AGE OF MOUNTAIN SYSTEMS. 



of the catastrophe was between the deposition of the strata b 

 and c. 



In like manner if it be found that while the strata a, b, and c 

 are all uplifted and inclined, d is horizontal, fig. 115, it is 

 inferred that the date of the catastrophe was between the periods 

 of the deposition of d and c, and so on. 



It will be evident that we have here assumed that the strata 

 a, &, c, d, &c. are in the natural succession of strata, in the order 

 of geological time, found in any complete section of the earth's 

 crust, in which no strata are deficient. 



197. It must also be observed that the direction of the inclina- 

 tion of the strata thus uplifted, corresponds with and determines 

 the direction of the ridge of mountains upon the flanks of which 

 they lie, and it has resulted from the extensive and profound 

 researches of M. Elie de Beaumont, that the chains of mountains 

 in general whose directions are parallel have the same geological 

 date, as is proved by the strata inclined upon their sides and 

 horizontal at their base. Mountain ranges, therefore, which 

 until the discovery of this important law were regarded as 

 geologically distinct and independent, are now brought into the 

 same system. Each mountain system, therefore, must be 

 regarded not as a single chain, but as a number of parallel chains 

 which may be near or distant from each other within any as- 

 signable limits. It. may also be observed, that the parts even of 

 the same chain are not always continuous, but may be broken by 

 intervals along which, as it were, the crust sinks to the level of 

 the surrounding plain. 



198. The systems of mountains which have thus been grouped 

 according to their geological dates have usually received denomi- 

 nations from some remarkable locality in which their prevalence 

 is most conspicuous. Thus one is called the system of the 

 Pyrenees, another the system of the principal Alps, another the 

 system of the western Alps, and so on. 



The different convulsions which have taken place upon the 

 surface of the globe, and which have produced the several 

 mountain systems, seem to have been always sudden. In effect 

 at some distance from the place where the discordance of the 

 stratification manifests former convulsions, the same strata are 

 found concordant and horizontal, from whence it follows, that in 

 such cases the sedimentary deposition has not been suspended, 

 the disturbance of the crust has been local, and the interval 

 during which it has prevailed has necessarily been short. 



199. During each successive geological period, the earth has 

 been differently divided into land and water, the continents and 

 islands of one period being submerged during another, and the 



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