12 HEPOET— 1886. 



I wisli to formulate these principles as distinctly as possible, and as 

 the result of all the long series of observations, calculations, and discus- 

 sions since the time of Werner and Hutton, and in which a vast number 

 of able physicists and naturalists have borne a part, because they may be 

 considered as certain deductions from our actual knowledge, and because 

 they lie at the foundation of a rational physical geology. 



We may popularise these deductions by comparing the earth to a 

 drupe or stone-fruit, such as a plum or peach, somewhat dried up. It has 

 a large and intensely hard stone and kernel, a thin pulp made up of two 

 layers, an inner more dense and dark-coloured, and an outer less dense 

 and lighter-coloured. These constitute the under-crust. On the outside 

 it has a thin membrane or over-crust. In the process of drying it has 

 slightly shrunk, so as to produce ridges and hollows of the outer crust, 

 and this outer crust has cracked in some places, allowing portions of the 

 pulp to ooze out — in some of these its lower dark substance, in others its 

 tipper and lighter material. The analogy extends no farther, for there is 

 nothing in our withered fruit to represent the oceans occupying the lower 

 parts of the surface or the deposits which they have laid down. 



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 

 much a depression of the crust of the earth as a flattening of it, and this, 

 as recent soundings have shown, with a slight ridge or elevation along its 

 middle, and banks or terraces fringing the edges, so that its form is not 

 so much that of a basin as that of a shallow plate with its middle a little 

 raised. Its true permanent margins are composed of portions of the over- 

 crust folded, ridged up and crushed, as if by lateral pressure emanating 

 from the sea itself. We cannot, for example, look at a geological map of 

 America without perceiving that the Appalachian ridges, which intervene 

 between the Atlantic and the St. Lawrence valley, have been driven 

 bodily back by a force acting from the east, and that they have resisted 

 this pressure only where, as in the Gulf of St. Lawrence and the Catskill 

 region of New York, they have been protected by outlying masses of very 

 old rocks, as, for example, by that of the island of Newfoundland and that 

 of the Adirondack Mountains. The admirable work begun by my friend 

 and fellow-student Professor James Nicol, followed up by Hicks, Lap- 

 worth, and others, and now, after long controversy, fully confirmed by 

 the recent observations of the Geological Survey of Scotland, has shown 

 ihe most intense action of the same kind on the east side of the ocean in 

 the Scottish highlands ; and the more widely distributed Eozoic rocks of 

 Scandinavia may be appealed to in further evidence of this.^ 



If we now inquire as to the cause of the Atlantic depression, we 



' Address to the Geological Section, by Prof. Judd, Aberdeen Meeting, 1885. 

 According to Kogers, the Crumpling of the Appalachians has reduced a breadth of 

 158 miles to about 60. 



