542 BENEFICIAL EFFECTS OF UNDULATIONS. 
position, the thin strata of Coal are worked with 
greater facility than if they had been horizontal ; 
but as this inclination has a tendency to plunge 
their lower extremities to a depth that would 
be inaccessible, a series of Faults, or Traps, is 
interposed, by which the component portions of 
the same formation are arranged in a series of 
successive tables, or stages, rising one behind 
another, and elevated continually upwards to- 
wards the surface, from their lowest points of 
depression. (See PI. 05. Fig. 3. and PI. ()G. 
Fig. 2.) A similar effect is often produced by 
U7idulations or contortions of the strata, which 
give the united advantage of inclined position 
and of keeping them near the surface. The 
JBasin-shaped structure which so frequently oc- 
curs in coal fields, has a tendency to produce 
the same beneficial consequences. (See PL 65. 
Figs. 1. 2. 3.) 
But a still more important benefit results from 
the occurrence of Faults or Fractures,^' without 
which the contents of many deep and rich mines 
* ''Faults," says Mr. Conybeare, "consist of fissures tra- 
versing the strata, extending often for several miles, and pene- 
trating to a depth, in very iew instances ascertained ; they are 
accompanied by a subsidence of the strata on one side of their 
line, or (which amounts to the same thing) an elevation of them 
on the other ; so that it appears, that the same force which has 
rent the rocks thus asunder, has caused one side of the fractured 
mass to rise, or the other to sink. — The fissures are usually filled 
by clay." Geology of Encfland and Wales, Part I. 348. 
