the Earth's Contraction from Cooling t 53 



larger ranges of uplifts and effects of heat occur on the oceanic 

 slope of the principal border mountain-chain instead of the con- 

 tinental slope, favouring the view that this lateral thrust was 

 more effective in the direction from the ocean against the con- 

 tinents than in the opposite. Finally, there is the fact that the 

 disturbances or effects of lateral thrust have been very much the 

 greatest on the border of the largest oceans. 



But has this greater effectiveness of lateral thrust from the 

 direction of the ocean been due to a proportionally greater con- 

 traction and subsidence of the oceanic crust than the continental ? 

 — the sinking causing the oceanic arch to press against the sides 

 of the basin. I formerly made this the chief means of moun- 

 tain-lifting ; and now, while not giving it so great prominence, 

 I believe it to be a true cause. It is certain that the depressing 

 of the ocean's bed, like the raising of the continental areas, has 

 been in progress through the ages. The great principal rise of 

 the continent and continental mountains took place after the 

 Cretaceous period or during the Tertiary, and some of it even 

 in the Quaternary ; and this is almost positive demonstration 

 that the bottoms of the oceans were tending downward contem- 

 poraneously. It is not possible in the nature of contraction 

 that it should have been all accomplished in these basins at the 

 beginning of their existence — a point I shall further illustrate 

 when discussing the nature of the earth's interior. Moreover 

 the mobile waters that occupy the oceanic depressions would 

 have given important aid in the cooling of the underlying crust. 

 It is to be noted also that the distance between the axis of the 

 Appalachians in North America and the opposite (African) side 

 of the Atlantic is 4000 miles, and that between the axis of the 

 Rocky Mountains and the opposite (Australian) coast of the 

 Pacific is over 7000 miles ; while between the axis of the Appa- 

 lachians in Virginia and that of the Rocky Mountains in the 

 same latitude the distance is hardly 1500 miles. Hence the 

 contraction was absolutely greatest over the oceanic areas inde- 

 pendently of any result from special causes ; and if the generated 

 pressure was not expended in uplifts over the oceanic areas 

 themselves, it would be in uplifts on its borders. 



In addition to the above advantage which the oceanic areas 

 have had in the making of border oscillations, the lower position 

 of the oceanic crust, and the abruptness with which the sides 

 fall off, give it an opportunity to push beneath the sides of the 

 continents; and this would determine the production of such 

 mountains and just such other effects of pressure on the conti- 

 nental borders as actually exist, even if contraction were equable 

 over the globe — that is, were alike in rate over the oceanic and 

 continental areas. It puts the oscillations over the continents 



