482 



NATURE 



[September 13, 1900 



geology is by definition a geologist. Our only regret is, not that 

 physicists occasionally invade our borders, but that they do not 

 visit us oftener and make closer acquaintance with us. 



Early History of the Earth : First Critical Period, 

 If I am bold enough to assert that cosmogony is no longer 

 alien to geology, I may proceed further, and taking advantage 

 of my temerity pass on to speak of things once not permitted to 

 us. I propose, therefore, to ofifer 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 lor astronomers. We 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 observed to penetrate our atmo- 

 sphere and to fall on to the surface of tha 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 5000 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 radia- 

 tion would be in excess. This might of itself lead to convec- 

 tional currents in the molten ocean. The efTect 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 

 trom 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 fore- 

 shadowings 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 homo- 

 geneous : it was constantly undergoing changes in composition 

 as 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 north-east to 

 south-west, 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 satel- 

 lite, 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 in- 

 clinedatsome 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. Prof. Darwin's story of the 

 moon is certainly one of the most beautiful contributions ever 



NO. 161 I, VOL. 62] 



made by astronomy to geology, and we shall all concur with 

 him when he says, "A theory reposing on vera cansce, which 

 brings into quantitative correlation the length of the present 

 day and month, the obliquity of the ecliptic, and the inclinatioD 

 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, of iron. Lord Kelvin has admitted the probable 

 existence of sorne such nucleus, and lately Prof Wiechert has 

 furnished us with arguments — " powerful " arguments Prof. 

 Darwin terms them— in support of its existence. The interior 

 of the earth for four-fifths of the radius is composed, according 

 to Prof. Wiechert, chiefly of metallic iron, with a density of 

 8 '2; the outer envelope, one fifth of the radius, or about 400 

 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, 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 3 "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 5000 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 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, which 

 sometimes descend upon our planet, are but portions of its owr> 

 envelope returning 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 stage 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 

 surface 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 move- 

 ments of the atmosphere should be unsteady, and definite 

 regions of high and low pressure might have l)een 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 emlx)ssed." ^ 



Third 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 become 

 filled with superheated water, and the pools so formed would 

 expand and deepen, till they formed the oceans. This is the 

 third critical stage in the history of the earth, dating, according 

 to Prof Joly, from between eighty and ninety millions of years 

 ago. With the growth of the oceans the distinction between 

 land and sea arose— in what precise manner we may proceed to 

 inquire. If we revert to the period of the "consistentior 

 status," when the earth had just solidified, we shall find, ac- 

 cording to Lord Kelvin, that the temperature continuously 

 increased from the surface, where it was II70°C., down to a 

 depth of twenty-five miles, where it was about 1430° C, or 

 260° C. above the fusion point of the matter, forming the crust. 



1 It would be difficult to discuss with sufficient brevity the probable dis- 

 tribution of these inequalities, 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. 



