350 Proceedings of the Royal Society of Victoria. 



(i). That the relative movement of the hanging wall side has 

 been downward on the line of dip of the fault (that is, that it is 

 represented on plan by ON). 



(ii). That there has been no relative rotation about an axis 

 perpendicular to the plane of the fault. 



(iii). That there has been no alteration of shape which would 

 make the two blocks no longer correspond to one another. 



A modified rule has been suggested by Mr. J. T. Freeland 

 (Trans. Amer. Inst. M.K., vol. xxi.). "After cutting through 

 the fault prospect on that side of the line of intersection of the 

 known parts of the vein and fault, indicated by the relative 

 motion of the opposite block of ground for the extension of the 

 vein and the second line of intersection." The first line of 

 intersection is here drawn towards the unknown ground, and the 

 " relative motion of the opposite block " is indicated Ijy its 

 projection on the horizontal plane of the diagram (the line OQ on 

 the figures). 



This completely disposes of the first assumption, and hence 

 gives the correct direction in the case of Fig. 2. The second 

 assumption is also disposed of if it is possible to ascertain the 

 direction of the line of intersection (on plan) of the fault with 

 the lost part of the deposit. 



It seems better, however, to use a geometrical construction 

 without putting it as a rule, as this tends to the forgetting of 

 assumed conditions. 



The lode and fault being plotted as DO, OF, draw OQ repre- 

 senting the projection on the horizontal plain of the direction of 

 movement, and QR parallel to the projection of the line of 

 intersection of the fault and the lost portion of the lode. Let 

 QR meet FO in R, then OR is the direction in which to seek the 

 lost lode, and if OQ represents the amount of the movement, OR 

 represents, on the same scale, the distance to be driven. 



The absence of rotation in stratified rocks is usually at once 

 ascertainable as soon as settled ground is reached beyond the 

 fault. If there has been rotation, the line of intersection is 

 easily plotted in the case of bedded rocks, or of a lode parallel to 

 the beds, but for other cases may be more easily given by another 

 construction following. If the rotation is small it can often be 

 neglected, but may aflfect the direction to be driven when the line 



