HOW THE COAL BEDS LIE.
The process of growth, deposition, submergence, and burial, described
in the preceding chapter, continued throughout the Carboniferous age.
Each period of inundation and of the covering over of beds of vegetable
deposit by sand and silt is marked by the layers of stratified rock
that intervene between, and that overlie the separate seams of coal in
the coal measures of to-day. The number of these coal seams indicates
the number of periods during which the growth and decay of vegetation
was uninterrupted. This number, in the anthracite coal regions, varies
from ten to thirty or thereabouts, but in the bituminous regions it
scarcely ever exceeds eight or ten. The thickness of the separate
coal seams also varies greatly, ranging from a fraction of an inch up
to sixty or seventy feet. Indeed, there are basins of small extent
in the south of France and in India where the seam is two hundred
feet thick. It is seldom, however, that workable seams of anthracite
exceed twenty feet in thickness, and by far the largest number of
them do not go above eight or ten, while the seams of bituminous coal
do not even average these last figures in thickness. Neither is the
entire thickness of a seam made up of pure coal. Bands of slate called
“partings” usually run horizontally through a seam, dividing it into
“benches.” These partings vary from a fraction of an inch to several
feet in thickness, and make up from one fifth to one seventh of the
entire seam.
The rock strata between the coal seams range from three feet to three
hundred feet in thickness, and in exceptional cases go as high as five
or six hundred feet. Perhaps a fair average would be from eighty to one
hundred feet. These rock intervals are made up mostly of sandstones
and shales. The combined average thickness of the coal seams of
Pennsylvania varies from twenty-five feet at Pittsburgh in the western
bituminous region to one hundred and twenty feet at Pottsville in the
eastern anthracite district, and may be said to average about one
fiftieth of the entire thickness of the coal measures, which is placed
at 4,000 feet.
Some conception may be had of the enormous vegetable deposits of the
Carboniferous era by recalling the fact that the resultant coal in
each seam is only from one ninth to one sixteenth in bulk of the woody
fibre from which it has been derived, the loss being mainly in oxygen
and hydrogen. It is probable that the coal seams as well as the rock
strata had attained a comparative degree of hardness before the close
of the Carboniferous age. It was at the close of this age that those
profound disturbances of the earth’s crust throughout eastern North
America took place which have already been referred to. Hitherto,
through the long ages of Paleozoic time, there had been comparative
quiet. As cooling and contraction of the earth’s body were still going
on, there were doubtless oscillations of surface and subsidence of
strata in almost continuous progress. But these movements were very
slow, amounting, perhaps, to not more than a foot in a century. Yet
in Pennsylvania and Virginia the sinking of the crust up to the close
of the Carboniferous age amounted to 35,000 or 40,000 feet. That the
subsidence was quiet and unmarked by violent movement is attested by
the regularity of strata, especially of the carboniferous measures,
which alone show a sinking of 3,000 or 4,000 feet. Neither were the
disturbances which followed violent, nor were the changes paroxysmal.
Indeed, the probability is that they took place gradually through
long periods of time. They were, nevertheless, productive of enormous
results in the shape of hills, peaks, and mountain ranges. These
movements in the earth’s crust were due, as always, to contractions
in the earth’s body or reductions in its bulk. On the same principle
by which the skin of an apple that has dried without decay is thrown
into folds and wrinkles, the earth’s crust became corrugated. There
is this difference, however: the crust, being hard and unyielding,
has often been torn and broken in the process of change. Naturally
these ridges in the earth’s surface have been lifted along the lines
of least resistance, and these lines seem to have been, at the time of
the Appalachian revolution, practically parallel to the line of the
Atlantic coast, though long spurs were thrown out in other directions,
isolated dome-shaped elevations were raised up, and bowl-shaped valleys
were hollowed out among the hills.
The anthracite coal beds were in the regions of greatest disturbance,
and, together with the rock strata above and below them, assumed new
positions, which were inclined at all angles to their old ones of
horizontality. More than this, the heat and pressure of that period
exerted upon these beds of coal, which up to this time had been
bituminous in character, resulted in the expulsion of so large a
portion of the volatile matter still remaining in them as to change
their character from bituminous to anthracite. Although the strata,
in the positions to which they have been forced, are at times broken
and abrupt, yet as a rule they rise and fall in wave-like folds or
ridges. These ridges are called _anticlinals_, because the strata slope
in opposite directions from a common plane. The valleys between the
ridges are called _synclinals_, because the strata slope from opposite
directions toward a common plane. One result of this great force of
compression exerted on the earth’s crust was to make rents in it across
the lines of strata. These rents are called _fissures_. Sometimes
the faces of a fissure are parallel and sometimes they inclose a
wedge-shaped cavity. This cavity, whatever its shape, is usually filled
either with igneous rock that has come up from the molten mass below,
or with surface drift or broken rock fragments that have been deposited
there from above. Where there is displacement as well as fracture, that
is when the strata on one side of a fissure have been pushed up or have
fallen below the corresponding strata on the other side, we have what
is known as a _fault_. Sometimes the displacement seems to have been
accomplished with little disturbance to the sides of the fissure; at
other times we find, along the line of fracture, evidences of great
destruction caused by the pushing up of strata in this way. A fault
may reach a comparatively short distance, or it may traverse a country
for miles. The vertical displacement may be only a few inches, or it
may amount to hundreds or thousands of feet. In the bituminous coal
regions, where the strata lie comparatively undisturbed, faults are
but little known. In the anthracite districts they are common, but not
great.
[Illustration: VERTICAL SECTION THROUGH SOUTHERN COAL FIELD.]
[Illustration: VERTICAL SECTION THROUGH NORTHERN COAL FIELD.]
Besides the great folds into which the earth’s crust was crowded, there
are usually smaller folds corrugating the slopes of the greater
ones, sometimes running parallel with them, oftener stretching across
them at various angles. A marked instance of this formation is found in
the Wyoming coal basin, the general coal bed of which is in the shape
of a canoe, about fifty miles long, from two to six miles broad, and
with a maximum depth of perhaps one thousand feet. Running diagonally
across this basin, in practically parallel lines from one extremity to
the other, is a series of gentle anticlinals, dividing the basin into
some thirty smaller synclinal valleys or sub-basins.
The irregularities produced by folds, fissures, faults, and partings
are not the only ones with which the miner has to deal. So far we
have supposed the coal seams to have been laid down in horizontal
layers of uniform thickness, with smooth and regular under and upper
surfaces. This is true only in a large sense. As a matter of fact
each separate seam varies greatly in thickness, and its roof and
floor are often broken and irregular. The beds of clay on which the
deposits were laid were pushed up unevenly by the exuberant growth
of vegetation from them. The action of waves and ocean currents made
hollows in them, and laid down ridges and mounds of sand on them,
around and over which the decaying vegetation rose and hardened. The
same forces, together with the action of running streams, made channels
and hollows in the upper surfaces of these beds of incipient coal,
which cavities became filled by sand and gravel, and this also hardened
into rock. These irregularities are found by the miner of to-day in
the floor and roof of the coal seam, and are called _rolls_, _horses_,
or _horse-backs_. When the coal seam thins out so rapidly that the
floor and roof come nearly together, this state of things is called a
_pinch_, or _squeeze_, though the latter term is more properly applied
to the settling of the roof rock after the coal has been mined out.
The inequalities of a coal seam that have now been mentioned, although
perhaps but a small portion of those that are daily met with in the
process of mining, are neverthless characteristic of the whole.
The hills and mountain ranges that were thrown up at the close of the
Carboniferous age were many times higher and broader then than they
are to-day. Heat and cold and the storms of a thousand centuries,
working by disintegration and erosion, have worn away their substance,
the valleys and low lands are filled with it, and the rivers are
always carrying it down to the sea. The peaks and the crests have
been the portions of the elevations that have suffered most. It is
often as though the tops of the anticlinal folds had been sliced off
for the purpose of filling the valleys with them to the level of the
decapitated hills. A great part of the coal measures have thus wasted
away; in some portions of the anthracite district by far the greater
part, including many valuable coal seams.
When a fold or flexure of the earth’s crust has been decapitated in
the manner mentioned, the exposed edge of any stratum of rock or
coal is called its _outcrop_. The angle of inclination at which any
stratum descends into the earth is called its _dip_. The direction of
a horizontal line drawn along the face of a stratum of rock or coal is
its _strike_. It is obvious that the strike must always be at right
angles to the dip. That is, if the dip is downward toward the east or
toward the west, the direction of the strike must be north and south.
It is now apparent that if one begins at the outcrop of a coal seam
and traces the course of the seam downward along the line of dip, his
path will lie down the inclination for a longer or shorter distance,
until the bottom of the synclinal valley is reached. This is known as
the _basin_ or _swamp_. Here the seam may be comparatively level for
a short distance; more often it has a mild vertical curve, and starts
up the dip on the other side of the valley, which inclination may be
followed till the outcrop is reached. If now the decapitated portion of
the fold could be replaced in its natural position, we could trace the
same seam up to and over the anticlinal axis and down upon the other
side. As it is, we must cross on the surface from the outcrop to the
place where the corresponding seam enters the earth. In the southern
and eastern anthracite coal districts of Pennsylvania decapitation of
folds to a point below the coal measures is general; the coal seams
dip into the earth with a very sharp pitch, and the coal basins are
often very deep and very narrow, striking into the earth almost like
a wedge. In the northern or Wyoming district decapitation is not so
general, the angle of inclination of strata is mild, and the basins are
wide and comparatively shallow. In the bituminous districts, where the
disturbance to the earth’s crust has been slight, the coal beds lie
very nearly as they were formed, the dip seldom exceeding an angle of
five degrees with the horizon. The exposures here are due generally to
the erosive action of water.
[Illustration: OLD OPENING INTO AN OUT-CROP OF THE BALTIMORE VEIN.]
The carboniferous measures are the highest and latest geological
formation in the great coal fields of the United States. Therefore
where the strata have not been disturbed by flexure the coal seams
lie near the surface. This is generally the case in the bituminous
districts, and it is also partially true in the northern anthracite
coal field. Deep mining is necessary only in the middle and southern
anthracite coal fields, where the folds are close and precipitous,
and the deep and narrow basins formed by them have been filled with
deposits of a later geologic age.
Some of the difficulties to be met and overcome in mining coal will by
this time have been appreciated by the reader. But some of them only.
The inequalities of roof and floor, the pitching seams, the folds
and faults and fissures, all the accidents and irregularities of
formation and of location, make up but a few of the problems which
face the mining engineer. But the intellect and ingenuity of men have
overcome most of the obstacles which Nature placed in the way of
successful mining when she hardened the rocks above her coal beds,
crowded the earth’s crust into folds, and lifted the mountain ranges
into the air.
It will not be out of place at this time to make mention of those
localities in which coal is found. Indeed, there are few countries on
the globe in which there are not carboniferous deposits of greater
or less extent. Great Britain, with Ireland, has about 12,000 square
miles of them. In England alone there is an area of 8,139 square miles
of workable coal beds. In continental Europe the coal fields are
numerous, but the character of the deposit is inferior. Coal is found
also in the Asiatic countries, in Australia, and in South America; and
in Nova Scotia and New Brunswick there is an area of 18,000 square
miles of coal measures. The combined areas of coal measures in the
United States amount to about 185,000 square miles. The Appalachian
or Alleghany region contains about 60,000 square miles, included in
the States of Pennsylvania, Virginia, West Virginia, Maryland, Ohio,
Kentucky, Tennessee, Georgia, and Alabama. The Illinois and Missouri
region contains also about 60,000 square miles, and has areas not only
in the States named, but also in Indiana, Iowa, Kentucky, Kansas,
and Arkansas. Michigan has about 5,000 and Rhode Island about 500
square miles. There are also small areas in Utah and Texas, and in the
far West there are workable coal fields in Colorado, Dakota, Indian
Territory, Montana, New Mexico, Washington, Wyoming Territory, Oregon,
and California. The entire coal area of the United States, with the
exception of that in Rhode Island and a few outlying sections in
Pennsylvania, contains coal of the bituminous variety only. Both the
area and supply are therefore practically without limit. In the coal
regions of Rhode Island the disturbances affecting the earth’s crust
have been very violent. The motion, heat, and compression have been so
great as to give the rocks associated with the coal measures a true
metamorphic or crystalline structure, and to transform the coal itself
into an extremely hard anthracite; in some places, indeed, it has
been altered to graphite. The flexures of the coal formation are very
abrupt and full of faults, and the coal itself is greatly broken and
displaced. Its condition is such that it cannot be mined with great
profit, and but little of it is now sent to market. The only areas
of readily workable anthracite in the United States are therefore in
Pennsylvania. These are all east of the Alleghany Mountains, and are
located in four distinct regions. The first or Southern Coal Field
extends from the Lehigh River at Mauch Chunk, southwest to within a
few miles of the Susquehanna River, ending at this extremity in the
form of a fish’s tail. It is seventy-five miles in length, averages
somewhat less than two miles in breadth, and has an area of one hundred
and forty square miles. It lies in Carbon, Schuylkill, and Dauphin
counties. The second or Western Middle field, known also as the Mahanoy
and Shamokin field, lies between the eastern headwaters of the Little
Schuylkill River and the Susquehanna River. It has an area of about
ninety square miles, and is situated in the counties of Schuylkill,
Columbia, and Northumberland. It lies just north of the Southern field,
and the two together are frequently spoken of as the Schuylkill Region.
The Eastern Middle or Upper Lehigh field lies northeast of the first
two fields, and is separated into nine distinct parallel canoe-shaped
basins. These extend from the Lehigh River on the east to the Catawissa
Creek on the west, and comprise an area of about forty miles. They
are principally in Luzerne County, but extend also into Carbon,
Schuylkill, and Columbia counties. The Northern or Wyoming field is a
crescent-shaped basin about fifty miles long and from two to six miles
broad, with an area of about two hundred square miles. Its westerly
cusp is just north of the Eastern Middle field, and it extends from
that point northeasterly through Luzerne and Lackawanna counties, just
cutting into Wayne and Susquehanna counties with its northern cusp. It
lies in the valleys of the Susquehanna and Lackawanna rivers, and in
it are situated the mining towns of Plymouth, Wilkes Barre, Pittston,
Scranton, and Carbondale. There is also a fifth district, known as
the Loyalsock and Mehoopany coal field, lying in Sullivan and Wyoming
counties. It is from twenty to twenty-five miles northwest of the
Wyoming and Lackawanna field, its area is limited, and its coals are
not true anthracite.
It will thus be seen that aside from this last field the anthracite
coal area of Pennsylvania contains about four hundred and seventy
square miles.