{)art 1] aKniyersary address of the president. Ixxxiil 



but it is not possible to place a limit on tliose which could origi- 

 nate in a similar manner, and it is, at least, not impossible that 

 the movements revealed by faulting may have originated in some 

 analogous way, and that the direction of the forces acting immedi- 

 ately on either side of the fault may have been very different from 

 that of the ultimate influence to which they were due, 



G-eologists have recognized that to some such cause we must 

 , attribute the origin of the fractures which traverse all rocks, 

 frequently with a remarkable parallelism and regularity of 

 direction ; but they have not sufficiently recognized that the same 

 cause which gives rise to the fractures may equally be the cause 

 of movement along the surface of the fracture, this movement 

 taking place simultaneously in opposite directions on opposite sides 

 of the fault. Generally faults are too numerous, and form too 

 complicated a S3^stem, to allow of such a cause as has been mentioned 

 being recognized in the effect ; but all who have had experience of 

 geological survey must have come across instances of faults that 

 can only be detected by close and detailed survey, or by under- 

 ground workings in mines and quarries. In these cases the 

 disturbance of the even course of the boundary-lines is limited to 

 the immediate neighbourhood of the fault, dying out on either 

 side, just as the displacements in 1906 were localized to the 

 immediate neighbourhood of the San Andreas Fault. 



It is no part of my present aim to enter on a discussion of the 

 physics of faulting, the purport of such reference as has been made 

 being to elucidate the meaning, and the limitation of meaning, of 

 the words which we use in a special sense, and to point out that 

 in using the words 'upthrow' and 'downthrow' we must be careful 

 to avoid any implication that the displacement was restricted to one 

 side of the fault ; for it may Avell have taken the form of a simul- 

 taneous movement in the same direction, but of different amount, 

 or in opposite directions, upwards on one side and downwards on 

 the other, of the surface of separation. 



I have already referred to two classes of technical terms, one of 

 which is a word not used outside some particular department of 

 knowledge, and consequently meaningless to the uninitiated, the 

 other a word in common use, to which a special limited meaning 

 is given when used as a technical term. A third class is that of 

 compound words, in Avhich one or all of the components may be in 

 general use, though the compound is confined to one special branch 



