748 



SCIENCE. 



[N. S. Vol. XII. No. 307. 



surrounded by an atmosphere of great 

 depth, the larger part of which was con- 

 tributed by the water of our present oceans, 

 then existing in a state of gas. This atmos- 

 phere, which exerted a pressure of some- 

 thing like 5,000 pounds to the square inch, 

 must have played a very important part in 

 the evolution of our planet. The molten 

 exterior absorbed it to an extent which de- 

 pended on the pressure, and which may 

 some day be learnt from experiment. Un- 

 der the influence of the rapid rotation of 

 the earth the atmosphere would be much 

 deeper in equatorial than polar regions, 

 so that in the latter the loss of heat by 

 radiation would be in excess. This might 

 of itself lead to convectional currents in 

 the molten ocean. The effect on the at- 

 mosphere is very difficult to trace, but it is 

 obvious that if a high-pressure area origi- 

 nated 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 es- 

 cape 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 con- 

 stantly undergoing changes in composition 



as it reacted chemically with the inter- 

 nal metallic nucleus ; its currents would 

 streak the different portions out in directions 

 which in the northern hemisphere would 

 run from northeast to southwest, and thus 

 the differences which distinguish particu- 

 lar petrological regions of our planet 

 may have commenced their existence at a 

 very early stage. Is it possible that as our 

 knowledge 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 ? Oar planet was profoundly dis- 

 turbed 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 con- 

 nection with the earth and to fly ofi' as the 

 infant moon. This event maj' be regarded 

 as making the first critical period, or catas- 

 trophe 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 parturi- 

 tion 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 terrestial 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 as- 

 tronomy to geology, and we shall all concur 

 with him when he says, " A theory reposing 

 on vercB causoe, which brings into quantita- 

 tive 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 



