September 13, 1900] 



NATURE 



483 



That the crust at this depth was not molten but solid is to be 

 explained by the very great pressure to which it was subjected — 

 just so much pressure, indeed, as was required to counteract the 

 influence of the additional 260° C. Thus if we could have 

 reduced the pressure on the crust we should have caused it to 

 liquefy ; by restoring the pressure it would resolidify. By the 

 lime the earth's surface had cooled down to 370° C. the depth 

 beneath the surface at which the pressure just kept the crust 

 solid would have sunk some slight distance inwards, but not 

 sufficiently to affect our argument. 



The average pressure of the primitive atmosphere upon the 

 crust can readily be calculated by supposing the water of the 

 existing oceans to be uniformly distributed over the earth's 

 surface, and then by a simple piece of arithmetic determining 

 its depth ; this is found to be 1718 miles, the average depth of 

 the oceans being taken at 2-393 miles. Thus the average pres- 

 sure over the earth's surface, immediately before the formation 

 of the oceans, was equivalent to that of a column of water 

 1 718 miles high on each square inch. Supposing that at its 

 origin the ocean were all ' ' gathered together into one place," and 

 *' the dry land appeared," then the pressure over the ocean floor 

 would be increased from 1718 miles to 2-393 miles, while that 

 over those portions of the crust that now formed the land would 

 be diminished by I'7i8 miles. This difference in pressure 

 would tend to exaggerate those faint depressions which had 

 arisen under the primitive anti-cyclonic areas, and if the just 

 solidified material of the earth's crust were set into a state of 

 flow, it might move from under the ocean into the bulgings which 

 were rising to form the land, until static equilibrium were esta- 

 blished. Under these circumstances the pressure of the ocean 

 would be just able to maintain a column of rock 0886 miles in 

 height, or ten twenty-sevenths of its own depth. It could do no 

 more ; but in order that the dry land may appear some cause 

 must be found competent either to lower the ocean bed the 

 remaining seventeen twenty-sevenths of its full depth, or to raise 

 the continental bulgings to the same extent. Such a cause may, 

 I think, be discovered in a further effect of the reduction in 

 pressure over the continental areas. Previous to the condensa- 

 tion of the ocean, these, as we have seen, were subjected to an 

 atmospheric pressure equal to that of a column of water 1 718 

 miles in height. This pressure was contributory to that which 

 caused the outer twenty-five miles of the earth's crust to become 

 solid ; it furnished, indeed, just about one fortieth of that pres- 

 sure, or enough to raise the fusion point 6°C. What, then, 

 might be expected to happen when the continental area was 

 relieved of this load ? Plainly a liquefaction and corresponding 

 expansion of the underlying rock. 



But we will not go so far as to assert that actual liquefaction 

 would result ; all we require for our explanation is a great ex- 

 pansion ; and this would probably follow whether the crust were 

 liquefied or not. For there is good reason to suppose that when 

 matter at a temperature above its ordinary fusion point is com- 

 pelled into the solid state by pressure, its volume is very 

 responsive to changes either of pressure or temperature. The 

 remarkable expansion of liquid carbon dioxide is a case in point : 

 120 volumes of this fluid at — 20° C. become 150 volumes at 

 33° C. ; a temperature just below the critical point. A great 

 change of volume also occurs when the material of igneous rocks 

 passes from the crystalline state to that of glass ; in the case of 

 diabase ^ the difference in volume of the rock in the two 

 states at ordinary temperatures is 13 per cent. If the relief of 

 pressure over the site of continents were accompanied by volume 

 changes at all approaching this, the additional elevation of 

 seventeen twenty-sevenths required to raise the land to the sea- 

 level would be accounted for,'* How far down beneath the sur- 



1 C. Barus so names the material on which he experimented ; apparently 

 the rock is a fresh dolerite without olivine. 



2 Prof. Fitzgerald has been kind enough to express part of the preceding 

 explanation in a more precise manner for me. He writes : " It would re- 

 (|u!re .a very nice adjustment of temperatures and pressures to work out in 

 the simple way you state it ; but what is really involved is that in a certain 

 state diabase (and exprything that changes state with a considerable 

 change of volume) has an enormous isothermal compressibility. Although 

 this is very enormous in the case of bodies which melt suddenly, like ice, 

 it would also involve very great compressibilities in the case of bodies even 

 which melted gradually, if they did so at all quickly,/.^, within a small 

 range of temperature. What you postulate, then, is that at a certain depth 

 diabase is soft enough to be squeezed from under the oceans, and that, 

 being near its melting point, the small relief of pressure is accompanied by 

 an enormous increase in volume which helped to raise the continents. Now 

 that I have written the thing out in my own way it seems very likely. It 

 is, anyway, a suggestion quite worthy of serious consideration, and a pro- 

 cess that in some places must almost certainly have been in operation, and 

 maybe is still operative. Looking at it again, I hardly think it is quite 



NO. 161 I, VOL. 62] 



face the unloading of the continents would be felt it is difficult to 

 say, though the problem is probably not beyond the reach of 

 mathematical analysis ; if it affected an outer envelope twenty- 

 five miles in thickness, a linear expansion of 4 per cent, would 

 suffice to explain the origin of ocean basins. If now we refer to 

 the dilatation determined by Carl Barus for rise in temperature 

 in the case of diabase, we find that between 1093° and 1112° C. 

 the increase in volume is 3*3 per cent. As a further factor in 

 deepening the ocean basins may be included the compressive 

 effect of the increase in load over the ocean floor : this increase 

 is equal to the pressure of a column of water 0-675 n^i'^ 'f* 

 height, and its effect in raising the fusion point would be 2°C., 

 from which we may gain some kind of idea of the amount of 

 compression it might produce on the yielding interior of the 

 crust. To admit that these views are speculative will be to 

 confess nothing ; but they certainly account for a good deal. 

 They not only give us ocean basins, but basins of the kind we 

 want, that is, to use a crude comparison once made by the late 

 Dr. Carpenter, basins of a tea-tray form, having a somewha 

 flat floor and steeply sloping sides ; they also help to explain 

 how it is that the value of gravity is greater over the ocean than 

 over the land. 



The ocean when first formed would consist of highly heated 

 water, and this, as is well known, is an energetic chemical re- 

 agent when brought into contact with silicates iike those which 

 formed the primitive crust. As a result of its action saline solu- 

 tions and chemical deposits would be formed ; the latter, how- 

 ever, would probably be of no'- great thickness, for the time 

 occupied by the ocean in cooling to a temperature not far re- 

 moved from the present would probably be included within a 

 few hundreds of years. 



The Stratified Series. 

 The course of events now becomes somewhat obscure, but 

 sooner or later the familiar processes of denudation and the de- 

 position started into activity, and have continued acting uninter- 

 ruptedly ever since. The total maximum thickness of the 

 sedimentary deposits, so far as I can discover, appears to amount 

 to no less than 50 miles, made up as follows : — 



Feet 



Geologists, impressed with the tardy pace at which sediments 

 appear to be accumulating at the present day, could not con- 

 template this colossal pile of strata without feeling that it spoke 

 of an almost inconceivably long lapse of time. They were led 

 to compare its duration with the distances which intervene 

 between tl^e heavenly bodies ; but while some chose the distance 

 of the nearest fixed star as their unit, others were content to 

 measure the years in terms of miles from the sun. 



Evolution of Organisms. 

 The stratified rocks were eloquent of time, and not to the 

 geologist alone ; they appealed with equal force to the biologist. 

 Accepting Darwin's explanation of the origin of species, the 



likely that there is or could be much squeezing sideways of liquid or other 

 viscous material from under one place to another, because the elastic yielding 

 of the inside of the earth would be much quicker than any flow of this 

 kind. This would only modify your theory, because the diabase that ex- 



f)ands so much on the relief of pressure might be that already under the 

 and, and raising up this latter, partly by being pushed up itself by the 

 elastic relief of the inside of the earth and partly by its own enormous ex- 

 pansibility near its melting point. The action would be quite slow, because 

 It would cool itself so much by its expansion that it would have to be 

 warmed up from below, or by tidal e.irth-squeezing, or by chemical action, 

 before it could expand isotbermally." 



