The South Staffordshire Coalfield/Preface
PREFACE TO THE SECOND EDITION.
It was not till the month of October 1858 that I could make arrangements for such a re-examination of the South Staffordshire Coal-field as should enable me to bring out a second edition of this Memoir. Many new mines and cuttings of different kinds having been opened during the preceding nine years, this re-examination, brief as it was, afforded fresh information on some points that had previously been obscure.
My colleagues, Messrs. Hull and Howell, under the direction of Professor Ramsay, have completed the examination of the details of the New Red Sandstone of the Midland counties, and Professor Ramsay has now contributed (pp. 3 to 8) an account of their results.
I have added also to the description of the Permian rocks an abstract of Professor Ramsay's remarkable speculations on the origin of the large angular blocks in the Permian breccia (pp. 13 to 15).
In the coal-field, the new pits at Essington and those on Cannock Chase gave additional data for the co-relation of the Wyrley and Essington district with the remainder of the field. The identity of the separated coals of Wyrley and Essington, (from the Old Robins coal down to the Bentley Hay coal), with those which, in the central part of the coal-field, unite to produce the Thick coal, is thus put beyond doubt. There was, indeed, no doubt on my own mind of this fact even ten years ago. A diagram, illustrative of the method of this expansion of the measures and separation of the coals, is now added to the general description of the Coal-measures.
An important change has been made, both in the Memoir and in the latest edition of the maps and sections, as regards the classification of certain red clays and sandstones which occur at Walsall Wood. Essington Wood, and at other localities about or within the coal-field. These were at first supposed to belong to the New Red Sandstone, afterwards were believed to be Permian, but are now decided to be true Coal-measures. I find in my own manuscript notes, made during the survey of the coal-field, the strongest expressions of opinion that these were in reality Coal-measures; but as that conclusion involved practical consequences which might, if erroneous, have led some persons into fruitless expenditure, it was thought safer to colour them as Permian until more evidence could be procured.
The opinion that they are upper Coal-measures, however, is now so strongly supported by my colleagues, and by several resident gentlemen of practical experience, and seems so far confirmed by the facts learnt in sinking the pits at Coppy Hall by the Rev. Baily Williams, that there is no longer any necessity for reserve in expressing the opinion, or for hesitation in altering the colouring of the maps and sections accordingly. It adds several square miles to the area of the northern part of the coal-field.
The driving of the tunnel beneath the Rowley Hills, which by a rather unfortunate misnomer is called the Netherton tunnel, added somewhat more of precision to our ideas respecting the "position and lie," of the Rowley basalt than we previously possessed. I regret that I was not aware till it was completed that this so-called Netherton tunnel was being driven through the base of the Rowley Hills, and thus was not led to visit it during the operation. This and the analyses of two specimens of the trap rocks by Mr. Henry, of London, has enabled me to give a little more complete account of the igneous rocks of the district. An equally good analysis of the "green rock" or "greenstone" of the district is still a desideratum.
I regret that an injustice was done in the first edition of this Memoir to the memory of Mr. Keir, whose excellent account of the coal-field, so far as it was known in his day, was published in Shaw's History of Staffordshire. Not only were his labours altogether ignored, but some of his materials were used at second hand without any acknowledgement of the true source whence they were derived. I need hardly say that this injustice was unintentional, and arose from forgetfulness, chiefly the result of the circumstances under which the Memoir was composed. The documentary materials for it were collected from different persons at different times, and were reduced to order at intervals in different parts of England. Wales, and Ireland. Anything like research into books was almost necessarily precluded, and the attention was fixed on the materials which were at hand, and which could be carried about in the portmanteau.
This absence of research into the labours of previous or contemporaneous writers is a defect which it is difficult for those to avoid who are almost constantly engaged in field work and debarred from access to libraries, except at rare and short intervals. It has been indeed hitherto sufficiently difficult for the officers of the Geological Survey to find time to give any written account even of their own Labours, the results of which have been chiefly published in the form of maps and sections.
In the following Memoir the attention is confined solely to the facts observed in the district; from some questions that have been put to me, however. I believe that a few preliminary words respecting the relation of the Carboniferous rocks of South Staffordshire to those of the neighbouring districts will not be out of place.
I have been asked especially as to the reason of the absence of the groups of rock known as Millstone grit. Carboniferous limestone, and Old Red sandstone, between the Coal-measures and the Upper Silurian rocks of South Staffordshire.
If we proceed from the South Staffordshire to the North Staffordshire coal-field, a distance of only 20 or 25 miles, we find a vast difference in the constitution of the Carboniferous rocks. In South Staffordshire the Carboniferous rocks consist of Coal-measures only, the maximum thickness of which does not seem ever to have much exceeded 1,000 feet. In North Staffordshire these beds are far more numerous, and attain an aggregate thickness four or five times greater. Thick masses of sandstone occur about their base, which are grouped together under the name of the Millstone grit. Below this, as we go towards Derbyshire, we find thick beds of black shale, in some places interstratified with thin limestones, and from underneath these comes out an assemblage of beds of pure limestone several hundred feet in thickness, forming all the beautiful hill country that spreads from Dovedale to Matlock and thence to Castleton and Buxton. This group of limestone is called the Mountain or Carboniferous Limestone, and the black shale between it and the Millstone grit is known as the Upper Limestone Shale.
The Carboniferous Limestone, either simple as a group of beds of pure limestone or complicated by being more or Jess interstratified with shales and sandstones, extends from Derbyshire to the borders of Scotland with a mean thickness of probably 2,000 feet at least; it re-appears as a simple limestone in North Wales, in Flintshire, and Denbighshire, and in the same form in South Wales and the adjacent parts of England, varying in thickness from 500 to 1,000 feet. The Coal-measures of the South Welsh coal-field are certainly 7,000 feet, even if they are not in some places 12,000 feet, in aggregate thickness, and have a thick group of sandstone beds at the base, which may be called the Millstone Grit, as well as another higher up, known as Pennant Grit. But from underneath the Carboniferous Limestone, which forms the thick enamelled lining, as it were, of the basins of the South Welsh and Forest of Dean coal-fields, there rises on all sides a great group of red and brown sandstones and red marls, known as the Old Red Sandstone. This Old Red Sandstone forms whole mountains of 2,700 feet in altitude (as the Vans of Brecon), and cannot in some places have a less aggregate thickness than about 10,000 feet.
Now these vast formations of Old Red Sandstone and Carboniferous limestone (to say nothing of the lower part of the Coal-measure series or Millstone grit) are altogether absent in South Staffordshire, neither is there the slightest reason for supposing that any part of them ever existed in that district. For not only are they absent in South Staffordshire, but there is a band of country running E. and W. across England from Leicestershire, through Warwickshire, South Staffordshire, North Shropshire, into Montgomeryshire, along which they are equally deficient.
In Leicestershire, the Carboniferous limestone, thinning out from Derbyshire, ends on the north side of Charnwood Forest, while the Coal-measures overlap it and rest upon the Cambrian rocks. In Warwickshire no Carboniferous limestone makes its appearance. In South Staffordshire the Coal-measures rest directly upon Upper Silurian rocks. In the Coalbrookdale coalfield the Coal-measures resting towards the south on the Old Red Sandstone, overlap its termination towards the north, and repose upon Upper Silurian rocks, and further west, towards Church Stretton and the Breiddens, upon Lower Silurian and Cambrian rocks. Thin scraps of Carboniferous limestone, indeed, show themselves about Lillieshall, as if spreading just so far from Derbyshire, and also at the Clee Hills, as if dying out from South Wales, and the Old Red Sandstone likewise stretches from the latter direction, but thins out and terminates before reaching the Severn. It is clear then that the absence of the Old Red Sandstone and Carboniferous limestone from the narrow band of country before indicated, is due to causes operating during the deposition of those formations. They each died away and terminated as they approached it both from the north and the south.
Now these formations were deposited under water, the Carboniferous limestone certainly under the sea, as is shown by its being almost entirely made up of the remains of marine animals, it seems natural then to suppose that the area towards which they thus both thin out and terminate was not under water, but formed dry land. It is, in fact, almost certain, that whilst the sea flowed deep over the remainder of that space where England and Wales now exist, during the periods when the great formations of Old Red Sandstone and Carboniferous limestone were in course of deposition, & narrow promontory, or an island, or a group of closely connected islands ran in an east and west line across the district before pointed out. This land, however, must itself have been depressed either wholly or in part while the Coal-measures were being deposited, and as it slowly sank beneath the water, sheet after sheet of Coal-measures extended over it, till, perhaps, the whole neighbourhood was finally buried under one wide-spread subaqueous Coal-measure plain. Since that time it has not only been lifted up again, but broken, dislocated, and contorted by forces of disturbance acting from below, and worn and eroded by denuding agencies acting from above; fresh formations have been deposited over it, and these also with the subjacent rocks broken and again denuded, till all likeness of its former state has been utterly destroyed, and its former condition left to be guessed at solely by deductions drawn from the relations which can be traced between the formations of which it is composed.
51, Stephen's Green, Dublin,
6th September 1859.