Rev. 0. Fisher — On Faulting, Jointing, and Cleavage. 205 



(2.) If a trial line cuts any given bed twice, the fault is an 

 " upcast " or " reversed " fault and the amount of vertical throw 

 is equal to the thickness of the beds which are twice encountered 

 by it ; and the strata will be on the whole compressed horizontally 

 and stretched vertically. 



From these two self-evident propositions many results more com- 

 plicated immediately follow. 



§ I.— Of THrect Faults. 



(3.) If there are two direct faults of vertical throws a and h (that 

 is, of a and b feet or other unit of measure), a trial line cutting both 

 faults will have missed a thickness a of beds as soon as it has crossed 

 the first fault, and an additional thickness b of beds as soon as it has 

 crossed the second fault. It will therefore on the whole have missed 

 a thickness a + & of beds when it has crossed both faults. 



It follows immediately that, if two faults intersect one another, the 

 trial line through their intersection will have crossed both of them 

 at once, and will therefore miss a thickness of beds equal to a + 6 

 at that one place. This however, being a case of a " trough " fault, 

 is deserving of somewhat fuller consideration. 



(4.) Trough faults. — Let there be two "direct" clean-cut faults 

 hading inwards towards the same vertical. Their outcrops may, or 

 may not, intersect. And let the first fault a a be faulted by the 

 second b b. 



The figure is supposed to be a vertical section. The masses of 

 rock A, X, B, Y, were originally unsevered. When the fault a a 

 occurred, A and X moved towards the left, and also got a throw a 

 downwards with respect to B and F, which remained stationary : 

 when the second fault b b occurred, 

 B and X moved to the right, and 



got a throw b downwards with re- \ a 



ference to A and Y, which this 

 time remained stationary. Now 

 although the two downward 

 throws, which have affected X, ^ 

 might have been the result of a 

 single movement, yet its horizontal 

 movements, having been in op- 

 posite directions, could not have been simultaneous. This shows 

 that the fault b b must have been subsequent in time to the fault 

 a a, although the interval may have been either short or long. 



It will be observed that the mass Y has remained stationary 

 during both movements, and may therefore serve as a base, to 

 which the throws may be measured. 



Then the tlirow of A with respect to F is «. 



X „ Y „ a + b. 



The throw of ... X „ A „ b. 



,, ,, X. ,, B ,, a. 



A ,, B „ a ~ b 



