])ai-t 1] ANNIVERSARY ADDRESS OF THE PHESIDEXT. Ixxxvii 



to chano-e of form will not be ecjual in o])posite directions, 

 and so the dislocations wliich i-esnlt will not be symmetrical 

 Avith reference to the apparently Hxed and movable parts of the 

 receptacle ; but tliis want of symmetry must be attributed to in- 

 equality of distribution of the resistance, not to an unsymmetrical 

 disposition of pressure or movement. 



In Nature the conditions are further complicated by the fact 

 that the material involved is much less uniform in character 

 throughout the disturbed tract than in the small-scale experiments, 

 and the character of the deformation even less dependent on the 

 direction of the compression, so that if this were in a north-and- 

 south direction the strike of the resulting folds or overthrusts may 

 depart very considerably from the general east-and-west direction. 

 A further complication is introduced by the fact that the rigidly- 

 fixed sides and bottom of the box-shaped receptacle used in the 

 experiment are not i-epeated in Nature, so that there is a possi- 

 bility of relief being found laterally or dowmvards, instead of only 

 upwards ; or there may be compression simultaneously exerted in 

 different, possibly widely different, directions. 



To unravel all these conditions in detail is beyond our ])ower, in 

 the present state of knowledge, but the important point to be 

 remembered is that we may not deduce fi-om the character of the 

 deformation which rocks have undergone anj" conclusion of absolute 

 movement of one side of the compressed tract or of the other ; all 

 that we can learn, from observation within the disturbed tract, is 

 that the horizontal dimensions have undergone diminution, but 

 whether by movement of one side only or of both, measured 

 relatively to some point outside the tract, cannot be determined. 

 The same reasoning and conclusions apply with equal force to the 

 compression indicated by overthrusts, and the deformations Avhich 

 have, at one time or other, been taken as evidence that the upper 

 mass moved over the lower, or the lower under the upper, are seen 

 to be merely disputation about words, for the structures appealed 

 to are the expression of the resistance offered to deformation by 

 the rocks in which the}^ occur. 



As in the case of the direction of movement, it may be a con- 

 venience to accept the common usage, incorrect though it be, when 

 referring to the cause to which the displacements are due, so long 

 as the language is understood to be merely descriptive, and so long 

 as we do not allow ourselves, in further following up the train 

 of thought, to be influenced by the words in which the facts of 



