THE HISTORY OF THE NORTH ATLANTIC 65 



tritus has been laid down upon it, and where, consequently, the 

 crust has been softened and depressed. We must beware, in 

 this connection, of exaggerated notions of the extent of con- 

 traction and of crumpling required to form mountains. Bonney 

 has well shown, in lectures delivered at the London Institu- 

 tion, that an amount of contraction, almost inappreciable in 

 comparison with the diameter of the earth, would be sufficient ; 

 and that, as the greatest mountain chains are less than -g^^th 

 of the earth's radius in height, they would, on an artificial 

 globe a foot in diameter, be no more important than the slight 

 inequalities that might result from the paper gores overlapping 

 each other at the edges. This thinness of the crushed crust 

 agrees with the deductions of physical science as to the 

 shallowness of the superficial layer of compression in a cooling 

 globe. It is perhaps not more than five miles in thickness. 

 A singular proof of this is seen by the extension of straight 

 cracks filled with volcanic rock in the Laurentian districts of 

 Canada. 1 The beds of gneiss and associated rocks are folded 

 and crumpled in a most complex manner, yet they are crossed 

 by these faults, as a crack in a board may tear a sheet of 

 paper or a thin veneer glued on it. We thus see that the 

 crumpled Laurentian crust was very thin, while the uncrushed 

 sub-crust determined the line of fracture. 



(7) The crushing and sliding of the over-crust implied in 

 these movements raise some serious questions of a physical 

 character. One of these relates to the rapidity or slowness 

 of such movements, and the consequent degree of intensity 

 of the heat developed, as a possible cause of metamorphism 

 of rocks. Another has reference to the possibility of changes 

 in the equilibrium of the earth itself, as resulting from local 

 collapse and ridging. These questions in connection with the 



1 As, for instance, the great dyke running nearly in a straight line from 

 near St. Jerome along the Ottawa to Templeton, on the Ottawa, and be- 

 yond, a distance of more than a hundred miles. 



