88 
ELEMENTS OF GEOLOGY. 
in his subterraneous journey, by a derangement in the stra- 
' ta, 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. 74, sometimes oblique 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 
Faults. A B perpendicular, C D oblique to the horizon. 
particular direction along the face of the others. In this 
motion they have sometimes preserved their parallelism, as 
in Fig. 74, so that the strata on each side of the faults A B, 
C D, continue parallel to one another; in other cases, the 
strata on each side are inclined, as in a, c, d (Fig. 75), 
though their identity is still to be recognized by their pos¬ 
sessing the same thickness and the same internal charac¬ 
ters.”* 
In Coalbrook Dale, says Mr. Prestwich,f deposits of sand¬ 
stone, shale, and coal, several thousand feet thick, and occu¬ 
pying an area of many miles, have been shivered into frag¬ 
ments, and the broken remnants have been placed in very 
discordant positions, often at levels differing several hun¬ 
dred feet from each other. The sides of the faults, when 
perpendicular, are commonly several yards apart, and are 
sometimes as much as 50 yards asunder, the interval being 
filled wfith broken debris of the strata. In following the 
* Playfair, Illiist. of Hutt. Theory, § 42. 
t Geol. Trans., second series, vol. v,, p. 452. 
