1198

THE COLLIERY GUARDIAN.

June 23, 1916.

contours ranges eastward by South Milford before it
takes the north-eastward turn which the accepted inter-
pretation of the Selby borehole demands.

The contours of the 100, 200, 300, and 400 yds. may be
considered together. In the south they course round the
Mansfield anticlinal, taking on an outline not unlike the
body of a swan, the neck of which is extended north-
westward along the ridge of Brimington. The line of
200 yds. comes in again at Renishaw, and, followed by
those of 300 and 400 yds., demonstrates the continuation
of the Norton-Ridgway anticline in that of Barlborough.
The double inflection of the 500-yd. line about Worksop
may be an accidental feature; but it draws attention to
the circumstance that, if the disturbance which affects
the permian outcrop at Kiveton Park were flattened out,
the bend of the present contours of the Top Hard coal
below it would be considerably accentuated. In South
Yorkshire the contour of 200 yds. appears only as topping
the crest of the Bon fault disturbance between Parkgate
and New 'Stubbin. The 300-yd. line, and with it the
400 and 500 yd. lines, follow the course of the Bon from
Attercliffe almost to Kilnhurst before they bend round
somewhat sharply westward and form the southern lip
of the Brierley basin. The increase of cover shown by
the crowding of five contours into the mile or so of
country between Roundwood and Silverwood is evidence
enough of the abruptness with which at its north-
western edge the bowl of the Maltby basin ends off
against, or is cut off by, the Bon fault disturbance.
From Tankersley to Ossett the contour of 400 yds. passes
along or to the westward of the outcrop of the Barnsley
coal, and only the deeper lines for 500, 600, and 700 yds.
appear within our area to indicate the shape of what
was evidently the basin with the widest westward exten-
sion in the North Midland coal fields.

The eastward bend of the lines north of. Barnsley
appears to be determined by the north-western anticline
through Wharncliffc Woodmoor, which makes an
evident bulge in the side of the main basin. The ridge
through Syndale makes a similar bulge, but the embay-
ment which succeeds it to the east fails to affect the
contours of thicknesses less than 500 yds., which as far
as the northern boundary through Bedstone Luck (pre-
viously discussed) course very regularly east and west
across all that part of the exposed coal field north of a
line joining Wakefield and Pontefract. Further east,
within the magnesian limestone area, the contours of
small thicknesses must lose their parallelism, and in
agreement with the thicknesses of coal measures
actually sunk through at Askern Alain and other
pits north of Boncaster, those for thicknesses greater
than 500 yds. sweep round southward and ultimately
sooth-westward, and so complete the encircling of
the south side, of the Brierley basin. How the
lines for thicknesses smaller than 500 yds. behave
in the region north of Askern Main is a problem
which the evidence before the writer does not
solve; and though the lines as plotted have been
obtained by joining the requisite points on the net of
intersections, the contours for the surface of the Barnsley
bed, which form one set of strands of that net, were
sketched in on such insufficient evidence that one must
wait for that more direct information, which can only be
obtained from boreholes or pits which may be sunk
within the area in question. In any case, if the lines
are to be made to tally with the published results of the
boring at Selby, it is evident that somewhere about
Pollington they must bend back north-eastward rather
sharply. Thus bending, they may be taken to suggest
the beginnings of a further north-eastern trough or basin
centred in the district between Snaith and Goole, con-
cerning which, though rumour says that it contains a
considerably greater thickness of coal measures newer
than the Barnsley bed than has yet been placed on
record, no trustworthy information has yet been made
public.

In discussing the probable eastward limits of the con-
cealed coal field, Br. Gibson* has well emphasised the
importance of the general eastward thinning of the coal
measures below the permian in the district north of
Boncaster. The ranging of the contours of equal thick-
ness among the pits of the district shows well the truth
of his contention, notwithstanding that Bullcroft Main
and Askern Alain have both greater thicknesses than
Brodsworth Main, which lies a mile or so to the west of
both of them. The contours also show that the maxi-
mum rate of variation is not by any means always strictly
to the east. A tongue of thinner measures, as indicated
by the 400-yd. line, seems to occur within the district
ringed round by the sites of the boreholes at Thorne,
Armthorpe, Cantley, and South Car, and adjoined by the
pits of Hatfield Alain and Bentley; but whether this
promontory is a detached outpost or is really a spur pro-
jecting in from the eastern margin of the whole coal field,
further exploration alone can show. Aleanwhile, the
hypothesis may be put forward that beyond Thorne the
400 yd. contour turns again eastward, and, forming a col
with its neighbour somewhere about Sykehouse, gives
space for other lines which will indicate thicker measures
in the trough or basin of the iSnaith and Goole district.

For the determination of the eastern limits of the
Maltby basin, the evidence is hardly yet before us,
although from the results obtained at the sinkings of
Yorkshire Main and Rossington Alain, and from the bore-
holes at South Car and Cantley, it seems reasonable to
express an opinion that the trend of the main extension
will be south-eastwards. The results obtained by the
boring at South Leverton are quite in keeping with the
same suggestion, and it is reasonable to expect that in
the region some few miles south of Gainsborough the
measures containing the Barnsley bed may be continued
eastward beyond the line of the river Trent. Concerning
the borings over the intermediate district south of
Alaltby, little information has been made public since the
time of the memoir on “ The Concealed Coal Field,”
and except to remark that the Wallingwells borehole has

* Concealed Coal Field Memoir, p. 44, and fig. 3, p. 45.

confirmed the continued great thickness of the coal
measures between the permian and the Barnsley bed
south-eastward from Binnington, the writer will leave
the question of the thicknesses of the measures which
separate the horizon of the Barnsley from the base of the
permian in the central part of the coal field which lies
further to the south.

Interpretation of Statistical Results.

So far, in dealing with the measured differences of
depths and thicknesses proved in mining, the writer has
treated them merely as inert numbers distributed
fortuitously at defined points scattered over a deter-
minate area. By drawing contour lines among these
numbers, he has given graphic expression to their
ordered arrangement, and therefore to the laws which
govern their distribution, which the maps have proved
to be neither intricate nor yet over simple. It now
remains to interpret these statistical results in terms of
tectonic structure and the geological history of the area
dealt with.

Assuming that the surface on which the permian now .
rests was once a uniform and horizontal plane, it must
follow that the general slope and all the irregularities
which the contours of the base of the permian now show
have been produced by tilting and by various incidental
crumpling movements, all of later date than the time
of the deposition of the permian rocks; and, since the
causes of earth movement are deep-seated, and no space
can long remain open within the crust of the earth, it
must also follow that with the permian rocks when they
were tilted, bent, and broken, the coal measure rooks
beneath them were also affected, and raised or lowered
vertically to the same extent. On this assumption the
contours drawn to record the distribution of the differ-
ences of level between the base of the permian and the
Barnsley bed must be identical in form with the contours
which had been impressed upon the Barnsley bed before
the permian was laid down, and fig. 4 may be regarded
as a fair expression of what is known of the form which
tire Barnsley bed had taken at the beginning of permian
time. If contour lines drawn for equal thicknesses of the
beds between the Barnsley coal and the base of the
permian surface level above or below datum level be
accepted as the contour lines of the Barnsley bed in pre-
permian time, then do these same contour lines become
the strike lines, and are all lines drawn at right angles to
them in the direction of the full dip of the coal measures
as they lay at the time when the permian was laid down?

Whether in actual fact the pre-permian surface at the
beginning of permian time can be accepted as a flat area
quite devoid of general slope, is a matter which must be
settled by the accumulation of geological evidence, and
one which provides a subject for much further research;
but certainly neither the recent work of the Geological
Survey in Berbyshire and Nottinghamshire, nor any
observations which the writer has been able to make in
Yorkshire, has provided any suggestion of irregularities
--'other than those due to evident and well-recognised
post-permian folding or faulting—large enough to be
shown by contours drawn at vertical intervals of 100 yds.
on a map of so small a scale as -J-in. to the mile.

Such evidence as there is of original irregularity in the
pre-permian surface is provided by the patches of sand
which occur at intervals along the permian outcrop; but
these, although they certainly rest in hollows eroded into
the surface of the coal measures, are rarely more than a
score of feet in thickness, and the hollows in which the
sand occurs are quite minor features, having little
relationship to the general form of the pre-permian
country as a whole. The question of the general slope of
the surface is rather a wider one, and the evidence
bearing on it must be sought in the study of the uncon-
formity and overlap of the permian rocks over a very
wide stretch of country.

Comparisons of records of total thicknesses of permian
rocks proved at far distant points along a north and
south line—for example, Nottingham (50 ft.) and Hartle-
pool (900 ft.)—120 miles apart on a north-south line,
show an average northerly thickening of these rocks of
about 7’5 ft. per mile. Similar comparison of the records
along an east-and-west line, as Alanton (400 ft.) and
South Leverton (526 ft.), which are 12 miles apart, prove
an easterly increase of thickness of about 10 ft. per mile,
and there is, therefore, reason to expert a general north-
easterly thickening of the rocks at the rate of
about 12’5 ft. per mile. From this.it is possible to argue
that, since such north-easterly thickening of the permian
rocks may be due to overlap by successive beds or sheets
of permian rocks which overstepped one another south-
westwards against a sinking short-line; a correction for
this north-eastwardly slope ought to be applied, and that
until such correction has been made the forms of the
contours obtained by the statistical process of subtrac-
tion can hardly be accepted as really representing the
pre-permian contours of the Barnsley bed. However,
it does not appear that the consequent refinement is
worth the trouble of reconstructing the map, and there-
fore fig. 4 has been allowed to go uncorrected.

Considering this map- as showing the pre-permian con-
tours of the Barnsley bed, it is perhaps worth while
again to draw attention to the general form of some of the
leading structures, and especially to the north-western
and south-eastern alignment of the great basin structures
of Yorkshire as they lie on either side of the Bon fault
disturbance.

A line joining the centres of the basins at Brierley and
Maltby continued north-westwards passes through
Leeds. 'Continued south-eastwards, it passes between
S tee tie y and Alanton, a little east of, and not far from,
Welbeck, and crosses the southern boundary of the map
between Farnsfield and Kirklington; and it will be
interesting to know whether the troughs which intervene
between the anticlinal swells of Barlborough and of
Alansfield also contain their maximum thickness of
coal measures along this line. The north-west and south-
east direction of the alignment of the basins is in itself

interesting, in that it is parallel with a line drawn
through the inliers of the old rocks near Ingleton, and
the perhaps even more ancient rocks of Charnwood
Forest; a line along which the pre-Cambrian rocks of
Charnwood in pre-Cambrian time were compressed in
tight folds, cleaved and over-faulted—the line, in fact,
of “ Charnian ” trend.

In comparing the Barnsley contours of the two main
basins as they were in pre-permian time (fig. 4) with the
arrangement of the contours of the same bed as it is at
present (fig. 3), one notices that the crowding of
contours towards the western outcrop of the coal is no
longer a feature which strikes the eye. Crowding of
contours about the large basins is, in fact, a resultant
effect produced by the double tilting in two stages, the
one before and the second after the deposition of the
permian, and in viewing the contours of any coal seam
as it is at the present day, one must always remember
that slopes which have a component in the direction
20 degs. north of east have been increased by something
like 150 ft. per mile, while those which are level or have
a slope with any component in the direction 20 degs.
■south of west have been first tilted in the one direction
and then by a later tilting in a different direction, have
had their eastward slope correspondingly reduced.
Where, within the central areas of the basins, the
measures in pre-permian times were fairly flat, the dip
and strike which they have at present must be approxi-
mately identical with that of the magnesian limestone;
and conversely in the West Yorkshire districts, where
block-faulting is the rule, the dip, now inconstant in
direction and hardly measurable, must in pre-permian
time have had a general direction north-westwards,
and have been equal in amount to the present north-
easterly dip of the permian as it is in the district between
Pontefract and 'Selby.

Flanking the main Charnian trough-line on its south-
western side, there is some evidence of subsidiary folding
parallel to the same direction. In the Yorkshire district
these flanking folds are generally wide and shallow, but
most of them are situate westward of the present line of
outcrop of ’the Barnsley bed and its equivalents. The
Whinmoor, Beeston, and Shirtcllffe seams, some 400 yds.
lower in the coal measure series, provide thegreatest
number of scattered spot levels available for the drawing
of contours in this western and north-western district.

Southwards of Sheffield, the flanking anticlinals are
of greater power and amplitude than in the northern
district, and those of Norton, Ridgway, Barlborough,
Whitwell, Holmesfield, Brimington, Tibshelf, Alansfield,
and Rufford are certainly to be regarded as major
features in determining the structure in the Berbyshire
and Nottinghamshire part of the coal field. As the axial
line of each of these anticlinal features is continued
south-eastwards, it changes its direction; and as the
effects of the swells encroach upon the central trough-
line of the district, the main line of their trend has
curved round until it is approximately east and west.
The swan-like form of the Brimington anticline has
already called for comment. The Norton-Ridgway-
Barlborough ridge in pre-permian time was not
dissimilar in form. The anticline which brings up the
lower carboniferous rocks in the midst of the millstone
grit at Ashover and at Crich in the midst of the millstone
grit area further to the west, belong to the same family
of flanking anticlinals also related to the Charnian axis
of the Pennine chain.

In order to emphasise and suggest to the eye the not
inconsiderable part played by Charnian movements in
determining the lie of all the coal fields about the
Pennine chain, the key map (fig. 1), which shows the
area in relation to other coal fields, was prepared in a
particular way, each set of formations being ruled over
with lines parallel to the direction of their dominant
structures. The direction chosen for the lines which
indicate the area over which the carboniferous rocks
come to outcrop was obtained by joining Ingleton and
Charnwood.

It needs only a glance at this or any other map, which
brings out the distribution of the coal fields of the South
Pennine country, to enable one to appreciate the aptness
of the description of the central anticline of the Pennines
as breaking up southwards into smaller folds which
diverge like the outstretched fingers of a hand, in the
hollows between the fingers of which lie the coal basins
of the Southern Midlands.

The eastward bending of the Charnian anticline
through Norton, Ridgway, Barlborough, and Whitwell,
as also that of Brimington, through Tibshelf and Rufford
may be interpreted as the swing of flanking folds, follow-
ing in sympathetic parallelism the trend of the palmate
spreading which marks the structural passage of the main
Pennine anticline into the South Midland coal field
district beneath the triassic rocks of the central Midland
plain.

As a suggestion which may throw light upon the causes
which produced the eastward curving of the north-west
and south-east folds as they pass into the southern part
of the district, one may point out that not far from the
southern limits of this area was the great east-and-west
ridge of lower palaeozoic and old rocks, which during
almost the whole of the carboniferous period stood up as
land and separated a southern region of South Wales,
Bristol, and Belgium from a northern area of deposition,
which extended over almost the whole area, and even
beyond the limits of the region represented on the key
map (fig. 1). Against this ridge the coal measures and
older carboniferous rocks thin off, and if earth move-
ments producing compression advanced from the north-
eastwards, and gave rise to Charnian folds, the northern
district, where the soft and freshly-deposited carbonifer-
ous rocks were thick, might offer less resistance to a
given stress, and might therefore be driven forward more
rapidly than the resistant old rocks under their thin
covering of carboniferous rocks in the neighbourhood of
the ridge to the south. If differential rates of advance
were set up at different points along the anticlinal ridges,