798 



DYNAMICAL GEOLOGY. 



sions, which the presence of water made oceanic areas. Facts prove 

 that they were more or less submerged during the larger part of geo- 

 logical time. But they also announce that the continents and oceans 

 have never changed places ; and, on the contrary, that the continental 

 plateaus and the oceanic depressions have always been working in op- 

 posite directions. The explorations of the ocean's depths have led Sir 

 Wyville Thomson, the chief explorer of these depths, to the conclu- 

 sion, long sustained by the author, that the continents have been a 

 gradual growth through successive sedimentary accumulations over 

 continental, and not over oceanic seas, and by means of other gradual 

 changes — deep-sea deposits having striking peculiarities not found 

 among the strata of the continents. Moreover he regards it as highly 

 probable that the oceanic depressions have only in comparatively re- 

 cent times reached a depth, through the progressing changes, which 

 permitted the formation of the abyssal beds. 



Continents as they now are, in their finished state, instead of being 

 half submerged plateaus, are combinations of two or more polygenetic 

 mountain chains, one on either border, the axis of the chains usually 

 within five hundred miles of the adjoining ocean, and the larger of the 

 mountain chains stands facing the larger of the oceans (p. 23). 



But while the extreme stages — the earliest and latest — are so di- 

 verse, there is fundamental unity, since the courses of mountain ranges 

 and chains over the continents, and the trends of coast-lines and of 

 oceanic islands, have the same general directions with those of the first 

 incipient emerging land of Archaean time. 



3. ORIGIN OF MOUNTAINS, AND OF THE ATTENDANT PHE- 

 NOMENA 



The sources of the phenomena exhibited in mountain regions are 

 primarily dependent on movements in the earth's crust. But many 

 of them have special causes, which experiment and observation illus- 

 trate ; and these may be first considered, before proceeding to the part 

 of the subject that is more or less conjectural. 



1. Explanations of some of the Subordinate Phenomena of Moun- 



tain-making. 



The subordinate phenomena here considered are : 1. Flexures ; 



2. Fractures, Faults, Joints ; 3. Earthquakes ; 4. The Inequilateral 

 Character of Mountains. 



1. Flexures. 



1. Facility of Flexure. — No material is so solid that, when in 

 broad tabular masses, it will not become flexed by lateral pressure 



