to Section C {Geology). 451 



might of itself lead to convectional currents in the molten ocean. 

 The effect on the atmosphere is very difficult to trace, but it is 

 obvious that if a high-pressure area originated over some cooler 

 region of the ocean, the winds blowing out of it would drive before 

 them the cooler superficial layers of molten material, and as these 

 were replaced by hotter lava streaming from below, the tendency 

 would be to convert the high- into a low-pressure area, and to 

 reverse the direction of the winds. Conversely, under a low- 

 pressure area, the in-blowing winds would drive in the cooler 

 superficial layers of molten matter that had been swept away from 

 the anticyclones. If the difference in pressure under the cyclonic 

 and anticyclonic areas were considerable, some of the gas absorbed 

 under the anticyclones might escape beneath the cyclones, and in 

 a later stage of cooling might give rise to vast floating islands of 

 scoria. Such islands might be the first foreshadowings of the future 

 continents. Whatever the ultimate effect of the reaction of the 

 winds on the currents of the molten ocean, it is probable that some 

 kind of circulation was set up in the latter. The universal molten 

 ocean was by no means homogeneous : it was constantly undergoing 

 changes in composition aa it reacted chemically with the internal 

 metallic nucleus : its currents would streak the different portions 

 out in directions which in the northern hemisphere would run from 

 N.E. to S.W., and thus the differences which distinguish particular 

 petrological regions of our planet may have commenced their 

 existence at a very early stage. Is it possible that as our know- 

 ledge extends we shall be able by a study of the distribution of 

 igneous rocks and minerals to draw some conclusions as to the 

 direction of these hypothetical lava currents ? Our planet was 

 profoundly 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 verm caiisce, 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, 



