750 



SCIENCE. 



[N. S. Vol. XII. No. 307. 



ical stage in the history of the earth, dating 

 according to Professor Joly, from between 

 eighty and ninety millions of years ago. 

 With the growth of the oceans the distinc- 

 tion between land and sea arose — -in what 

 precise manner we may proceed to inquire. 

 If we revert to the period of the ' consist- 

 entior 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 

 1,170° C, down to a depth of twenty-five 

 miles, where it was about 1,430° C, or 

 260° C. above the fusion point of the matter 

 forming a crust. That the crust at this 

 depth was not molten but solid is to be ex- 

 plained 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 be- 

 neath the surface at which the pressure 

 just kept the crust solid would have sunk 

 some slight distance inwards, but not suffi- 

 ciently to aifect our argument. 



The average pressure of the primitive at- 

 mosphere 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 being 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 oceans were all ' gathered 

 together into one place,' and ' the dry land 

 appeared,' then the pressure over the ocean 

 floor would be increased from 1.718 miles 



to 2.393 miles, while that over those por- 

 tions of the crust that now formed the land 

 would be diminished by 1.718 miles. This 

 difference in pressure would tend to exag- 

 gerate 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 es- 

 tablished. Under these circumstances the 

 pi-essure of the ocean would be just able to 

 maintain a column of rock 0.886 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 ex- 

 tent. Such a cause may, I think, be dis- 

 covered in a further effect of the reduction 

 in pressure over the continental 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 contributory to that 

 which caused the outer twenty-five miles 

 of the earth's crust to become solid ; it fur- 

 nished, 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 liquefac- 

 tion and corresponding expansion of the 

 underlying rock. 



But we will not go so far as to assert that 

 actual liquefaction would result ; all we re- 

 quire 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 compelled into the 



