TRANSACTIONS OF SECTION C. 715 



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 

 theearth,datiDg:,accordingtoProfessorJoly, from hetween 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, according to Lord Kelvin, that the temperature continuously increased from 

 the surface, where it was 1170° C, down to a depth of twenty-five miles, where 

 it was about 1430° C, or 260° C. above the fusion poiut of the matter forming 

 the crust. 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 time the earth's 

 surface had cooled down to 370°C. the depth beneatli 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 1'718 miles, the average depth of the 

 oceans beinsr taken at 2-393 miles. Thus the average pressure 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 1'71S miles 

 to 2-393 miles, while that over those portions of the crust that now formed the land 

 would be diminished by 1-718 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 establi.'-hed. Under these 

 circumstances the pressure of the ocean would be just able to maintain a column 

 of rock 0-886 mile in height, or ten twenty-sevenths ol its own depth. It could do no 

 more ; but in order that the dry land may appear some cause must be found com- 

 petent 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 efl'ect of the reduction in pressure over the con- 

 tinental areas. Previous to the condensation 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 contributor}' 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 pressure, 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 expansion : 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 

 compelled 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 difl'erenee in volume of the rock in the two 



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

 rock is a fresh doleritc without olivine. 



