264

THE COLLIERY GUARDIAN.

February 11, 1916.

days, by these coal fields is not in itself an evidence
that they were always so detached. Everybody ’knows
that the coal measures, after the upheaval of the Her-
cynian range, were exposed, especially the topmost beds,
to the denudation which follows the building up of a
mountain range. Such inliers as the Glee Hills coal
fields, standing as landmarks on the Shropshire platform,
afford classical examples of “ erosion witnesses ”
(“ Temoins d’erosion ”). It has been alluded to the
absence of upper measures in these hills, while they are
well represented in the next coal field. It is easy to see
that these hills owe their existence to a cap of hard
eruptive rock, which prevented the underlying strata
from being swept away, like the overlying upper
measures and the surrounding middle measures, unpro-
tected by such a cap. Therefore, we have many a good
reason to believe that the Midland coal fields are parts of
a former main coal field.

.North Wales-Brabant Anticline.—While its northern
margin was sinking, in the Midlands we have ground to
believe that its southern margin, according to locality,
was either sinking, motionless, or slightly rising. In
Belgium it was rising, and by erosion getting rid of its
cover of carboniferous rocks; the writer found pebbles
in the conglomerates and in the coal seams of the middle
coal measures, while in the transition measures the
erosion laid bare the old siluro-cambrian floor, which
yielded pebbles and grains in the conglomerates of
Noeux. Likewise, in South Wales, pebbles of cannel
coal, iron stone, anthracite, and conglomerate were
found in coal seams by Logan, Strahan, and De La
Beche, well up in the coal measures. These facts show
that the lower measures had been subjected to upheaval
and denudation on the rising anticline. On the con-
trary, the conspicuous overstepping of the middle coal
measures in North Pembrokeshire gives clear evidence
of the gradual sinking of the anticline in that district.
Similarly in the Pas-de-Calais coal field there is a gradual
overstepping north-westwards of each division of the
middle coal measures, until finally, in the Boulonnais
coal field, the very high measures came to rest on the
Visean.

South Wales-Namur Syncline.—The most important
event in the history of this syncline is its invasion by the
middle measures in Kent. But we are still unaware as
to their extent and relationship with the surrounding
strata. Except on both sides of the Dover Straits, and
for some parts of the Pendleside and millstone grit series
in South-West England, therefore, the writer believes
there is a continuous sequence in this syncline from the
top of the Visean to the transition measures in Eastern
Westphalia; on the Upper Lippe the middle coal
measures have been found in borings overstepping the
middle devonian.

The Devon-Dinant Syncline. — In Devonshire and
Cornwall the correlation of the carboniferous rocks has
been much obscured by Hercynian disturbances, while
in the Dinant basin the strata have been so deeply
eroded that we are unable to draw any useful conclusion
from the structures of both these areas.

The Transition and Upper Coal Measures Period.—
Noteworthy earth movements mark the end of the fore-
going period. The formation of coal went on unchanged
in some areas (France and Belgium), while in other
areas (South Midlands) it went on slowly, or even
stopped.

Great Britain.—The most interesting movement we
have evidence of is the folding which gave rise to the
so-called Symon fault of the Coalbrookdale coal field,
for, thanks to the exhaustive study of Clarke, we have
valuable information regarding it. Between the middle
and transition measures a gentle folding took place along
the Shropshire borderland, giving rise to a series of folds,
striking north-east to south-west, and which increased
in importance towards the north-west. The north-west
wing of the synclines has a steeper dip than the corre-
sponding wing. These characters give clear evidence
that this folding is a posthumous Caledonian movement,
originating in the north-west. The results of this fold-
ing seem to have been felt as far as South Staffordshire
and Warwickshire, but with characters pointing to a
diminishing energy westwards. In the former coal field
R. Kidson recorded an unconformity between the middle
and the transition measures, where, moreover, the
Etruria marls are wanting. Likewise, R. G. Vernon has
shown that the Etruria marls (Nuneaton clays) are
locally wanting between the transition and the middle
measures in the Warwickshire coal field. Round the
Pennine chain there is a conformable sequence every-
where, according to W. Gibson. In the Cumberland
coal field an unconformity is also supposed to exist on
the top of the middle coal measures (Arber), but the
question seems to be controversial. In Ayrshire the
unconformity which is supposed to exist between the
productive measures and the upper red measures, has
been shown recently to be much less important and
questionable. After this folding, general subsidence took
place in the British Isles, especially along the Wales
anticline. The Welsh uplands and the Mercian high-
lands were overlapped by the transition and upper
measures. A row of small inliers of these measures,
starting from the Wyre Forest coal field, may be seen
along the Malvernian range, either between the pro-
truding detached pieces of the range, or even on the
summit (Woodbury Hill). The last member of this row
is the Newent coal field, whose characters, according to
N. Arber, show much more connection with the Shrop-
shire coal field than with the southern coal field. There-
fore, we may suppose that a carrier existed south of the
Newent coal field, and this carrier no doubt was the
North Wales-Woolhope anticline and its prolongation
through May Hill. During a long period this anticline
nlayed an important part in the history of this district.
Its upheaval is no doubt post-devonian, for the dis-
severance of the upper old red sandstone of the Glee Hills
from the strata of the same age in Gloucestershire and

Glamorganshire, was produced by this upheaval. The
writer believes that the true age of this upheaval is
Tournaisian, its slow rise compelling the southwards
retreat of the Tournaisian sea quoted above. From the
great difference of sediment and sequences of both sides
of this anticline, we get conclusive evidence of its con-
tinuous existence throughout the upper coal measures
period, and that it was never covered by deposits of this
age. In the Kent coal field, transition measures are
fairly well developed, but we have very few indications
regarding their extent or their relationship with other
strata. ' In South-West England, an important over-
stepping took place, for, in the short distance separating
the South Wales and Bristol coal fields from the Forest
of Dean, the upper coal measures overstep the whole
thickness of strata from the bottom of the Visean up to
the upper measures, and all the measures of the Forest
of Dean coal field belong to the upper coal measures, as
pointed out long ago by H. De la Beche, and confirmed
by N. Arber, and they rest unconformably on lower
carboniferous sandstone (Sibley). Therefore, the
southern, as well as the northern, margin of the Wales
anticline was sinking during this period. The writer has
already dealt with the possibility of the extension of
transition or upper coal measures above the Westmore-
land anticline; but we are unable to trace the history
of the Northumberland-Cumberland syncline during this
period. In Northumberland the upper red measures
have not yet been submitted to modern palaeontological
and stratigraphical study, while in Cumberland and in
the Canonbie Colliery coal field much obscurity still pre-
vails about these important points. Large sheets of
unmetamorphosed coal and loose upper coal measures
were distributed and uplifted by the Hercynian folding,
and afterwards exposed to denudation, and swept away.
The evidences of overstepping displayed by the upper
measures afford ground to believe that at the end of the
period England was covered with deposits which were
not so extensive as the Visean formation, but were,
nevertheless, very important. The transition and upper
coal measures of England display two very distinct types
—which one may designate as the South Wales and the
Midland types. The South Wales type is developed in
the South Wales-Namur syncline, and (except for a
bed of red shales in the Bristol coal field) is the normal
type of productive coal measures, with a marked pre-
dominance of arenaceous sediment (Pennant grit) in
the lower part. The Midland type presents the follow-
ing characters : (1) A predominance of red and greenish
(Espley rocks) sediments; (2) occasional beds of con-
glomerate and breccias; (3) coal seams, scarce, thin, and
often yielding sulphurous coal; (4) occasional, but not
uncommon, thin beds of Spirorbis limestone. All these
characters point out very peculiar physiographical con-
ditions during this period, which are by no means easy
to solve. The presence of red rocks may be ascribed,
like in other red’rock formations (permian, trias, etc.),
to climatic influences, such as desert conditions and
extreme changes of temperature. The presence of
breccias should give some support to this hypothesis.
But the writer cannot reconcile the existence of desert
conditions and extreme temperature in Shropshire and
South Staffordshire with the fact that typical and rich
coal measures flora flourish close and all around in the
Forest of Dean, Bristol, and near Manchester. More-
over, this abnormal type is already displayed by the
middle coal measures of the Clee Hills and Wyre Forest
coal field. Red measures are also existing in the mill-
stone grit of some coal field in North Staffordshire. But
it is a curious fact—not a mere coincidence, indeed—
that red measures are also very often barren measures.
In the Midlands the red colour of the sediments may
perhaps be ascribed to the abundance of volcanic detrital
material, some beds showing under the microscope a
good deal of volcanic tuff. But what then of the
breccias? It must be admitted that an explanation of
these abnormal characters is still to be found.

The Continent.—The writer has already spoken of the
overstepping of the transition measures of North France.
In Belgium there are many hundred metres of strata on
the top of the Hainault coal field still waiting for the
determination of their age; and there is little doubt that
we have there beds coeval with the transition measures
of the North of France. In both countries they are
productive and of the South Wales typo. In Belgium
the upper beds do not display any sign of overstepping.
Likewise, the age of the uppermost beds discovered in
Westphalia by the last borings is still unknown. On the
supposed northern margin of this , coal field upper coal
measures are outcropping in small inliers near
Osnabruck. From the very low percentage of volatile
matter which they contain, namely, ranging as low as
7 per cent., we mav infer that the older coal measures
divisions are wanting below, and that these upper
measures overstep a still unknown pre-carboniferous
floor. In the Saarbruck coal field conditions are quite
different from any other part of the great carboniferous
belt, for a conformable sequence from higher divisions
in the permian to lower divisions in the middle coal
measures occurs, resting probably on lower devonian
rocks.

Denudations on the Coal Fields.

A thorough knowledge of the tectonics of a country
and of the inroads of the sea is the best means of study-
ing the denudations. As the phenomena of erosion was
already at work during the formation of the coal fields,
and are still active to-day, the subject is so extensive
that the writer feels compelled to content himself with
a short sketch of the periods following immediately tlm
coal measures during which these phenomena was most
active.

1. Stephanian Period.—This division has been intro-
duced in the geological chronoloev bv the • French
geologists, but it may be questioned if this period does
not coincide more or less with the upper coal measures
period of English geologists. As a brief summary of
the foregoing periods, we may say ■ that the leading

Hercynian anticlines had nearly disappeared in the
British Isles. Some Charnian anticlines were slightly
emerged in the South of England, while the Caledonian
highlands were still standing, much worn and lowered,
overlooking the ruins of the Dumfries anticline. On the
Continent, the Brabant anticlines had perhaps emerged,
the Hartz anticline still more, and the main Hercynian
ridge was already built up. During the upheaval of the
Hercynian range the anticlines and synclines were built
up, some of them as new features, others rejuvenated
or blocked out old tectonic unities, while others were
modified or re-adapted structures. For a while North-
West Europe became a mountain range, in which at
last, under the impulse of a pressure from the south, a
giant thrust plane originated, stretching from the Rhine
to Ireland, along the Mendip-Condroz anticline, which
was built up with accumulated overthrusted masses.
The geological structure displayed by the geological map
of the North-West European palaeozoic deposits is a
good deal the result of these powerful earth movements.

2. Post-Carboniferous Periods. — The history of the
ruin of the Hercynian range is the textual repetition
of the history of the disappearance of the Caledonian
range. During the foregoing periods, earth movements
took place, which, although on a much less scale than
the foregoing ones, are by no means unimportant. Fold-
ing and faulting went on, followed by denudation
through these posthumous disturbances. The tectonic
net of North-West Europe, already very complicated,
became still more so by features of less importance. ■ As
a general result of these post-carboniferous phenomena,
the great North-West European belt was separated into
detached basins, and the old core of the intervening
anticlines was laid bare. We may add that since the
Keuper period up to the present folding and faulting,
erosion and sedimentation have never ceased to work on
the palaeozoic floor of North-West Europe. The impor-
tant faults of the Wurm and Dutch Limburg coal field
are still shaking the ground. Beneath the plains of
Holland and Campine the coal measures are buried low
down in the earth’s crust. The Molembeersel boring
was still in the Oligocene at a depth of 3,442 ft. in the
Campine. At the Rothem boring the coal measures
were found at a depth of about 4,000 ft.

Local Descriptions.

The author then discusses the physical geology of the
British Isles in its-bearing on the several coal fields.

The Kent Coal Field.—Since the discovery of this
coal field by B. Dawkins, good work has been done to
prove the characters and particulars of this new coal
field, and an important mass of information has been
gathered. Even from the palaeontological side, this coal
field, through the exertions of N. Arber and H. Bolton,
is better known than many older productive fields.
Nevertheless, much indeed remains to be done to get a
thorough knowledge of this curious coal field. The
writer intends to study the problems of the district,
which are very interesting for the purpose of this paper,
under the following headings :—

Position of the Kent Coal Field in the South-West
Namur Syncline.—N glance at the map of the coal fields
of this syncline shows at once that there are two distinct
forms among them. For instance, the pear-shaped
Bristol coal field is very different from the canoe-shaped
Belgian coal field. All the British and Irish coal fields
of this syncline belong, more or less, to a round-shaped
type, while all the Continental ones belong to the elon-
gated type. The difference between both types is not
merely one of shape only, but of structure also, the
elongation being due to an intense longitudinal folding
keeping pace with many disturbances and thrust faults.
Although the limits of the Kent coal field are still
obscure, there is no doubt that it does not exhibit the
shape of a narrow east and west trough, nor the sharp
folds of the Continental type. The horizontality, or
nearly so, of the strata in the records of most of the
borings and pits, the gentle dip along north and north-
south cross sections, the slight undulations displayed in
the colliery workings, bear no resemblance with the
structure of the nearest French coal field, but they
remind us of the structure of the South Wales coal field
(north-east corner). Therefore, the limit between both
types must be somewhere in the Straits of Dover.

Relationship of the Kent Coal Fields with the Other
Coal Fields.—From the abundance of marine beds in its
strata, and from the identity of the fauna with the
marine fauna of the middle coal measures of the other
coal fields, we know that these measures were deposited
in a basin, having open and important connections with
all the other coal fields of North-West Europe, where
such marine beds have been found. On the eastern
shore of the Straits of Dover the middle measures are
wanting, or much reduced. Therefore, the connection
with other coal fields was doubtless during this period
throughout the west. ’ There is now a series of borings
from Brabourne to Ghilham, Bobbington, and Cl'ffe,
where silurian rocks have been proved. From the
known north to ‘south width of the coal field there is
little or no hope that it .might cross the silurian barrier
already pointed out by B. Dawkins. Therefore, the
former western extent of the coal field has been cut by
upheaval and denudation. This barrier may be a branch
of the Pennine chain, or a peculiar transversal ridge.
Behind the barrier westwards, the numerous borings
scattered on the London area (Strahan) all prove pre-
carboniferous rocks, and, in consequence, little hope is
left to find an actual connection in this direction with
the coal fields of South-West England. A northern
margin of carboniferous limestone is well proved by the
results of borings. The writer will deal later on "with
the southern margin. As to the eastern margin, French
geologists have taken into account the direction of the
Artois axis and west-north-west trend of the Bouloinais
coal field, and formerly looked for a westwards prolonga-
tion of this coal field in Sussex; and from the same idea
no doubt the Sub-Wealden borings and the Pluckley