EVOLUTIONAL GEOLOGY. 293 



planet was profoundl}^ di.stiirlxxl ))y tides produced b}- 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 ofl' 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 ll"^ or 12'-' to the ecliptic. The 

 time wdiich has elapsed since the moon occupied a position nine terres- 

 trial radii distant from the earth is at least fifty-six to fift}^ -seven mil- 

 lions of years, but mav have been nmch 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 vera? causte, 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, 1 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 argu- 

 ments — "powerful" arguments, Professor Darwin terms them — in 

 support of its existence. The interior of the earth for four-fifths of 

 the radius is composed, according to Professor Wiechert, chiefly of 

 metallic iron, with a density of 8.2; the outer envelope, one-fifth of 

 the radius, or about -±00 miles in thickness, consists of silicates, such 

 as we are familiar with in igneous rocks and meteorites, and possesses 

 a density of 3. 2. It was from this outer envelope when molten that the 

 moon was trundled off", 27 miles in depth going to its formation. The 

 density of this material, as we have just seen, is supposed to be 3.2; 

 the density of the moon is 3.39, a close approximation, such difference 

 as exists being completely explicable by the comparatively low tem- 

 perature of the moon. 



The outer envelope of the earth which was drawn off to form the 

 moon was, as we have seen, charged with steam and other gases under 

 a pressure of 5,000 pounds to the square inch; but as the satellite wan- 

 dered away from the parent planet ttis pressure continuously dimin- 

 ished. Under these circumstances the moon would become as explosive 

 as a charged bomb, steam would burst forth from numljerless volca- 

 noes, and while the face of the moon might thus have acquired its 

 existing features, the ejected material might possibly have been shot 

 so far away from its origin as to have acquired an independent orbit. 

 If so, we may ask whether it may not be possible that the meteorites, 



