706 



SCIENCE. 



[N. S. Vol. IX. No. 229. 



PROBABLE ORIGIN OP CONTINENTS AND OCEAN 

 DEPTHS OF THE EARTH. 



§ 31. If the shoaling of the lava ocean up 

 to the surface had taken place everywhere 

 at the same time, the whole surface of the 

 consistent solid would be the dead level of 

 the liquid lava all round, just before its 

 depth became zero. On this supposition 

 there seems no possibility that our present- 

 day continents could have risen to their 

 present heights, and that the surface of the 

 solid in its other parts could have sunk 

 down to their present ocean depths, during 

 the twenty or twenty-five million years 

 which may have passed since the con- 

 sistentior status began or during any time 

 however long. Eejecting the extremely 

 improbable hypothesis that the continents 

 were built up of meteoric matter tossed 

 from without, upon the already solidified 

 earth, we have no other possible alternative 

 than that they are due to heterogeneous- 

 ness in different parts of the liquid which 

 constituted the earth before its solidifica- 

 tion. The hydrostatic equilibrium of the 

 rotating liquid involved only homogeneous- 

 ness in respect to density over every level 

 surface (that is to say, surface perpen- 

 dicular to the resultant of gravity and cen- 

 trifugal force); it required no homogeneous- 

 ness in respect to chemical composition. 

 Considering the almost certain truth that 

 the earth was built up of meteorites falling 

 together, we may follow in imagination the 

 whole process of shrinking from gaseous 

 nebula to liquid larva and metals, and 

 solidification of liquid from central regions 

 outwards, without finding any thorough 

 mixing up of different ingredients, coming 

 together from different directions of space — 

 any mixing up so thorough as to produce 

 even approximately chemical homogene- 

 ousness throughout every layer of equal 

 density. Thus we have no difiiculty in 

 understanding how even the gaseous 

 nebula, which at one time constituted the 



matter of our present earth, had in itself a 

 heterogeneousness from which followed by 

 dynamical necessity Europe, Asia, Africa, 

 America, Australia, Greenland and the 

 Antarctic Continent, and the Pacific, At- 

 lantic, Indian and Arctic Ocean depths, as 

 we know them at present. 



§ 32. We may reasonably believe that a 

 very slight degree of chemical heterogene- 

 ousness could cause great differences in the 

 heaviness of the snow shower of granules 

 and crystals on difi'erent regions of the bot- 

 tom of the lava ocean when still 50 or 100 

 kilometers deep. Thus we can quite see 

 how it may have shoaled much more 

 rapidly in some places than in others. It 

 is also interesting to consider that the solid 

 granules, falling on the bottom, may have 

 been largely disturbed, blown as it were 

 into ridges (like rippled sand in the bed of 

 a flowing stream, or like dry sand blown 

 into sand-hills by wind) by the eastward 

 horizontal motion which liquid descending 

 in the equatorial regions must acquire, 

 relatively to the bottom, in virtue of the 

 earth's rotation. It is, indeed, not im- 

 probable that this influence may have been 

 largely effective in producing the general 

 configuration of the great ridges of the 

 Andes and Rocky Mountains and of the 

 West Coasts of Europe and Africa. It 

 seems, however, certain that the main de- 

 termining cause of the continents and 

 ocean-depths was chemical differences, per- 

 haps very slight differences, of the material 

 in different parts of the great lava ocean 

 before consolidation. 



§ 33. To fix our ideas let us now suppose 

 that over some great areas such as those 

 which have since become Asia, Europe, 

 Africa, Australia and America, the lava 

 ocean had silted up to its surface, while in 

 other parts there still were depths ranging 

 down to 40 kilometers at the deepest. In 

 a very short time, say about twelve years 

 according to our former estimate (§ 24), the 



