GENERAL OBSERVATIONS. 401 



Silurian, or Green Mountain, revolution, the cause of the upturning 

 had the following characteristics : — 



1. The force acted at right angles to the general direction of the At- 

 lantic coast, the flexures being approximately parallel to the coast-line. 



2. It acted from the direction of the ocean, the flexures and meta- 

 morphism being greatest on the oceanic side, and fading out toward 

 the interior. 



3. It was slow in action and long continued, a result of movement 

 at the rate of a few feet or yards in a century, the flexures having 

 taken place without obliterating, and hardly obscuring, the stratifica- 

 tion. There may have been sudden starts, and earthquakes beyond 

 modern experience ; but the general course of progress must have 

 been quiet. 



4. Heat was concerned in the changes, or produced by the move- 

 ment ; for several thermal springs exist in Virginia, situated, accord- 

 ing to Rogers, along the axes of the Appalachian folds, as if some 

 traces of the heat still remained. 



5. The force was the same in kind, and also in direction, judging 

 from the identity of results, with that which produced the flexures 

 and other changes that closed Archaean time (p. 155), as well as those 

 of the Medio-Silurian disturbance, and caused the oscillations through 

 the progressing Paleozoic ages required for the completion of the 

 succession of rocks ; the same that occasioned the deep subsidences 

 along the Appalachian region. When the Appalachian subsidences 

 were about to cease, then began the new movement that flexed and 

 stiffened the rocks of the Atlantic border. 



Although there is no proof, in the flexures or the metamorphism, 

 of any emergence of the strata from the ocean during their progress, 

 there is sure evidence that, when the revolution ceased, it left the 

 Appalachian chain with nearly the present elevation. The evidence 

 of this final result of the moving forces is afforded by the strata of 

 Mesozoic time, which come next under consideration. 



In North America, from the close of the Paleozoic, there was a 

 great change in the scene of geological progress, so that the regions 

 are no longer the Eastern Border, the Appalachian, and the great In- 

 terior Continental; but, instead, the Atlantic Border, the Gulf Border, 

 the Western Interior, or interior west of the Mississippi, and the Pacific 

 Border. The Appalachian region and the eastern part of the Interior 

 basin no longer participate in the rock-making. The new regions co- 

 alesce ; the third is but a continuation of the Gulf region to the 

 northwest, over the area of the Rocky Mountains, which was still low 

 or submerged, and it is probable that it communicated directly with 

 the Pacific 



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