714 REPORT— 1900. 



since tbe moon occupied a position nine terrestrial radii distant from the eartli is 

 at least fit\y-six to lifty-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 quanti- 

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

 ecliptic, and the inclinatiou and eccentricity of the lunar orbit, must, I think, 

 have strong claims to acceptance.' 



The majority of geologists have long hanlcered 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, according to Professor Wiechert, chiefly of metallic 

 iron, with a density of 8'2 ; tlie outer envelope, one fifth of the I'adius, or about 

 400 miles in thicljuess, consists of silicates, such as we are familiar with in 

 igneous rocks and meteorites, and possesses a density of 3'2. It was frorn this 

 outer envelope when molten that the moon was trundled oft", twenty-seven 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 o-.39, a close approximation, 

 such difference as exists being completely explicable by the comparatively low 

 temperature 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 lb. 

 to the square inch ; but as the satellite wandered away from the parent planet 

 this pressure continuously diminished. Under these circumstances the moon 

 would become as explosive as a charged bomb, steam would burst forth from 

 numberless volcanoes, and while the face of the moon might thus have ac- 

 quired 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, which sometimes 

 descend upon our planet, are but portions of its own envelope returniug to it. The 

 facts that the average specific gravity of those meteorites which have been seen to 

 fall is not much above 3'2, and that they have passed through a state of fusion, 

 are consistent with this suggestion. 



Second Critical Period. ' Consistentior Status.' 



The solidification of the earth probably became completed soon after the birth 

 of the moon. The temperature of its suriace at the time of consolidation was about 

 1170° C, and it was therefore still surrounded by its primitive deep atmosphere of 

 steam and other gases. This was the second critical period in the history of the 

 earth, the stage of the ' consistentior status,' the date of which Lord Kelvin would 

 rather know than that of the Norman Conquest, though he thinks it lies between 

 twenty and forty millions of years ago, probably nearer twenty than forty. 



Now that the crust was solid there was less reason why movements of the 

 atmosphere should be unsteadv, and deflnite regions of high and low pressure 

 might have been established. Under the high-pressure areas the surface of the 

 crust would be depressed ; correspondingly under the low-pressure areas it would 

 be raised; and thus from the first the surface of the solid earth might be dimpled 

 and embossed.^ 



TJiird Critical Period. Origin of the Oceans. 



The cooling of the earth would continuously progress, till the temperature of 

 the surface fell to 370° C, when that part of the atmosphere which consisted of 

 steam would begin to liquefy; then the dimples on the surface would soon 



' It would be difficult to discuss with sufficient brevity the probable distribution 

 of these inequafities, but it may be pointed out that the moon is possibly responsible, 

 and that in more ways than one, for much of the existing geographical asymmetry. 



