82 Lord Kelvin on the Age of the 



of space — any mixing up so thorough as to produce even 

 approximately chemical homogeneousness throughout every 

 layer of equal density. Thus we have no difficulty in under- 

 standing 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 neces- 

 sity Europe, Asia, Africa, America, Australia, Greenland, and 

 the Antarctic Continent, and the Pacific, Atlantic, Indian, 

 and Arctic Ocean depths, as we know them at present. 



§ 32. We may reasonably believe that a very slight degree 

 of chemical heterogeneousness could cause great differences 

 in the heaviness of the snow shower of granules and crystals 

 on different regions of the bottom of the lava ocean when 

 still 50 or 100 kilometres 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 improbable 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 determining cause of the continents and 

 ocean-depths was chemical differences, perhaps 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 kilometres at the deepest. In 

 a very short time, say about twelve years according to our 

 former estimate (§ 24) the whole lava ocean becomes silted 

 up to its surface. 



§ 34. We have not time enough at present to think out 

 all the complicated actions, hydrostatic and thermodynamic, 

 which must accompany, and follow after, the cooling of the 

 lava ocean surrounding our ideal primitive continent. Bv 

 a hurried view, however, of the affair we see that in virtue 

 of, let us say, 15 per cent, shrinkage by freezing, the level 

 of the liquid must, at its greatest supposed depth, sink six 

 kilometres relatively to the continents : and thus the liquid 



