BIRTH OF THE OCEAN 11 



move from under the ocean into the bulgings which were 

 rising to form the land, until static equilibrium were 

 established. 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 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 

 tw r enty-sevenths of its full depth or to raise the conti- 

 nental 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 

 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 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 re- 

 lieved of this load ? Plainly a liquefaction and corre- 

 sponding 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 expla- 

 nation 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 tem- 

 perature 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 



