44 THE POPULAR SCIENCE MONTHLY. 



emission of material from below the over-crust has caused this crust 

 to press downward, and therefore laterally, and so to effect great 

 bends, folds, and plications ; and these, modified subsequently by sur- 

 face denudation, constitute mountain-chains and continental plateaus. 

 As Hall long ago pointed out, such lines of folding have been pro- 

 duced more especially where thick sediments had been laid down on 

 the sea-bottom. Thus we have here another apparent paradox — 

 namely, that the elevations of the earth's crust occur in the places 

 where the greatest burden of detritus 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 contraction and of crumpling required to form mountains. Bonney 

 has well shown, in lectures delivered at the London Institution, 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 one six-hundredth 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. 



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 pos- 

 sibility of changes in the equilibrium of the earth itself as resulting 

 from local collapse and ridging. These questions in connection with 

 the present dissociation of the axis of rotation from the magnetic 

 poles, and with changes of climate, have attracted some attention, and 

 probably deserve further consideration on the part of physicists. 



In so far as geological evidence is concerned, it would seem that 

 the general association of crumbling with metamorphism indicates a 

 certain rapidity in the process of mountain-making, and consequent 

 development of heat, and the arrangement of the older rocks around 

 the Arctic basin forbids us from assuming any extensive movement of 

 the axis of rotation, though it does not exclude changes to a limited 

 extent. I hope that Professor Darwin will discuss these points in his 

 address to the Physical Section. I wish to formulate these princijjles 

 as distinctly as possible, and as the result of all the long series of ob- 

 servations, calculations, and discussions since the time of Werner and 

 Ilutton, and in which a vast number of able physicists and naturalists 

 have borne a part, because they may be considered as certain deduc- 

 tions from our actual knowledge, and because they lie at the founda- 

 tion of a rational physical geology. 



Keeping in view these general conclusions, let us now turn to 

 their bearing on the origin and history of the North Atlantic. 

 Though the Atlantic is a deep ocean, its basin does not constitute so 



