|)art ij ANiflVURSAEy ADi>KESS OF THE PEESlDENT. IxxxV 



disquisitions of pure philosophy on the question of the possibility 

 of there being such a thing as absolute motion, it is certain that it 

 can only be expressed in terms of displacement, relative to some 

 point which has to be accepted as fixed, and this point must 

 necessarily lie outside the body regarded as in movement. From 

 this it results that we cannot determine the direction of dis- 

 placement of the masses, on either side of the plane of separation, 

 b}^ observations within the region of the displacements ; and, as 

 we have no means of reference to some external point, which can 

 be regarded as unaffected, it results that we may onl}^ speak with 

 certainty of the relative movements within the region of the 

 overthrust. We are justified in speaking, or writing, for purely 

 descriptive pm'poses, of an eastward or westward movement of the 

 upper block over the lowei*, in order to avoid the long periphrases 

 and digressions Avhich would be unavoidable if the true meaning of 

 the observations were always to be expressed in full ; but in this 

 case it is desirable to observe uniformity of j^i'actice, and always 

 to regard the upper block as having moved relatively to the lower, 

 and especially to remember that the expression is used merely 

 as a convenience in description, not as implying any assertion of 

 displacement or fixity relative to B,nj point outside the area of the 

 overthrust. 



Having shown that argument as to the direction of movement 

 is merely discussion of the words in which the facts are to be 

 represented, I come to the question of the direction from which 

 the pressure, to which the movement is attributed, was exerted. 

 This discussion, again, involves a widespread fallacy that pressure 

 can be one-sided ; it permeates the great work of Suess, in which 

 we find repeated reference to earth-waves advancing against 

 resistant blocks, and in which the forms of the folds are repeatedly 

 invoked as evidence of the direction from which the pressure came. 

 The deservedly great influence of this work on geological thought 

 has sei'ved to emphasize and perpetuate a very natural fallacy, 

 derived from an imperfect interpretation of everyday experience. 

 When a person pushes, for instance, against an unlatched door 

 which yields to the pressure, it is natural for that person to 

 attribute the result to the action of which he is conscious, and 

 to take no account of the inanimate subject of his activities ; 

 2)roperly considered, how^ever, both take an equal and opposite part, 

 and the door pushes back in exactl}^ the same degree as the person 

 pushes against it. This is easily recognized when the door is 



