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APPENDIX. 



motion on the surface of the molten matter upon which the earth's crust is supported. 

 In the first instance he supposes the strata of the region affected to have been sub- 

 jected to excessive tension, arising from the expansion of solid matter and of vapours ; this 

 tension is relieved by linear fissures, and the sudden release of pressure adjacent to 

 these lines of fracture produces violent pulsations on the surface of the liquid below. 

 A tangential force would take advantage of these regular undulations to produce folds 

 and reverse faults such as Mr. Rogers describes. The generalized section of the Alps (Eig. 

 24), as conceived by Mr. Rogers, exemplifies well this authov's views of the flexures of strata, 



Eig. 24. 







Generalised section of the Alps. — (Rogers.) 

 and their relations to mountain formation. In this figure there are four belts of closely folded 

 waves, each belt having its axis planes dipping towards the centres of their own high mountain 

 system. 



In Mr. Rogers' application of his theory to the Appalachian mountains he has omitted to 

 discuss a question of considerable importance. Each of the theories we have examined sup- 

 poses a zone of maximum intensity of action (geological effect). Indeed it seems to me a 

 necessary condition of every theory of elevation. As a consequence, and with any thing like 

 homogeneous conditions of resistance, we should expect some approach to a bilateral symme- 

 try in the resulting disturbance of the rocks. In the sections of the Wealden and of the Alps we 

 see examples of such a symmetry. The more a theory adopts the supposition of intense inter- 

 nal action and a consequent subordination of superficial influences of resistance, the more 

 does it involve this symmetry. Mr. Rogers' theory is eminently of this nature. But it is his 

 facts and not his theory of which we are to seek an explanation. In his section of the Alle- 

 ghanies there is a steadily increasing intensity of disturbance in the strata towards the east and 

 south-east border, even to beyond the limits of the chain itself. Both the form and the gradation 

 of intensity of the flexures point to. a region external to the chain as the axis of disturbance. 

 The Appalachian chain is therefore wanting in symmetry as a mountain chain, it is 

 essentially one-sided. If we are to accept Mr. Rogers' theory in its full development, 

 we must, I think, to be consistent with it and the facts, look upon the actual Appalachian 

 chain as but a shred of a once far more mighty mountain system, of which the central 

 region of maximum disturbance and elevation stood over what is now the Atlantic slope 

 of the North American continent, the eastern declivities of that chain having been long 

 since removed and depressed beneath the present ocean area. Any one, who had only read 

 Mr. Rogers' reasoning upon the undulation and elevation of strata, would look upon the 

 gneissic and intrusive rocks to the south-east of his Appalachian section as the physical 

 equivalent or. analogue of the similar rocks in the middle region of the Alps, — as the result of 

 the great central upheaval, fracture, and protrusion upon which his theory so strongly 

 insists, and of which the undulations of the Appalachian strata are, according to it, but a 



