1900; 

 Nature, 13th Sept., 1900, p. 482. 



