IxXXviii PEO0EEDI?5"GS OF THE GEOLO&ICAL SOCIETY, [vol. Ixxvii, 



observation have been presented, rather than by the facts them" 

 selves. Yet it would be better that we should abandon this mode 

 of expression altogether, for it is not only fraught Avith danger to 

 ourselves, and likely to lead to erroneous reasoning, but also it is 

 liable to misunderstanding by ungeological readers who, attaching 

 a different meaning to the words from that which was intended, 

 will conclude that we do not understand the subject with which we 

 are dealing, and so our Philistines be led to scoff. 



From the consideration of these two matters Avhich have given 

 rise to controversy, concerned almost entirely with words, by 

 Avhich things that really matter may be described, I now come 

 to one which is a vital one, for it may involve a modification, and 

 in some respects a radical change, iii some of the fundamental 

 principles, which have rather been tacitly accepted than definitely 

 proved. In discussions, as in descrij^tions, of the phenomena or of 

 the origin of these overthrusts, the masses involved have generally 

 been regarded as passive, moving under the influence of external 

 forces in the production of which they took no j^art. The notion 

 is a natural one, it is the simplest and easiest way of interpreting 

 the facts of observation ; but its general acceptance must be very 

 largely attributed to the influence of experiments on a small scale, 

 which have themselves been suggested and directed by the hypo- 

 thesis Avhich they were intended to illustrate and investigate. In 

 these we have an inert mass, variously composed to imitate, more 

 or less, the rocks of the Earth's crust, and this mass is subjected 

 to deformation by the application of external forces. In this way 

 many of the structures which have been worked out by geological 

 observations in the field were imitated on a small scale in the 

 experiment, and the resemblance was accepted as evidence that 

 the large-scale structures, met with in Nature, were produced, like 

 the small-scale structures of the experiment, b}^ the application of 

 external forces. Difficulties, however, arise when we consider the 

 conditions which are introduced by an increase of dimensions to 

 the scale of Nature; and, Avhen the mechanics of overthrusts are 

 investigated, these difficulties become insuperable. 



When one body is pressed against another by any force at right 

 angles to the surface of contact, it may be caused to move by 

 another force acting at right angles to the first, and the magnitude 

 of tbe second force needful to produce movement bears a definite 



