July 10, 1914. THE COLLIERY GUARDIAN 91 The Geology of the Northern Part of the Derbyshire Coalfield.* In the preface to this memoir Dr. Teall says that the area described comprises the northern portion of the Derbyshire coalfield with underlying carboniferous rocks on the west and overlying permian and triassic rocks on the east. The prolongation of the coalfield south- ward in sheet 125 (Derby) was described in the memoir on that sheet published in 1908 (“ The Southern Part of the Derbyshire and Nottinghamshire coalfield ”), so that this and the present memoir together contain an account of the whole of the exposed Derby-Notts, coal- field, while the concealed eastward extension of the same field beneath the newer rocks is dealt with in the recently-issued (1913) memoir entitled “ The Concealed Coalfield of Yorkshire and Nottinghamshire.” The area now described was originally surveyed on the one-inch scale by W. W. Smyth and J. Phillips more than 60 years ago, with some later revision by A. H. Green, J. C. Ward, and J. R. Dakyns, and the work was published on the old-series one-inch quarter- sheets 82 NW. and 82 SW. The carboniferous area has been re-surveyed on the six-inch scale by Dr. W. Gibson and Mr. C. B. Wedd; and the permian and triassic area by Mr. J. B. Hill, Dr. R. L. Sherlock and Mr. G. W. Lamplugh. Part of the re-survey in sheet 112 was done under the superintendance of the late C. Fox-Strangways and the remainder under that of Mr. Lamplugh who has acted as editor for this memoir. The Yorkshire portion of new-series sheet 100 had been mapped • on the six-inch scale in the previous survey, and the description of this tract was included in the memoir on “ The Geology of the Yorkshire Coal- field,” by A. H. Green and others, published in 1878. That area is not dealt with in the present memoir, but the previous geological lines are reproduced on the map (sheet 100) now issued. While the broad outlines of the structure remain as shown on the old-series maps, the new maps bring out many features of interest not previously indicated. In the carboniferous limestone areas the volcanic and other igneous rocks have been classified in accordance with the valuable work of Dr. H. H. Arnold-Bemrose. The different members of the millstone grit series have been differentiated by colour-distinctions and their correlation has been amended. In coal measures, the chief sandstones have for the first time been separately mapped and the structure of the coalfield thus more clearly demonstrated. The great amount of additional mining information now available has allowed the details of the faulting, etc., to be more fully shown, while an important modification of the previous map is brought about by the elimination of the supposed boundary- fault (‘‘ Trinity Chapel fault ”) at the western margin of coalfield in the neighbourhood of Ashover. In the permian rocks, the lower marly portion has been separately distinguished as the “ Basement Beds.” The Coal Measures, their General Characters, and Structure.—The western margin of the broad sheet of coal measures is clearly defined by the outcrop of the millstone grits, and the eastern boundary by the bold and conspicious escarpment of the magnesian limestone. On the north, the coal measures are con- tinuous with those of South Yorkshire; and on the south, with those of the southern part of the Derbyshire coalfield. In character, the coal measures show a relation to the sequence in South Yorkshire, and to that in the Ere wash Valley. Thus the thickly-bedded and some- what coarse sandstones, conspicuously developed in the upper part of the series east of Rotherham, gradually diminish as they approach the county-border, but though of inferior thickness they are more persistent than in the Erewash Valley. Similarly the sandstones in the middle and lower parts, which in the south are fine-grained and of irregular occurrence, slowly assume northward a coarser texture and more massive character so that individual beds can.be identified with those of the Yorkshire sequence. Argillaceous rocks form a large and important group. When mixed with sand, as is frequently the case, they are known to the collier as “ stone-binds ” or “ rock- binds ” to distinguish them from the purely argillaceous members called “ binds.” For the beds of unstratified clays and for the seat-earths or underclays of the coals he usually applies the term “ clunch.” Beds of coal occur at intervals, but the thickest (5 to 7 ft.) and most valuable seams lie in the middle part of the sequence, which is also the location of the chief ironstones, termed “ rakes,” extensively worked up to the middle of last century. The complete succession is wanting both at the out- crop and in the deeper sinkings in the east, since every- where the red rocks of the upper coal measures and part of the underlying grey measures were denuded before the deposition of the permian strata. At Bolsover Castle, where some of the highest grey measures crop out beneath the magnesian limestone, the thickness amounts to 4,100 ft. At Maltby, 14 miles north-north-east of Bolsover, where the upper coal measures are present, it is no less than 5,000 ft., while at Oxton, 14 miles south-east of Bolsover, the complete ♦Memoirs of the Geological Survey, England and Wales, Explanation of Sheet 112 and the Southern Part of Sheet 100. By W. Gibson, D.Sc., and C. B. Wedd, B.A., with contributions by G. W. Lamplugh, F.R.S., J. B. Hill, R.N., and R. L. Sherlock, D.Sc., and an appendix by L. Moysey, B.A., M.B. sequence of grey measures does not much exceed 3,000 ft. These are estimated measurements, as in no one place have the coal measures been penetrated by a shaft or boring; but if, as in the following table, the proved thickness in feet of grey measures above the seam of coal known as the Top Hard at Oxton is compared with that above the same seam at Maltby, it is seen that the chief increment occurs above this coal. Oxton Maltby Divisions and description, ^ing Bovver Colliery district. district. Upper coal measures— Red measures without coal .. Middle coal measures— Grey measures, several coals Top Hard coal Grey measures, several coals Silkstone c< al Lower coal measures— Grey measures, some coals... Present.... Absent. 1,280 ... 1,550+. 1,000 1,100 : 1,100 ... 1,450 . ..Present. .. 2,250 1,100 .. 1,650 3,380 ... 4,100 + .. 5,000 The coal-measure deposits form a continuous series of which one part is not markedly differentiated from another by distinctive lithological characters, while the vertical distribution of the fossils has not been sufficiently worked out to serve as a guide in classifi- cation. Since, however, the subdivision of the coal measures into the upper, middle, and lower series, given in the table above, was adopted by Green (‘‘York- shire Coalfield,” p. 75) for the contiguous district the same classification is retained for the present area. It also has some advantages, for the Silkstone coal, which was taken by Green as marking the dividing line in Yorkshire between lower and middle coal measures, can be easily traced throughout the present district and possesses some characters which distinguish it from any other coal. Also, relative to the seams of the middle coal measures, those of the lower series are few Fig. 1.—Section across the Brimington Anticline, illustrating the Middle Coal Measures. ki.L K iteP c I MILE Robin Brook SEA LEVEL SEA LEVEL Unstone Colliery SSAV VERTICAL SCALE c rina rnt horizontal Prince Albert —Pits HORIZONTAL SCALE /OOO 2000 3000 0-000 3000 FEET N-NZE.|S.W Apperkndwle Colliery Fig. 2.—Section showing Structure of Coal Measures S.E. of Sheffield. in number and of far less commercial value; but this distinction does not hold good either for the southern part of the Derbyshire coalfield, where the Kilburn coal, 400 ft. below the Silkstone coal, is a thick and important seam, or for the northern part of the York- shire coalfield, in which the Beeston and Better Bed coals are highly esteemed. So far as the scanty palaeontological evidence admits a comparison, a greater change in the character of the fauna occurs at the horizon of the Top Hard than at the Silkstone coal. Structure.—The coal measures occupy a central position in a big synclinal fold, which contains the united coalfields of Yorkshire, Derbyshire and Notting- hamshire. The western edge of the fold is exposed in the present area, but its eastern margin is concealed beneath the mesozoic formations. The axis of the fold trends north and south with a general inclination of the strata to the east; but several minor folds cross and break the symmetry of the major syncline. Of these, the most important, from its effects on coal mining, is the conspicuous anticline, usually known as the “ Brimington anticline,” which extends from Holmes- field eastward to Unstone, where it bends south to continue as a tract of elevated ground past Brimington to Calow. The uplift is sufficient to bring the top beds of the lower coal measures, here unproductive, to the surface, and thus to introduce a tract of barren ground in the heart of the coalfield; but the loss to mining is in part compensated for by the double outcrop of the lower and better-class seams of the middle coal measures on the flanks of the anticline. Although, to the south-east of Calow, the fold loses its character of a well-defined anticline and becomes obscured by faulting, it nevertheless continues across the visible coalfield up to the escarpment of the permian formation west of Skegby. Throughout this distance it can be distinctly traced by means of the underground workings for coal, as well as, though less clearly, by the outcrops of the coal measure sandstones. Whether the disturbance extends still farther to the south-east beneath the triassic rocks has not yet been proved, but the evidence afforded by the borings at Thurgarton and Kelham* sugests its extension far beyond the limits of the map. The central and most conspicuous part of the anticlinal fold thus introduced is illustrated by the section shown on fig. 1. It will be seen that the easterly dip on the east side of the saddle is sharply ♦“The Concealed Coalfield of Yorkshire and Notting- hamshire.” Mem. Geol. Surv., 1913, p. 39. accentuated, and that it rapidly diminishes from about 20 degs. east of Arkwright station to somewhat under 4 degs. before the coal measures disappear beneath the magnesian limestone of Bolsover. It therefore happens that the chief workable seams of the middle coal measures are within easy mining depths beneath the permian cover. A complementary synclinal fold, with its centre at Sheepbridge, occurs to the west of the Brimington anticline, between Hasland and a short distance north of Barlow Brook, and another to the north around Dronfield, where it is clearly defined by the outcropping sandstones above the Silkstone and Deep Hard coals. North of the Dronfield syncline, the anticline trending a little south of east, which brings up the lower coal measures in the districts of Norton and Ridgeway, pitches to the east, so that the lowest strata crop out to the west, where they are deeply trenched by the River Sheaf down to the horizon of the Alton Coal. This and the Brimington anticline form the two arms of the horse-shoe-shaped anticline enfolding the Dronfield syncline. The faults crossing the coalfield are neither numerous nor of great magnitude, compared with those of the coalfields on the western side of the Pennines. They appertain to two systems : one trending north-west; and the other north-east. Both are intimately related to the folding, as is conspicuously shown along the line of the Brimington anticline between Tibshelf and Teversal, where the faults coincide with the change in strike of the coal measures. The main faults in the neighbourhood of Barlborough, Renishaw and Ecking- ton, and the large fault crossing Birley Moor are also in alignment with the Norton-Ridgeway anticline. The relative age of the two systems remains uncertain, but that the major part of both the faulting and folding is of pre-permian age is clearly shown between Hucknall-under-Huthwaite and Teversal, where the undisturbed and unbroken magnesian limestone crosses the faulted Brimington anticlinal uplift. Characteristic of the coalfield generally, and in great contrast to other Midland coalfields, the complete absence of boundary-faults may be noted. On the west the millstone grits rise at gentle angles from under the lower coal measures to their outcrop, and the sup- posed faulted junction between Wheatcroft and Stubbing Edge Hall, along the line of the “ Trinity Chapel fault,”j- has been proved to be an error in the interpretation of the local stratigraphy! Beneath the magnesian limestone the middle coal measures also incline gently eastward, and there is no occurrence of the post-triassic faulting so conspicuously developed on the west side of the Pennine uplift. Palceontological Characters.—In the list of fossils drawn up by Dr. Moysey the poverty of the flora and fauna of the lower doal measures is at once noticeable, and it will be gathered that in the fuller record from the middle coal measures the localities are chiefly situated in the southern part of the coalfield. The limited flora and fauna of the lower coal measures may represent an actual barrenness of the strata; but the more extensive knowledge of the palaeontology of the middle coal measures in the southern part of the coal- field is due to the active work of local geologists in that region, while in the present area hardly any attempt has been made in the past to examine the material obtained in sinking shafts or from the temporary opening up of clay pits. Owing to this very imperfect knowledge of the organic remains of the area the follow- ing account considers only two aspects of the enquiry : (1) to what extent does the vertical distribution of the fossils afford a means of distinguishing one part of the sequence from another? and (2) under what conditions, judging from the character of the organic remains, were the sediments deposited? (1.) Since the reliability of fossil evidence in the establishment of fixed zones depends on exhaustive and systematic research, it is not safe in the present case to conclude that a horizon can be recognised, either by the rarity or absence of certain fossils; or by the apparent restriction to it, dr the abundance, of particu- lar genera and species. A few broad generalisations, however, seem admissible, especially when they are in conformity with the evidence from adjacent areas and from other Midland coalfields. Among the plants, which are more abundantly repre- sented than other fossils, there is the same absence of Pccoptcris arborescens in the lower and middle coal measures as occurs in the southern part of the coalfield, f “ Carboniferous Limestone, etc., of N. Derbyshire,” Mem. Geol. Surv., ed. 2., 1887, p. 84. 1 “ Summary of Progress for 1905,’’ Mem. Geol. Surv., 1906, pp. 14-18.