Phenomena in South America. 625 



and elevate unequally portions of it, could not have effected the observed 

 effects. It is, however, a subordinate question, whether there exist proofs 

 of paroxysmal violence in some mountain-chains; the important fact which 

 appears to me proved, is, that there is a power now^ in action, and which has 

 been in action with the same average intensity (volcanic eruptions being the 

 index) since the remotest periods, not only sufficient to produce, but which 

 almost inevitably must have produced, unequal elevation on the lines of frac- 

 ture. 



Theoretical Considerations on the slow Elevation of Mountain- Chains. 



The conclusion that mountain-chains are formed by a long succession of 

 small movements, may, as it appears to me, be rendered also probable by 

 simple theoretical reasoning. Mr. Hopkins has demonstrated, that the first 

 effect of equably elevating a longitudinal portion of the crust of the earth, is 

 to form fissures, parallel to the longer axis (with others transverse to them, 

 which may here be neglected) of the kinds represented in the accompanying 

 diagram (No. I.), copied from that published in the Cambridge Philosophical 



Transactions. But he further shows, that the square masses, now disjointed, 

 will, — from the extreme improbability of the force uplifting them, when sepa- 

 rate, equably, or from their settling down afterwards, — assume some such posi- 

 tion as that given in Diagram No. 2. In the Cordillera, which may be taken 



as a good example of the structure of a great mountain-chain, the strata in 

 the central parts are inclined more commonly at an angle above 45", than 

 beneath it; and very often they are absolutely vertical. The axis of the lines 

 of dislocation is formed of syenitic and porphyritic masses, which, from the 



