TRANSACTIONS OF SECTION C. 713 



Early History of the Earth: First Critical Period. 



If I am bold enough to assert tliat cosmogony is no longer alien to geology, I 

 may proceed fm-tlier, and taking advantage of my temerity pass on to speak of 

 things once not permitted to us. I propose therefore to oiler some short account of 

 the early stages in the history of the earth. Into its nebular origin we need not 

 inquire — that is a subject for astronomers. AVe are content to accept the infant 

 earth from their hands as a molten globe ready made, its birth from a gaseous 

 nebula duly certified. If we ask, as a matter of curiosity, what was the origin of 

 the nebula, I fear even astronomers cannot tell us. There is an hypothesis which 

 refers it to the clashing of meteorites, but in the form in which this is usually 

 presented it does not help us much. Such meteorites as have been ob.served (o 

 penetrate our atmosphere and to fall on to the surface of the earth prove on 

 examination to have had an eventful history of their own of which not the least 

 important chapter was a passage through a molten state; they would thus appear 

 to be the products rather than the progenitors of a nebula. 



We commence our history then with a rapidly rotating molten planet, not 

 impossibly already solidified about the centre and surrounded by an atmosphere 

 of great depth the larger part of which was contributed by the water of our 

 present oceans, then existing in a state of gas. This atmosphere, which exerted a 

 pressure of something like 5,000 lb. 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 depended on the pressure, and which may some day be learnt 

 from experiment. Under 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 atmo- 

 sphere 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 diflerence in pressure under the cyclonic and anti- 

 cyclonic areas were considerable, some of the gas absorbed under the anticyclone8 

 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 foreshadowiugs 

 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 as it reacted 

 chemically with the internal metallic nucleus : its currents would streak the 

 difierent portions out in directions which in the northern hemisphere would 

 run from N.E. to S.AV., 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 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? Oixr planet was pro- 

 foundly disturbed by tides, produced by the sun ; for as yet there was no moon ; 

 and it has been suggested that one of its tidnl 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 



