76 



LYELL'S ELEMENTS OF GEOLOGY. 



Faults. 



side of a fissure, thrown up above or down below the mass with 

 which it was once in contact on the other side. This mode of 

 displacement is called a shift, slip, or fault. " The miner," says 

 Playfair, describing a fault, " is often perplexed, in his subterra- 

 neous journey, by a derangement in the strata, which changes 

 at once all those lines and bearings which had hitherto directed 

 his course. When his mine reaches a certain plane, which is 

 sometimes perpendicular, as in A B, Fig. 76., sometimes oblique 



Fig. 76. 



B D 



Faults. A B perpendicular, C D oblique to the horizon. 



to the horizon (as in C D, ibid.), he finds the beds of rock broken 

 asunder, those on the one side of the plane having changed their 

 place, by sliding in a particular direction along the face of the 

 others. In this motion they have sometimes preserved their pa- 

 rallelism, as in Fig. 76., so that the strata on each side of the 

 faults A B, C D, continue paft,llel to one another ; in other cases, 

 the strata on each side are inclined, as in a, &, c, d, (Fig. 77.), 



Fig, 77. 



E F, fault or fissure filled with rubbish, on each side of which the shifted 

 strata are not parallel. 



though their identity is still to be recognized by their possessing 

 the same thickness, and the same internal characters."* 



We sometimes see exact counterparts of these slips, on a small 

 scale, in pits of fine loose sand and gravel, many of which have 

 doubtless been caused by the drying and shrinking of argillace- 

 ous and other beds, slight subsidences having taken place from 



* Playfair, Illust. of Hutt. Theory, $ 42. 



