Ch. XXIV.] 
OF MOUNTAIN-CHAINS. 
341 
If we then discover another chain, B, 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 * 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 
