The first three infer dislocation of the strata; the fourth changes in
the bed of coal itself.
1. A dike is a wall of extraneous matter, which divides all the beds in
a coal-field.
[Illustration: 809]
Dikes extend not only in one line of bearing through coal-fields for
many miles, but run sometimes in different directions, and have often
irregular bendings, but no sharp angular turns. When from a few feet to
a few fathoms in thickness, they occur sometimes in numbers within a
small area of a coal basin, running in various directions, and even
crossing each other. _Fig._ 809. represents a ground plan of a
coal-field, intersected with greenstone dikes. A B and C D are two dikes
standing parallel to each other; E F and G H are cross or oblique dikes,
which divide both the coal strata and the primary dikes A B and C D.
[Illustration: 810 811]
2. _Slips_ or _faults_ run in straight lines through coal-measures, and
at every angle of incidence to each other. _Fig._ 810. represents a
ground plan of a coal-field, with two slips A B and C D in the line of
bearing of the planes of the strata, which throw them down to the
outcrop. This is the simplest form of a slip. _Fig._ 811. exhibits part
of a coal-field intersected with slips, like a cracked sheet of ice.
Here A B is a dike; while the narrow lines show faults of every kind,
producing dislocations varying in amount of slip from a few feet to a
great many fathoms. The faults at the points _a_, _a_, _a_ vanish; and
the lines at _c_ denote four small partial slips called _hitches_.
[Illustration: 812]
The effects of slips and dikes on the coal strata appear more
prominently when viewed in a vertical section, than in a ground plan,
where they seem to be merely walls, veins, and lines of demarcation.
_Fig._ 812. is a vertical section of a coal-field, from dip to rise,
showing three strata of coal _a_, _b_, _c_. A B represents a dike at
right angles to the plane of the coal-beds. This rectangular wall merely
separates the coal-measures, affecting their line of rise; but further
to the rise, the oblique dike C D interrupts the coals _a_, _b_, _c_,
and not only disjoins them, but throws them and their concomitant strata
greatly lower down; but still, with this depression, the strata retain
their parallelism and general slope. Nearer to the outcrop, another dike
E, F, interrupts the coals _a_, _b_, _c_, not merely breaking the
continuity of the planes, but throwing them moderately up, so as to
produce a steeper inclination, as shown in the figure. It sometimes
happens that the coals in the compartment H, betwixt the dikes C and E,
may lie nearly horizontal, and the effect of the dike E, F, is then to
throw out the coals altogether, leaving no vestige of them in the
compartment K. “Such,” says Mr. Bald, from whom these illustrations are
borrowed, “are the most prominent changes in the strata, as to their
line of direction, produced by dikes; but of these changes there are
various modifications.”
[Illustration: 813]
The effect of slips on the strata is also represented in the vertical
section, _fig._ 813., where _a_, _b_, _c_ are coals with their
associated strata. A, B, is an intersecting slip, which throws all the
coals of the first compartment much lower, as is observable in the
second, No. 2.; and from the amount of the slip, it brings in other
coal-seams, marked 1, 2, 3, not in the compartment No. 1. C, D, is a
slip producing a similar result, but not of the same magnitude. E, F,
represents a slip across the strata, reverse in direction to the former;
the effect of which is to throw up the coals, as shown in the area No.
4. Such a slip occasionally brings into play seams seated under those
marked _a_, _b_, _c_, as seen at 4, 5, 6; and it may happen that the
coal marked 4 lies in the prolongation of a well-known seam, as _c_, in
the compartment No. 3., when the case becomes puzzling to the miner. In
addition to the above varieties, a number of slips or hitches are often
seen near one another, as in the area marked No. 5., where the
individual displacements are inconsiderable, but the aggregate
dislocation may be great, in reference to the seams of the 6th
compartment.
[Illustration: 814]
The results of dikes and slips on a horizontal portion of a field are
exemplified in _fig._ 814. Where the coal-measures are horizontal, and
the faults run at a greater angle than 45° to the line of bearing, they
are termed dip and rise faults, as A B, C D, E F.
[Illustration: 815]
Coal viewers or engineers regard the dislocations now described as being
subject in one respect to a general law, which may be thus
explained:--Let _fig._ 815. be a portion of a coal-measure; A, being
the pavement and B the roof of the coal-seam. If, in pursuing the
stratum at C, a dike D occurs, standing at right angles with the
pavement, they conclude that the dike is merely a partition-wall between
the beds by its own thickness, leaving the coal-seam underanged on
either side; but if a dike F forms, as at E, an obtuse angle with the
pavement, they conclude that the dike is not a simple partition between
the strata, but has thrown up the several seams into the predicament
shown at G. Finally, should a dike H make at I an acute angle with the
pavement, they conclude that the dike has thrown down the coal-measures
into the position of K.
The same important law holds with slips, as I formerly stated; only when
they form right angles with the pavement, the case is ambiguous; that
is, the strata may be dislocated either upwards or downwards.
Dikes and faults are denominated upthrow or downthrow, according to the
position they are met with in working the mine. Thus, in _fig._ 812., if
the miner is advancing to the rise, the dike A, B obviously does not
change the direction; but C, D is a downthrow dike of a certain number
of fathoms towards the rise of the basin, and E, F is an upthrow dike
likewise towards the rise. On the other hand, when the dikes are met
with by the miner in working from the rise to the dip, the names of the
above dikes would be reversed; for what is an upthrow in the first case,
becomes a downthrow in the second, relative to the mining operations.
[Illustration: 816]
3. We have seen that _hitches_ are small and partial slips, where the
dislocation does not exceed the thickness of the coal-seam; and they are
correctly enough called _steps_ by the miner. _Fig._ 816. represents the
operation of the _hitches_ A, B, C, D, E, F, G, H, on the coal-measures.
Though observed in one or two seams of a field, they may not appear in
the rest, as is the case with dikes and faults.