DISLOCATIONS OF STRATA. 101 



small beds, or even portions of rocks ; at others, 

 involving a large series of strata. The extent of such 

 fractures also varies; but is rarely very great, without 

 being attended by those motions which will imme- 

 diately be considered. 



It is much more common to find that the separated 

 portions of strata, thus fractured, no longer preserve 

 the game continuity of position ; and these changes 

 are attended with some variety. It is observed, in 

 the first place, that a distance intervenes between the 

 separated ends of the mass, in a line, vertical, or 

 nearly so, to the plane of the strata ; and the quan- 

 tity of this, or the extent of the slide, is often con- 

 siderable, producing those shifts which are of such 

 frequent inconvenience in the progress of a coal mine. 

 In such an event, the broken strata may continue 

 parallel, though no longer in the same plane : but it 

 also happens that an alteration of the angle occurs at 

 the same time ; so that the same series of rocks pre- 

 sents two different inclinations on the opposite sides 

 of the place of fracture. It needs scarcely here be 

 shown, that, in extreme cases of this nature, the 

 phenomena of opposing or convergent strata may be 

 produced ; or that fractures attended with a mere 

 change of inclination, may occur without being 

 accompanied by any sliding or complete separation of 

 the broken ends. To say that such fractures and 

 shifts may also be attended with flexures of the strata, 

 or that they may be the consequences of curvatures 

 exceeding the power of flexibility, would be only to 

 anticipate what is necessarily detailed in the following 

 chapter. 



It is usual, in the language of miners, to consider 

 a stratum as either elevated or depressed ; but this 

 expression is regulated by his casual progress along 



