FAULTS. 
151 
strata are actually vertical, and a shaft has 
been sunk for eighty fathoms in the same 
bed of coal. 
The beds are not only thus inclined, but 
they are also disturbed by vertical disloca¬ 
tions, similar to those which are occasioned 
in the ice of a canal when the water is 
raised or lowered. The disturbances are 
sometimes mere shifts of the strata up or 
down, so that its continuity on the same 
level is simply interrupted, and the miner 
has to explore above and below in order to 
find the beds on which he had been pre¬ 
viously working: at other times dense 
wedges of clay are interposed, and in other 
instances hard dykes or masses of igneous 
rock. There are places where the mineral 
composition of the bed becomes altered in 
the neighbourhood of these occurrences, 
and the rich bed of ore or coal, may grad¬ 
ually or suddenly become useless. A de¬ 
tail of these incidents, aptly termed by the 
colliers troubles,” will be found in treatises 
on mineral geology. They are all evidently 
the results of numerous fractures and move¬ 
ments accompanied by the ejection of gas 
