THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CX.

FRIDAY, NOVEMBER 19, 1915.

No. 2864.

The Fauna and Stratigraphy of the Kent Coal Field.

DISCUSSION BEFORE THE MANCHESTER GEOLOGICAL AND MINING SOCIETY.

Criticisms by Dr. WHEELTON HIND and Dr. STRAHAN.—Mr. BOLTON’S reply to Dr. ARBER.

In continuance of the discussion upon Mr. Herbert
Bolton’s paper on “ The Fauna and Stratigraphy of
the Kent Coal Field,” read at a meeting of the Man-
chester Geological and Mining Society (Colliery
Guardian, June 18 and 25), written contributions have
been submitted by Dr. Wheelton Hind, Mr. Watson
(Manchester University), and Dr. Strahan (London).

Dr. Hind congratulated the author on the large
number of facts recorded; they had now an up-to-date
knowledge of the fauna of the Kent coal field. As might
be expected, the Kent coal field had closer affinities with
the South Wales, British, Pas-de-Calais, and Belgian
coal fields than with those of the Midlands, owing to the
tectonic structure of the two areas; but the intervening
coal fields of Coalbrookdale and South Staffordshire
formed a half-way house and area of value in showing a
connection between both districts. It was, therefore,
not surprising that there should be no evidence of the
millstone grit in the Kentish borings. It was not suffi-
ciently appreciated by students of carboniferous geology
how very local a deposit the millstone grit was. Its
greatest thickness was in the North Midlands, and from
this point it rapidly thinned out north, west, and south,
and to the east nothing was known about it after it
dipped below the coal measures of the eastern Pennine
slope. In North Staffordshire it thinned out rapidly to
a very small thickness, and was not represented in South
Staffordshire; while in Scotland and North Wales it was
very feebly represented, and of a very different litho-
logical structure. The evidence of an unconformity
between the Kent coal measures and the carboniferous
limestone was small, but interesting. Our knowledge of
the British and South Wales coal field would, he
thought, lead one to expect such a condition. The
faunal evidence afforded by the examination of the bore-
holes was of the greatest importance; it spoke with no
uncertain voice as to the'horizon of the measures, and
enabled one to correlate them with a very high degree of
certainty with the coal measures of the Midlands. In
all sections the typical fossil was Anthracomya Phillipsi
and its relatives. He (Dr. Hind) was interested to note
that certain small species (A. Minima and A. Laevis) of
that genus appeared to be stages of growth of
A. Phillipsi. That fossil had a very definite distribu-
tion in the Midland coal fields, and indicated a high coal
measure horizon. In North Staffordshire the zone of
A. Phillipsi contained at least one marine band—that
associated with the Lady or Bay coal, and that band
contained Lingula mytiloides, Orbiculoidea nitida,
Productus scabriculus, and at least eight other species
of marine shells. That coal was 48 ft. above the
Knowles or Winghay coal. He did not understand why
the fossils Lingula, Orbiculoidea, and Productus should
be relied on as affording any evidence of horizon what-
ever. They all occurred, and, in the case of the former,
two at least specifically, some way down in the carboni-
ferous limestone and at various horizons in the Pendle-
side series, millstone grit, and coal measures, and there-
fore most certainly could never afford conclusive
evidence of horizon. They were present in practically
every marine band in the coal measures. The fauna,
as a whole, had a distinctly high facies, and this con-
clusion w’as strengthened by the absence of all but one
species of Carbonicola, and the rarity of that species. He
would say that the base of the Kentish coal measures,
as reported in the paper, did not pass much, if at all,
below the horizon of the Knowles coal of North Stafford-
shire. There was, therefore, if his views were correct,
no contradiction between the paleobotanical and the
faunal evidence, and he was absolutely in agreement
with Dr. Arber, but on totally different grounds, in his
view of the horizon of the Kentish coal measures. If

the fauna of the Belgian coal fields were compared with
that of the Kent coal measures, strong support was
afforded to their high horizon.

Mr. David M. S. Watson (Manchester University)
said the paper was an important addition to their
knowledge of the Kent coal field, but Mr. Bolton’s dis-
cussion of the evidence of its age raised some doubtful
points of general interest. The author’s correlation of
the Somerset and Kent coal fields rested largely on the
occurrence of occasional marine faunas in both.
Nothing whatever was known of the upper limit of the
range of those marine animals. That they did not
appear in the upper coal measures of Staffordshire, for
instance, was entirely dependent on the fact that these
measures were of entirely fresh water origin. Their
greater abundance in the lower coal measures than in
succeeding beds in Lancashire simply meant that during
the time when those beds were being deposited irrup-
tions of the sea were more frequent, a local feature
which had nothing to do with the real range of the animals
which were well known to recur in the Tame marine
band high in the middle coal measures. When Mr.
Bolton used the occurrence of Lingula, Orbiculoidea and
Productus to prove the occurrence of lower measures in
Kent he was simply gambling in the belief that irrup-
tions of the sea became infrequent at the same time
over England between Bristol and Canterbury. That
belief was possibly justified, but quite unproved.
Between Bristol and Lancashire it was almost certainly
correct. Mr. Bolton expressed his belief, which was
no doubt justified, that Anthracomya Minima and
Laevis were growth stages of A. Phillipsi, but, as Mr.
Prevost had pointed out, those two occurred lower than
A. Phillipsi, although they ranged up to its horizon.
Speaking without any serious investigation of the matter,
it seemed probable that A. Minima and A. Laevis were
really ancestors of A. Phillipsi reproduced in its life
history in accordance with the well known principle that
“ ontogeny repeats phylogeny.” If that were so, the
incoming of A. Phillipsi would be the turn of its real
origin, and would provide a definite horizon in the
districts in which it was known.

. Dr. Aubrey Strahan (London) wrote that he desired
to express his sense of the value of the additions made
by the author to the palaeontology of the Kent coal
measures. The work had been carried out under
unusual difficulties. Subdivision of the coal measures
on palaeontological grounds still lacked precision owing
to the great vertical range of most of the fossils, and
in this case the difficulties had been increased by the
scantiness and fragmentary character of the material
obtainable in boreholes. Possibly for these reasons the
“ lower series ” which the author claimed to have
differentiated appeared to lack clearness in definition.
The statement that the Kent coal measures included
representatives of the Pennant appeared to him to
require confirmation. That the millstone grit was
absent in Kent had long been known, and that there was
an unconformity between the coal measures and the
carboniferous limestone was probable. The trough,
however, which had been proved to exist in the lime-
stone surface was in the main, if not wholly, of a
structural character, and not the result of erosion before
the coal measures were deposited. The trough had
determined the limits of the Kent coal field, and its
form and position were, therefore, of much importance.
He was not wholly in agreement with the author on
some points. In the accompanying map in the text
the boreholes described by Mr. Bolton, and some others
as yet undescribed, had been laid down, and against
them the depth of the top of the limestone below
Ordnance datum had been entered, where known. It

became apparent that there was sufficient evidence for
the drawing of contour lines in the limestone surface, at
depths ranging from 1,000 to 3,500 ft. below Ordnance
datum. The contour lines on the north-eastern side
of the trough were evenly spaced, and indicated a
gradient of 2,500 in 7 J miles, or about 1 in 16. They
swept round from a north-westerly direction at the
coast to a westerly direction near Chislet. On a
south-westerly side of the trough a sufficient number of
these contour lines could be fixed to show that the
gradient was 2,156 ft. in three miles, or about 1 in 7-J-,
and that they ran in a direction S. 12 degs. E. The
trough, therefore, was steep-sided on its western margin,
filled up towards the north, and deepened towards the
south, the line of greatest depth presumably ranging
from near Dover Colliery to some point a little east of
Bourne. It was worth noting that parts of the margin
of the coal field could be located by superimposing upon
this contour map a similar map showing contours in
the floor upon which the secondary strata rested.
Wherever space was left between the two places, that
space must be occupied by coal measures, but wherever
the two planes came in contact the secondary rocks
must rest upon carboniferous limestone or some older
rock. That the trough was mainly structural, and not
due to erosion of the top of the limestone, was indicated
by the fact that rocks older than the limestone lay
next below the secondary strata to the north and west of
the trough. It was, therefore, a syncline, and only in
part, if at all, a trough of erosion. Much of that was
confirmatory of the conclusions formed by the author,
but he (Dr. Strahan) was unable to accept the state-
ment that the axis of Ferques ran through Hythe to a
little south of Ashford, and then began to swing round
to the west. Ferques was upwards of 50 miles away,
and although it was likely that the general structural
character of the west (or south-western) margin of the
coal measures might be similar at Ferques and in Kent,
it seemed inadvisable to speak of the axis as though it
had been proved to be continuous. Nor could he follow
the argument about the “ Ebbsfleet axis ” and its sug-
gested swing to the north-west. The axis had hardly
yet been proved, and there was no evidence of the
suggested swing. In any circumstances, as a movement
which was likely to have been of post-carboniferous
age, it would not affect the palaeontological relationship
of the Kent and the Midlands coal fields as was sug-
gested. Lastly, that the trough was an eastward exten-
sion of the South Wales syncline had long been
disproved. These points were submitted for the con-
sideration of the author, not as criticisms, but in the
hope that they would be of service to him in continuing
the work which he had so well begun.

Mr. Bolton, in the course of a long reply, said that
Dr. Arber’s remarks (Colliery Guardian, October 22,
1915) called for first consideration. Dr. Arber had dealt
with a number of matters which had nothing to do with
the questions under consideration; and whilst declaring
that he was not competent to judge of the value of the
palaeontological side of the paper, he based his chief
criticism upon the animal fossils. It was therefore
evident that he was somewhat inconsistent. Neverthe-
less, he would reply to Dr. Arber’s remarks in regular
order. With regard to the question of the presence or
absence of millstone grit in Kent, the late Mr. James
McMurtrie had clearly demonstrated the presence of
fully 1,000 ft. of millstone grit in the Bristol area, and
he (Mr. Bolton) knew of field evidence in the same area
which showed that the millstone grit entered into a
closer relation to the carboniferous limestone below than
was the case in more northerly districts. Whilst bear-
ing this in mind, he did not find evidence of millstone
grit in the Kent area he had worked over, and therefore
said so, but he was not prepared to say that millstone
grit would not be found somewhere in the Kent coal
field. His position was that he did not know. His
conclusion that the coal measures of Kent rested uncon-
formably upon the lower carboniferous rocks was a per-