168 



below b, c, dy e, and so on (in succession), on the right of the line 

 until, at the point of maximum depression, in the angle where the 

 upper limb of the Z goes off from the middle limb, a on the left, is 

 opposed to say w, x, or y on the other. We will call that a down- 

 throw on the left hand. From that point, in the direction of the 

 left hand extremity of the upper limb of the Z the displacement 

 diminishes, as before, and finally comes once more to nothing at 

 all, at the end of the incision. 



It is worth while to take a little trouble in order to comprehend 

 the phenomena I have endeavoured to illustrate by this, itself 

 somewhat-complicated experiment, because a distinct conception 

 of the structural character it is intended to elucidate, really affords 

 the key to the structure of one of the most important features in 

 the geology of the North of England, and it enables one, in 

 addition, to gain a better insight into the true nature of faults in 

 general than can very well be obtained by other means. And one 

 result of the imperfect conception of the true nature of what 

 people call " The Pennine Fault," has been that no correct general 

 description of it has yet been given. Yet the very existence of the 

 remnant of Coal Measure strata about to be referred to is owing 

 to one of the complex series of changes of throw, whose nature I 

 have just endeavoured to describe. 



A reference now to the annexed stereographic diagram will help 

 still further to explain these complex relations. This represents 

 in a highly generalized diagrammatic form the same phenomena 

 that I have endeavoured to bring before the mind by means of the 

 •experiments just described. In this figure the two planes meeting 

 at right angles on the left, and separating the quadrangular blocks 

 from the continuous portion, are supposed to coincide with the 

 planes of the faults represented in place by the middle and the 

 upper Umbs respectively of the Z. The surface of the continuous 

 part represents, in like manner, the Carboniferous uplands on the 

 outer side of the Faults, while the plane represented by the inclined 

 upper surface of the several detached blocks shews the general 

 shape of the lower surface presented by the outcrops of the rocks 

 on the inner, or western, side of the faults. 



