Ch. XXIV.] 



OF MOUNTAIN-CHAINS. 341 



If we then discover another chain, 13, in which we find not 

 only the formation b, but the group c also, disturbed and 



No. 83. 



thrown on its edges, we may infer that the latter chain is of 

 subsequent date to A ; for B was elevated after the deposition 

 of c, and before that of the group d ; whereas A originated 

 before the strata c were formed. 



In order to ascertain whether other mountain ranges are of 

 contemporaneous date with A and B, or whether they are 

 referrible to distinct periods, we have only to inquire whether 

 the geological phenomena are identical, namely, whether the 

 inclined and undisturbed sets of strata correspond to those in 

 the types above mentioned. 



Objections to M. de Beaumont's theory. Now all this rea- 

 soning is perfectly correct, so long as the particular groups of 

 strata b and c are not confounded with the geological periods 

 to which they may belong, and provided due latitude is given 

 to the term contemporaneous ; for it should be understood to 

 allude not to a moment of time, but to the interval, whether 

 brief or protracted, which has elapsed between two events, 

 namely, between the accumulation of the inclined and that of 

 the horizontal strata. 



But, unfortunately, the distinct import of the terms ' for- 

 mation ' and c period ' has been overlooked, or not attended to 

 by M. de Beaumont, and hence the greater part of his proofs 

 are equivocal, and his inferences uncertain; and even if no 

 errors had arisen from this source, the length of some of his 

 intervals is so immense, that to affirm that all the chains raised 

 in such intervals were contemporaneous, is an abuse of language. 



In order to illustrate our argument, let us select the 

 Pyrenees as an example. This range of mountains, says M. 

 de Beaumont, rose suddenly (a un seul jet) to its present 



