Ixxxiv PEOCEEDIXGS OF THE GEOLOGICAL SOCIETY, [vol. Ixxvii, 



of science ; and of tins a very t^^pical instance is proYiclecl by that 

 special form of reversed fanlt, commonly knoAvn as an ' overthrust.' 

 This AYord is distinctly a technical term special to geology, it is not 

 used as a noun in ordinary speech or writing, nor is it used in any 

 other branch of natural knowledge, yet, unlike the word ' hade,' it 

 is far from meaningless apart from its technical significance, for 

 it is compounded of two common, characteristically English, 

 words, and as such carries with it a whole group of connotations. 

 First, that the rocks now resting upon the surface of separation 

 ought not normally to occup}* that position, but hare been brought 

 there by displacement from that which they originally occupied : 

 secondl}^, that the displacement has taken place by a movement of 

 the upper mass over the lower ; and, thirdly, that this movement 

 was produced by some cause or force external to the area occupied 

 by the material displaced, which has been thrust as an inert 

 mass, iniluenced by, but taking no part in the production of, the 

 jDOwer by which it was moved. 



Doubtless the word Avas from the outset intended to carry with 

 it the whole of these implications, and so long as it is used only in 

 that extended sense and some other word made use of when 

 a different group of connotations is intended, no objection could 

 be raised ; but, if it is to be retained as one of our special technical 

 terms, it is eminently desirable that its meaning should be limited 

 to the first, which expresses a fact, and that it should cease to 

 impl}' the other two, which are of the nature of a theory of origin. 

 In practice, however, it has generally been used in the extended 

 interpretation, and on this has grown up a mass of controversy 

 as to whether the u^Dper mass was thrust over the lower, or the 

 low^r thrust under the upper, as to the direction from which the 

 impulse came, and as to the ultimate cause to which it Avas due ; 

 but the greater part of these controversial writings resolves itself, 

 on critical examination, into mere verbal dialectics, or is incon- 

 sistent Avith some of the fundamental principles of physics. 



Taking these points in order, Ave may fii-st consider that of 

 the direction of movement. In the Scottish Highlands it is quite 

 clear that the upper blocks have moved AvestAvards relative to the 

 lower, in Scandinavia the relative movement of the upper blocks 

 has been eastwards, and in the Alps northwards; but it AA'ould be 

 an ec|ually true statement of the facts to say that the movement 

 of the loAA'er masses had been in the opposite dii'ection, and more 

 than this it is impossible to say. Leaving on one side the abstract 



