BIRTH OF THE MOON 7 



foundly disturbed by tides, produced by the sun ; for as 

 yet there was no moon ; and it has been suggested that 

 one of its tidal waves rose to a height so great as to sever 

 its connection with the earth and to fly off as the infant 

 moon. 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 inclined 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. Professor Darwin's story of the 

 moon is certainly one of the most beautiful contributions 

 ever made by astronomy to geology, and we shall all 

 concur with him when he says, " A theory reposing on 

 verce causa, which brings into quantitative correlation the 

 length of the present day and month, the obliquity of the 

 ecliptic, and the inclination and eccentricity of the lunar 

 orbit, must, I think, have strong claims to acceptance." 



The majority of geologists have long hankered after a 

 metallic nucleus for the earth, composed chiefly, by 

 analogy with meteorites, of iron. Lord Kelvin has 

 admitted the probable existence of some such nucleus, 

 and lately Professor Wiechert has furnished us with 

 arguments "powerful" arguments, Professor Darwin 

 terms them in. support of its existence. The interior of 

 the earth for four-fifths of the radius is composed, accord- 

 ing to Professor Wiechert, chiefly of metallic iron, with a 

 density of 8'2 ; the outer envelope, one-fifth of the radius, 

 or about 800 miles in thickness, consists of silicates, such 

 as we are familiar with in igneous rocks and meteorites^ 



