5 CHAPTER II. TABLE AND LITHOLOGICAL DESCRIPTION OF THE GEOLOGICAL FORMATIONS. The rocks we shall have to describe belong, with the exception of Recent and Glacial deposits, to the Carboniferous system. The following table shows them in descending order: Recent and post-Glacial. Alluvium, River-gravels, &c. Carboniferous or Mountain Limestone.—This is a mass of limestone of very great but unknown thickness. Very thin partings of shale or clay are found in it here and there, and two or more beds of contemporaneous dolerite called Toadstone,* a corruption probably of todtstein, lit., dead, or unproductive stone (p. 122). In the upper portion the limestones are thinly bedded and somewhat earthy, and contain layers and nodules of chert. Below the cherty group comes a great thickness of exceedingly massive pure limestones, at times semi-crystalline, of white or pale grey colour; and underneath these there is a mixture of thickly- and thinly-bedded limestones. The number of the toadstone beds is still uncertain. In a pamphlet, printed for private circulation in 1834, the late Mr. W. Hopkins, the well-known mathematician, endeavoured with great ingenuity to explain all the facts bearing on the matter he could collect, on the hypothesis of there being only one toadstone; he does not seem to have been aware that two, and perhaps three, beds of toadstone have been sunk through in the same shaft; but besides this he was obliged to call into his aid faults, many of which have certainly no existence. White Watson, in his "Delineation of the Strata of Derbyshire" (1811), gives three beds of toadstone, and this was also Farey's view; see also Conybeare and Phillips' "Outlines of the Geology of England and Wales," p. 448. There seems to be no doubt that in places there are three beds, but that they are liable to thin away, as indeed is only likely; but we cannot surely say, till the limestone country has been more thoroughly examined, how many toadstones there are, and whether any of them spread uniformly over the whole district; for instance, it would be very rash to assume that the highest toadstone of Matlock and the highest toadstone of the neighbourhood of Buxton belong to one and the same bed. A summary of the evidence on which the toadstones are attributed to contemporaneous volcanic eruptions will be found on p. 123. *See Allport, Quart. Journ. Geol. Soc., vol. xxx. (1874), p. 551, for a description of the Toadstone. Yoredale Beds.-Under this head we include a mass of sandstones, shales, and limestones lying between the Mountain Limestone and the Lowest Millstone Grit. On the western side of the great anticlinal these beds were best shown in the neighbourhood of Leek, and were there originally subdivided as follows.* Subdivisions of the Yoredale Rocks, as formerly made. 1. Upper Group. 2. Middle Group. Yoredale Sandstones. 3. Lowest Group. la. Shales. 16. Yoredale or Shale Grit. Ic. Shales, with perhaps a few thin beds of limestone. Sandstones, hard and closely grained, here and there semi-crystalline quartz-rocks, with partings and thick beds of black shale. Black shales, with thin beds and nodules of earthy limestone. A more extended knowledge of the Carboniferous rocks of the north of England has shown reasons for modifying this classification, which are thus stated in the Geological Survey Memoir on the Yorkshire Coalfield (p. 33) :—" The Kinder Scout Grit has hitherto been taken by the Geological Survey for the base of the Millstone Grit, and all the beds beneath it have been placed among the Yoredale rocks. The Kinder Scout Grit is, in North Derbyshire, underlaid by a band of shale of variable thickness, but averaging perhaps between 200 and 300 feet, and beneath this there is a thick mass of sandstone with irregular bands of shale to which the name Yoredale Grit has been given. There is nothing to distinguish this bed from many of the sandstones of the Millstone Grit, and it would perhaps have been better to have included it in that group, especially as no sandstone of any importance occurs below it. This arrangement has, however, now become a matter of necessity. In Derbyshire there was some difference between the Kinder Scout and the Yoredale Grits; the first was, as a rule, coarse; the second almost invariably a fine rock; and the band of shale between the two was persistent. Further north, however, these means of distinction vanish; there is a numerous group of sandstones occupying the same general position as the Kinder Scout and Yoredale Grits of Derbyshire; but the bottom beds are frequently quite as coarse as any higher up on the series, and the shale bands are no longer persistent. There is nothing to do, then, but to include everything in the Millstone Grit down to the lowest thick sandstone." We will therefore transfer the bed called in the former edition of this Memoir the "Yoredale Grit" to the Millstone Grit group, and we may very fittingly designate it by the term applied to it by Farey, White Watson, and the older geologists, and call it the "Shale Grit." Since it forms the capping of Pendle Hill in Lancashire it is often called the "Pendle Grit." The existence of the beds marked lc was very doubtful about Leek, and we have not been able to recognise them on this side of the axis. *The Geology of the Country round Stockport, Macclesfield, Leek, and Congleton (Mems. of Geol. Survey,) p. 17. "This stone I call the Shale Freestone or Shale Grit, from the circumstance of its always alternating with shale." (Farey, General View of the Agriculture and Minerals of Derbyshire, vol. i., p. 228.) On the western side of the anticlinal the Yoredale Sandstones form the most striking member of the group, and are believed to reach in places the great thickness of 2,000 feet, but even there they were found to be very changeable, and sometimes had not more than a tenth of that thickness. It is somewhat doubtful whether they are represented on the eastern side, but we do find in places a group of shales and thinly bedded sandstones, underlying and apparently distinct from the Shale Grit, which may be their equivalents. These, though they nowhere reach so great a thickness as on the western side, are just as changeable. In the Edale valley they vary from 200 to 400 feet, and about Bamford are still thicker, but they are often wanting altogether, and we have not been able to trace them southwards farther than Hathersage. Also though the sandstones are for the most part hard and finely grained, we nowhere find on this side of the axis those peculiar quartz-rocks which are characteristic of this group about Leek and Macclesfield. Soine little hesitation in correlating these beds with the Yoredale Sandstones arises from the following consideration. They may be only a local form of the lower part of the Shale Grit, in which the shale bands that are always present in that sub-division, have swelled out till the sandstone element has become of subordinate importance. For the line drawn on the map for the lower boundary of the Shale Grit, and the same may said of the upper also, is a mere lithological boundary, showing in general way where sandstone ends and shale begins. Now the sandstone beds never keep for long the same thickness, but are wedgeshaped and dovetail. into the shales, so that thick masses of sandstone are often lost altogether, and their place taken by shale or by alternations of shale and sandstone within a small distance, and hence lines which merely separate sandstone from shale are often far from true geological horizons. Assuming, however, that we have in the present country representatives of the Yoredale Sandstones of North Staffordshire, the further question presents itself whether they are to be placed in the Yoredale Series or transferred to the Millstone Grit. Such an uncertainty is always liable to arise whenever we have only lithological characters to trust to for the purposes of classification. The black shales and impure limestones of the lowest Yoredale Group are unquestionably marine; there are very strong reasons for thinking that the Millstone Grit, though it contains marine intercalations, is in the main of fresh-water or estuarine origin. If then the Yoredale Sandstones contained fossils, these would decide whether they ought to be reckoned Yoredale or Millstone Grit; unfortunately no fossils have yet been found in them. There would also be considerable difficulty in drawing a line between them and the black shales and limestones which lie beneath them. It will therefore be convenient to retain them provisionally as Yoredale, but to look upon them as passage beds deposited irregularly in the interval during which the change took place from the marine conditions of the Carboniferous Limestone Series to the fresh-water or estuarine conditions of the Millstone Grit and Coal-Measures. This change seems to have been brought about very gradually, so that there is, in England at least, no wide spread unconformity between the marine Lower Carboniferous and the fresh-water Upper Carboniferous; but it may well have been accompanied by local oscillations of variable amount, and hence the passage beds deposited while it was going on may well vary in thickness from place to place and be in some places absent altogether. With this provisional classification, then, the Yoredale Series of the country now under consideration will consist of the two following members: Yoredale Sandstones.-Shales with thin beds of hard closely-grained sandstone. Lowest Group.-Black shales with thin beds and nodules of earthy limestone. The Lowest Group needs little description. Its upper part is almost wholly black shale, among which are a few nodules of earthy limestone with Goniatites: lower down, beds of black earthy limestone, which seem to become purer and more plentiful towards the bottom, are interstratified with the shales. It is not possible to form any very trustworthy estimate of the thickness of this division in the north of our district. Between Castleton and the Peak the beds roll so much that it is not safe to trust to an average dip, but the thickness cannot there be well less than 400 or 500 feet, and may be as much as 1,000 feet. In the Lady Wash Mine, near Eyam, the thickness of this group was proved to be 330 feet. Millstone Grit.-The chief members of the rocks of the present country, which are placed by the Geological Survey in this sub-formation, are shown in descending order in the left-hand column of the following table; the corresponding divisions for Staffordshire and Lancashire are given in the right-hand column :— The first five of these subdivisions are placed in the Lower CoalMeasures by some authors, who call our Third and Fourth Grits the First and Second Millstones. The great Carboniferous System is, as far as we know, conformable and without break from top to bottom,* and it would be idle therefore to argue that one method of subdivision was better than another, since all alike must be artificial and merely matters of convenience. The Shale (or Pendle) Grit reaches in the country round the Peak a thickness of 500 or 600 feet; though it thins away to the south, and in places dies out perhaps altogether, we get what seem to be traces of this bed even down to Duffield. In the north it is made up of thick massive sandstones, here and there passing into coarse grits and even conglo * Though something may be said in favour of a break at the base of the Middle Coal-Measures, the evidence for it is as yet very meagre. See Geology of the country round Bolton-en-le-Moors (Memoirs of the Geol. Survey), p. 7. In referring to this passage I must beg to be allowed to correct a slight mistake in it. I never saw the section there described.-A. H. G. merates, with many layers and a few thick masses of interbedded shale. When these shale-beds are traced across the country they are found sooner or later to die away, and fresh shale-bands set in on a slightly different horizon; hence we have not been able to make out any subdivisions of this complex mass that would be lasting, and the whole has been mapped and coloured as a single subdivision. The country occupied by this rock round the Peak is a broad plateau some 500 feet lower than the table of the Peak itself, deeply channelled by rivers and brook courses. The hills are heavy and flat-topped, with rounded edges and gently curving flanks, and along the steep sides of the valleys run little terraces marking the outcrops of the shale-bands. These physical features enable us at a glance, in this district, to distinguish between the Shale Grit and the Kinder Scout beds, the outcrops of the latter forming very generally bold craggy cliffs, often crowned with piles of rocks weathered into fantastic shapes. The shales below the Kinder Scout Grit have thin sandstones, and rarely thin beds of coarse grit in them. They vary in thickness from 30 to 300 feet. These beds are mostly found cropping out along a steep hill-side below the craggy escarpment of the Kinder Scout Grit. The Kinder Scout Grit consists in the north of Derbyshire of two thick beds of massive gritstone and conglomerate, parted by a thick mass of shale. As we follow this subdivision southwards we find that the lower gritstone dies away on Bamford Edge: the upper bed runs on, but becomes step by step thinner and more finely grained down to Chatsworth: hence southwards it gradually recovers itself, and about Rowsley, Matlock, and Ashover is again a very striking sandstone, by no means however so coarse and massive as the rock we started with in the north, except perhaps about Birchover. The features made in the landscape by the escarpments of these grits have already been mentioned. On the top of the Upper Kinder Scout Grit, and sometimes on the top also of the Lower, are traces of very thin seams of coal. They are of no economical value, but have an important bearing on the question of the changes that went on during the formation of the Grit Series. The beds between the Kinder Scout and Chatsworth Grits are mostly shale, but sandstones are found in them, one especially being often seen near the top of the division. The Chatsworth Grit is a rock very changeable in mineral character. About the centre of the district we are now concerned with it has, under its typical form, the upper and larger part red, massive, and coarse, often a conglomerate, while the lower beds are finely grained, flaggy sandstones. It is seen under this form at Crow Chine. The under flagstone is however often wanting, and the rock is coarse grit from top to bottom. The bed keeps this character pretty uniformly from Crow Chine to the south of Chatsworth, and along all this range it is further distinguished by its very marked and regular jointing. It is owing to this peculiarity in its structure that the Chatsworth Grit forms those escarpments, often running for miles in an unbroken wall of rock, which are the most noticeable features in the scenery of the Grit country; for as the shales beneath are undermined by the action of the weather, huge rectangular masses of the gritstone break off along joints, and the face of the cliff is thus from time to time renewed and so kept sharp and clean. Thus it happens that we may easily pick out, even from a distance, the escarpments of the Chatsworth Grit by the regular, wall |