XXXIV PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



• 



them. We may regard all the lines of which the formation may be 

 referred to a distinct geological period as belonging to one system. 

 The number of such systems would be equal to the number of 

 recognized geological periods of elevation. What may have been 

 the duration of each period of disturbance, it is impossible to deter- 

 mine. We can only show by geological observation, that those 

 periods must have been respectively comprised between epochs 

 defined by particular geological phsenomena, and may unquestionably 

 in many cases have been of sufficient duration to admit of a series 

 of many consecutive movements. M. de Beaumont, however, has 

 always, I believe, contended for one great, sudden, and instan- 

 taneous movement, which has produced all the phsenomena of 

 elevation referable to the period in which it occurred. I confess 

 myself somewhat surprised at his insisting on this point, which is 

 not essential, I conceive, to his mechanical views on the subject, and 

 his theory of parallelism may be as applicable to the result of a 

 succession of movements during a comparatively short definite 

 period, as to the effects of a single movement. Some of the move- 

 ments might reasonably be supposed to have been sufficiently energetic 

 to stamp at once their impress on the geological character of each 

 district, but I see no adequate reason why the theory should 

 altogether reject the idea of subordinate movements in the same 

 system. 



We have next to inquire ; are these two groupings of the lines of 

 elevation, — the one depending on the dii'ections, the other on the 

 relative ages of those lines, — identical with each other? M. de 

 Beaumont replies in the affirmative ; and it is on the truth of this 

 assertion that the geological value and importance of his theory 

 depend. 



I have already pointed out two essential sources of indetermi- 

 nateness in the grouping of lines of elevation according to their 

 directions, viz. the actual deviation from exact parallelism in the 

 lines of each system, and the imperfect determination of the actual 

 directions of the lines. We must now, in like manner, consider the 

 essential sources of indeterminateness in the grouping of these lines, 

 according to their relative ages. 



If a series of conformable horizontal beds be subjected to an 

 elevatory movement which gives to them a determinate strike, the 

 direction of the strike will be that which characterizes the move- 

 ment. Suppose these beds to be acted on by a second similar 

 movement, which, if the beds were horizontal, would give them a 

 strike (the characteristic direction of this second movement) different 

 from that resulting from the first movement. The actual strike 

 would be intermediate to these two directions, and would not be cha- 

 racteristic of either movement. In all cases in which the last move- 

 ment materially affecting the configuration of a district was of great 

 magnitude, the effects of all minor anterior movements would be 

 nearly obliterated, and those of greater movements considerably mo- 

 dified. Faults, on account of their approximate verticality, would be 

 generally much less affected in direction by any subsequent move- 



