EVOLUTIONAL GEOLOGY. 295 



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 beneath the surface at 

 which the pressure just kept the crust solid would have sunk some 

 slight distance inward, but not sufficientl}' 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 being taken at 2.303 

 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 dr}' land appeared," then the pressure over the ocean floor would 

 be increased from 1.718 miles to 2.393 miles, Avhile that over those 

 portions of the crust that now formed the land would be dmiinished 

 by 1.718 miles. This difference in pressure would tend to exaggerate 

 those faint depressions which had arisen under the primitive anticy- 

 clonic areas, and if the just solidilied 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 established. Under these circumstances the pressure of the 

 ocean would be just able to maintain a column of rock 0.880 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 l)ed the remaining seventeen 

 twenty-sevenths of its full depth or to raise the continental bulgings 

 to the same extent. Such a cause may, I thiidv. 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 contrib- 

 utory to that which caused the outer 25 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 liquefled 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 



