The North of England: Dykes, The Great Whin Sill—The Derbyshire Toadstones—The Isle of Man—East Somerset—Devonshire
The volcanic intercalations which diversify the Lower Carboniferous formations of Southern Scotland extend but a short way across the English Border, and although, over the moors and hills of the north of Cumberland and Northumberland, the Carboniferous sandstones, limestones and shales are well exposed, they present no continuation of either the plateau or puy-eruptions which play so prominent a part in the geology of Roxburghshire and Dumfriesshire. This deficiency is all the more noticeable seeing that the Carboniferous system is exposed down to its very base, in the deep dales of the North of England. Had any truly interstratified volcanic material existed in the system there, it could hardly fail to have been detected.
But while contemporaneous volcanic rocks are absent, the northern English counties contain many intrusive masses of dolerite, diabase, andesite or other eruptive rocks, which may be found traversing all the subdivisions of the Carboniferous system. These eruptive materials have taken two forms: in some cases they rise as Dykes, in others they appear as Sills.
Dykes.—With regard to the dykes, some are probably much later than the Carboniferous period, and consequently will be more appropriately considered in Chapters xxxiv. and xxxv. The great Cleveland dyke, for example, which runs across the Carboniferous, Permian, Triassic and Jurassic formations, is probably referable to the Older Tertiary volcanic period. One dyke known as the Hett Dyke, has been plausibly claimed as possibly of Carboniferous age. It runs in a W.S.W. direction from the Magnesian Limestone escarpment at Quarrington Hill, a few miles to the east of Durham, through the great Coal-field, across the Millstone Grit and Carboniferous Limestone, disappearing near Middleton in Teesdale. Its total length is thus about 23 miles. It varies in breadth from about 6 to about 15 feet, and appears to increase in dimensions as it goes westward.[1]
[1] Sedgwick, Trans. Geol. Soc. 2nd series, iii. part 1 (1826-28), p. 63; Trans. Cambridge Phil. Soc. ii. (1822), p. 21. Sir J. Lowthian Bell, Proc. Roy. Soc. xxiii. (1875), p. 543.
The age of this dyke cannot at present be satisfactorily fixed. It must be later than the Coal-measures through which it rises. Sedgwick long ago pointed out that though it reaches the escarpment of the Magnesian Limestone, it does not cut it; yet it is found in coal-mining to traverse the Coal-measures underlying the Limestone. He was accordingly inclined to believe it to be of older date than the Magnesian Limestone. At its western extremity it approaches close to the Great Whin Sill of Teesdale, though no absolute connection between the two has been established. Mr. Teall, however, has called attention to the similarity between the microscopic structure of the rock forming the Hett Dyke and that of the mass of the Whin Sill, and he is strongly inclined to regard them as belonging to the same period of intrusion.[2]
[2] Quart. Journ. Geol. Soc. xl. (1884), p. 230.
It is especially worthy of remark that in the course of its nearly rectilinear course across the Durham Coal-field, the Hett Dyke, where it crosses the Wear, is flanked on the north at a distance of a little more than two miles by a second parallel dyke of nearly identical composition. Between the two dykes, during mining operations, a sill about 20 feet thick has been met with, lying between two well-known coal-seams at a depth of about 60 fathoms from the surface, and extending over an area of at least 15 acres.[3] Microscopic examination of this sill by Mr. Teall proved that the rock presents the closest resemblance to that of the Hett Dyke.[4] In this case, it may be regarded as probable that the two dykes and the intermediate sill form one related series of intrusions, and the conjecture that the Hett Dyke may be connected with the Whin Sill thus receives corroboration. The age of the Whin Sill itself will be discussed a few pages further on.
[3] Sir Lowthian Bell, Proc. Roy. Soc. xxiii. (1875), p. 544.
[4] Quart. Journ. Geol. Soc. xl. (1884), p. 230.
Of the other dykes which may possibly be coeval with the Hett Dyke we may specially note those which follow the same W.S.W. trend, for that strike differs from the general W.N.W. direction of most of the dykes. Two conspicuous examples of the south-westerly trend may be seen, one near Morpeth, the other north of Bellingham. The former dyke, as regards microscopic structure, is more nearly related to the majority of the series in the North of England. But that north of Bellingham (High Green) presents affinities both in structure and composition with the Hett Dyke,[5] and may perhaps belong to the same period of intrusion.
[5] Mr. Teall, op. cit. p. 244. Quart. Journ. Geol. Soc. xxxix. (1884), p. 656, and Proc. Geol. Assoc. (1886). See also Prof. Lebour, Geology of Northumberland and Durham, chap. xi.
The Great Whin Sill.—The geologist who, after making himself acquainted with the abundant sills among the Carboniferous rocks in the centre of Scotland, finds his way into Northumberland, meets there with geological features that have become familiar to him further north. The sea-cliffs of Bamborough and Dunstanborough, the rocky islets of Farne, the long lines of brown crag and green slope that strike inland through the Kyloe Hills and wind across the cultivated lowlands and the moorlands beyond, remind him at every turn of the scenery in the basin of the Forth. But not until he has traced these ridges for many miles southwards and found their component rocks to form there an almost continuous sheet does he realize that nothing of the kind among the Scottish Carboniferous rocks can be compared for extent to this display in the North of England.[6]
[6] The Whin Sill has been the subject of much discussion, and a good deal of geological literature has been devoted to its consideration. The writings of Trevelyan, Sedgwick, W. Hutton, Phillips and Tate are especially deserving of recognition. The intrusive character of the Sill, maintained by some of these writers, was finally established by the mapping of the Geological Survey, and was discussed and illustrated by Messrs. W. Topley and G. A. Lebour in a paper in the 33rd volume of the Quart. Journ. Geol. Soc. (1877), in which references to the earlier observers will be found. See also Prof. Lebour's Outlines of the Geology of Northumberland, 2nd edit. (1886), p. 92. The petrography of the Whin Sill is fully treated by Mr. Teall in Quart. Journ. Geol. Soc. xl. (1884), p. 640, where a bibliography of the subject is also given.
From the furthest skerries of the Farne Islands southwards to Burton Fell on the great Pennine escarpment, a distance in a straight line of about 80 miles, this intrusive sheet may be traced in the Carboniferous Limestone series (Map I.). There are intervals where its continuity cannot be actually followed at the surface, but that it really runs unbroken from one end to the other underground cannot be doubted by any one who has examined the region. This singular feature in the geology and scenery of the North of England is known locally as the Great Whin Sill.[7] From the rocky islets and castle-crowned crags of the coast-line it maintains its characteristic topography, structure and composition throughout its long course in the interior. So regularly parallel with the sedimentary strata does it appear to lie, that it was formerly regarded by many observers as a true lava-sheet, poured out upon the sea-floor over which the limestones and shales were laid down. But its really intrusive character has now been clearly demonstrated. Not a vestige of any tuff has been detected associated with it, nor does it ever present the usual characters of a true lava-stream.[8] Its internal structure and the wonderful uniformity in its character mark it out as a typical intrusive sheet.
[7] "Whin" is a common term in Scotland and the North of England for any hard kind of stone, especially such as can be used for making and mending roads. "Sill" denotes a flat course or bed of stone, and was evidently applied to this intrusive sheet from its persistent flat-bedded position and its prominence among the other gently inclined strata among which it lies. It is from this example in the North of England that the word "sill" has passed into geological literature.
[8] On the coast at Bamborough and the Harkess Rocks the usual petrographical characters of the Whin Sill are exchanged for those of fine-grained amygdaloidal diabases arranged in distinct sheets, which in their upper parts are highly vesicular and show ropy surfaces—peculiarities suggestive of true lava-streams. But according to Professor Lebour the rocks are intrusive into limestone and shale (Geology of Northumberland and Durham, p. 98). Mr. Teall has expressed the suspicion that these rocks must have consolidated under conditions somewhat different from those which characterized the normal Whin Sill (Quart. Journ. Geol. Soc. xl. p. 643). They seem to be the only parts of the sill which present features that might possibly indicate superficial outflow.
Among the manifestations of the subterranean intrusion of igneous rocks in the British Isles the Great Whin Sill, next after the Dalradian sills of Scotland, is the most extensive. Its striking continuity for so great a distance, and the absence around it of any other trace of igneous action, save a few dykes, place it in marked contrast to the ordinary type of Carboniferous sills. The occasional gaps on its line of outcrop in the northern part of its course do not really affect our impression of the persistence of the sheet. They not improbably indicate merely that in its protrusion it had a wavy irregular limit, which in the progress of denudation has occasionally been not yet reached. For mile after mile the sill has been mapped by the Geological Survey in lines of crag across the moorlands, and as a conspicuous band among the limestones and shales that form the steep front of the Pennine escarpment, where it has long been known in the fine sections exposed among the gullies by which that noble rock-face has been furrowed.
Along its main outcrop, the sill dips gently eastwards below the portion of the Carboniferous Limestone series which overlies it. But so slight are the inclinations, so gentle the undulations of the rocks in this part of the country, that far to the east of that outcrop the sill has been laid bare by the streams which in the larger dales have cut their way through the overlying cake of Carboniferous strata down to the Silurian platform on which they rest (Fig. 176). Among these inland revelations of the eastward continuation of the sill under Carboniferous Limestone strata, the most striking and best known are those which have been made by the River Tees, and of which the famous waterfalls of the High Force and Cauldron Snout are the most picturesque features. The distance of the remotest of these denuded outcrops or "inliers" from the main escarpment is not less than 20 miles.
It is not possible to form an accurate estimate of the total underground area of the Whin Sill. In the southern half of the district, south of the line of the Roman Wall, where, the inclination of the strata being generally low, the same stratigraphical horizons are exposed by denudation far to the east of the main outcrops of the rocks, we know that the sill must have a subterranean extent of more than 400 square miles. Yet this is probably only a small part of the total area over which the molten material was injected. In the northern part of the district, the Carboniferous Limestone series is not exposed over so broad a stretch of country, and denudation has not there revealed the eastward extension of the sill. But there is no reason to suppose the sheet to be less continuous and massive there. We must remember also that the present escarpment has been produced by denudation, and that the intrusive sheet must have once extended westwards beyond its present limits at the surface. If, therefore, we were to state broadly that the Great Whin Sill has been intruded into the Carboniferous Limestone series over an area of 1000 square miles we should probably be still below the truth.
The rock composing this vast intrusive sheet is a dolerite or diabase, which maintains throughout its wide extent a remarkable uniformity of petrographical characters. In this and other respects it illustrates the typical features of sills. Thus it is coarsest in texture where it is thickest, and somewhat finer in grain towards its upper and lower surfaces than in the centre. Among the coarser varieties the component crystals of augite are not infrequently an inch in length and occur in irregular patches.[9] Occasional amygdaloidal portions are observable, but these are not more marked than those to be found in the "whin-dykes" of the same region.[10] The amygdaloidal and vesicular fine-grained rocks of the Bamborough district may possibly be quite distinct from the main body of the Whin Sill.
[9] Sedgwick, Cambridge Phil. Trans. ii. p. 166. Mr. Teall, Quart. Journ. Geol. Soc. xl. p. 643.
[10] Messrs. Topley and Lebour, Quart. Journ. Geol. Soc. xxxiii. p. 418.
Under the microscope the rock is seen to consist essentially of the usual minerals—plagioclase, augite and titaniferous magnetic iron-ore. An ophitic intergrowth of the augite and felspar is observable, likewise a certain quantity of micropegmatite which plays the part of groundmass between the interstices of the lath-shaped felspars. Full details of the characteristics of the component minerals and their arrangement are given by Mr. Teall in the paper already cited.
The main body of the sill is a sheet which sometimes diminishes to less than 20 feet in thickness and sometimes expands to 150 feet, but averages from 80 to 100 feet. It occasionally divides, as near Great Bavington, where it appears at the surface in two distinct beds separated by an intervening group of limestones and shales. Occasionally, as at Elf's Hill Quarry, it gives out branches which send strings into the adjacent limestone.[11]
Although in most natural sections it seems to lie quite parallel with the strata above and below, yet a number of examples of its actual intrusion have been observed. When traced across the country, it is found not to remain on a definite horizon, but to pass transgressively across considerable thicknesses of strata. Its variations in this respect are well shown in the accompanying table of comparative sections constructed by Messrs. Topley and Lebour.[12] It will be seen that while at Harlow Hill the sill is found overlying the Great Limestone of Alston Moor, at Rugley, five miles off it lies about 1000 feet lower down, far below the position of the Tyne-bottom Limestone. Still farther north, however, the sill west of Holy Island is said to lie 800 feet above the Great Limestone and to come among the higher beds of the Carboniferous Limestone series.[13]
The Whin Sill appears generally to thicken in an easterly or north-easterly direction. There are further indications that it was intruded from east to west. Thus, at Shepherd's Gap, on the Great Roman Wall, the dolerite, coming evidently from an easterly quarter, has broken up and thrust itself beneath a bed of limestone. Again, when the sill bifurcates the branches unite towards the east or north-east.[14] The sill can be proved to thin away to the west from Teesdale to the Pennine escarpment, and in Weardale the "Little Whin Sill" diminishes from 20 feet, till in three miles it disappears.[15]
The strata in contact with the Whin Sill, both above and below, have been more or less altered. Sandstones have been least affected; shales have suffered most, passing into a kind of porcellanite, with development of garnet and other minerals.[16] Limestone often shows only slight traces of change, though here and there it has become crystalline.
[16] Mr. Teall, op. cit. xxxix. (1884), p. 642, and authors cited by him.
No trace of any boss or neck has been detected in the whole region which might be supposed to mark a funnel of ascent for the material of the Whin Sill. The Hett Dyke and the High Green Dyke, already noticed, may, however, have been possibly connected with the injection of this great intrusive sheet. No other visible mass of igneous rock in the region has been even plausibly conjectured to indicate a point or line of emission for the sill.
It is certainly singular that in so wide a territory, where the whole succession of strata has been so admirably laid bare by denudation in thousands of natural sections, and where, moreover, much additional information has been obtained from lead-mining as to the nature of the rocks below ground, not a single vestige of tuff, agglomerate or interstratified lava has been up to the present time recorded, unless the Harkess rocks already alluded to can be so regarded.
Judging, however, from the analogy of the other districts of igneous rocks in Britain, we can hardly resist the conclusion that the Great Whin Sill is essentially a manifestation of volcanic action, that it was connected with the uprise of basic lava in volcanic orifices, and that the subterranean energy may quite probably have succeeded in reaching the surface and ejecting there both lavas and tuffs.
It appears to be certain that any vents which existed cannot have lain to the west of the present escarpment of the sill, for no trace of them can be found there piercing the Carboniferous or older formations. They must have lain somewhere to the east in the area now overspread with Millstone Grit and Coal-measures, or still farther east in the tract now concealed under the North Sea. The evidence of the sill itself, as we have seen, corroborates this view of the probable situation of the centre of disturbance.
The question of the geological age of the sill is one of considerable difficulty, to which no confident answer can be given.[17] The injection of the diabase must obviously be considerably later than the highest strata through which it has risen; that is, it must be younger than some of the higher members of the Carboniferous Limestone series. But here our positive evidence fails.
The Sill is traversed by the same faults which disrupt the surrounding Carboniferous rocks. It is therefore of older date than these dislocations. Its striking general parallelism with the shales and limestones probably proves that it was intruded before the rocks were much disturbed from their original horizontal position. But the manner in which the intrusive rock has been thrust into and has involved the shales and limestones seems to indicate that these strata had already become consolidated and lay under the pressure of a great thickness of superincumbent Carboniferous strata.
In the absence of all certainty on the subject it seems most natural to place the Whin Sill provisionally among the Carboniferous volcanic series with which petrographically and structurally it has so much in common. In Scotland the puy-eruptions continued till the time of the Coal-measures. If, before the close of the Carboniferous period, volcanic vents were opened somewhere to the east of the coal-fields of Northumberland and Durham, they might be accompanied with basic sills injected into the Carboniferous Limestone series, which was then lying still approximately horizontal under a thickness of from 3500 to 5000 feet of Carboniferous sedimentary deposits. These still undiscovered volcanoes seem to have been endowed with even more energy than those of Central and Southern Scotland, at least nowhere else among the Carboniferous records of Britain is there such a colossal manifestation of subterranean intrusion as the Great Whin Sill.
In the absence of any certain evidence that the Whin Sill belongs to the Carboniferous period, we must advance southward into the very heart of England before any clear vestiges can be found of contemporaneous volcanic eruptions among the members of the Carboniferous system. After quitting the lavas and tuffs of Roxburghshire and their brief continuations across the English border, we do not again meet with any truly bedded volcanic rocks in that system until we reach the middle of Derbyshire. In this picturesque district, famous for its lead-mines and its mineral waters, a feebly developed but interesting group of intercalated lavas, locally called "toadstones," has long been known. There is thus a space of some 150 miles across which, though the formations are there so fully developed and so abundantly trenched by valleys from the top to the bottom of the system, no volcanic vents nor any trace of Carboniferous volcanic ejections has yet been found. On the other hand, after the district of the "toadstones" is passed, the Carboniferous rocks are again destitute of any volcanic intercalations across the centre and south-west of England and over Wales, until after a space of about 150 miles they reappear in Somerset.
The volcanic group of Derbyshire thus stands out entirely isolated. Lying in the Carboniferous Limestone, where that formation is typically developed, it presents an admirable example of a thoroughly marine phase of volcanic action (Map I.).
One of the most prominent features in the geology of the centre of England is the broad anticlinal fold which brings up the lower portion of the Carboniferous system to form the long ridge of the Pennine chain that runs from Yorkshire to the Midland plain, and separates the eastern from the western coal-fields. This fold widens southwards until not only the Millstone Grit and Yoredale rocks, but the underlying Mountain Limestone is laid bare. A broad limestone district is thus exposed in the very heart of the country, ranging as a green fertile undulating tableland, deeply cut by winding valleys, which expose admirable sections of the strata, but nowhere reach the base of the system. The total visible depth of the limestone series is computed to be about 1500 feet; the Yoredale shales and limestones may be 500 feet more; so that the calcareous formations in which the volcanic phenomena are exhibited reach a thickness of at least 2000 feet.
It is not yet definitely known through what vertical extent of this thickness of sedimentary material the volcanic platforms extend, but where most fully developed they perhaps range through 1000 feet, lying chiefly in the Carboniferous Limestone, but apparently in at least one locality extending up into the lower division of the Yoredale group. The area within which they can be studied corresponds nearly with that in which the limestone forms the surface of the country, or a district measuring about 20 miles from north to south, with an extreme breadth of 10 miles in an east and west direction.
A special historical interest belongs to the Derbyshire "toadstones."[18] They furnished Whitehurst with material for his speculations, and were believed by him to be as truly igneous rocks as the lava which flows from Hecla, Vesuvius or Etna. But he thought that they had been introduced among the strata and "did not overflow the surface of the earth, according to the usual operations of volcanoes."[19]
[18] This word has by some writers been supposed to be corrupted from tod-stein, dead-stone, in allusion to the dying out of the lead veins there; by others the name has been thought to be derived from the peculiar green speckled aspect of much of the rock, resembling the back of a toad.
[19] An Enquiry into the Original State and Formation of the Earth, 1778, Appendix, pp. 149, et seq.
His views were published as far back as 1778, three years after Hutton read the first outline of his theory of the earth and made known his observations regarding the igneous origin of whinstones.[20] The first detailed account of the Derbyshire eruptive rocks was that given by Fairey,[21] which has served as the basis of all subsequent descriptions. Conybeare, in particular, prepared a succinct narrative from Fairey's more diffuse statements, and thus placed clearly before geologists the nature and distribution of these volcanic intercalations.[22] Subsequently the district was mapped by De la Beche and the officers of the Geological Survey, and the areas occupied by the several outcrops of igneous rock could then be readily seen.[23]
[20] Trans. Roy. Soc. Edin. i. p. 275, et seq. Hutton specially mentions the toadstone of Derbyshire as one of the rocks produced by fusion, p. 277.
[21] General View of the Agriculture and Minerals of Derbyshire (1811).
[22] Outlines of the Geology of England and Wales (1822), p. 448.
[23] See Sheets 71 N.W., 72 N.E., 81 N.E. and S.E. and 82 S.W. of the Geological Survey of England and Wales.
Though the "toadstones" were believed to form definite platforms among the limestone strata, and thus to be capable of being used as reliable horizons in the mineral fields of Derbyshire, they appear to have been generally regarded as intrusive sheets like the Whin Sill of the north. Thus De la Beche in his Manual of Geology, giving a summary of what was known at the time regarding intercalated igneous rocks, remarks with regard to the Derbyshire toadstones that they may from all analogy be considered to have been injected among the limestones which would be easily separated by the force of the intruded igneous material.[24] But the same observer, after his experience among the ancient volcanic rocks of Devonshire, came fully to recognize the proofs of contemporaneous outflow among the Derbyshire toadstones. In his subsequently published Geological Observer, he described the toadstones as submarine lavas that had been poured out over the floor of the sea in which the Carboniferous Limestone was deposited, and had been afterwards covered up under fresh deposits of limestone.[25] It is remarkable, however, that he specially comments on the absence, as he believed, of any contemporaneously ejected ashes and lapilli, such as occur in Devonshire. That true tuffs or volcanic ashes are associated with the toadstones was noticed by Jukes in 1861,[26] and afterwards by the Geological Survey.[27] Since that time geologists have generally recognized these Derbyshire igneous rocks as truly contemporaneous intercalations. But very little has recently been written on the structure of the district, our information regarding it being still based mainly on the early observations of Fairey and the mapping of the Geological Survey.
[26] Student's Manual of Geology, 2nd edit. (1863), p. 523. For a general résumé of the proofs of contemporaneity furnished by the toadstones, see "The Geology of North Derbyshire," by Messrs. A. H. Green and A. Strahan (Memoirs of the Geological Survey, 2nd edit. (1887), p. 123).
[27] In the first edition of the Memoir on the Geology of North Derbyshire, published in 1859, the authors of which were Messrs. A. H. Green, C. le Neve Foster and J. R. Dakyns.
The subject, however, has now been resumed by Mr. H. Arnold Bemrose, who in 1894, after a prolonged study of the petrography of the rocks, communicated the results of his researches to the Geological Society.[28] In his excellent paper, to which I shall immediately make fuller reference, he mentions the localities at which lava-form and fragmental rocks may be observed, but does not enter on the discussion of the geological structure of the region or of the history of the volcanic eruptions. Before the announcement of his paper, hearing that I proposed to make for the first time a rapid traverse of the toadstone district, for the purpose of acquainting myself with the rocks on the ground, he kindly offered to conduct me over it. My chief object, besides that of seeing the general nature of the volcanic phenomena of the region, was to examine more particularly the areas of the volcanic fragmental rocks, with the view of discovering whether among them some remains might not be found of the actual vents of discharge. In this search I was entirely successful. Aided by Mr. Bemrose's intimate knowledge of the ground, I was enabled to visit in rapid succession those tracts which seemed most likely to furnish the required evidence, and in a few days was fortunate enough to obtain proofs of six or seven distinct vents, ranging from the extreme northern to the furthest southern boundary of the volcanic district. Mr. Bemrose has undertaken to continue the investigation, and will, I trust, work out the detailed stratigraphy of the Carboniferous Limestone so as eventually to furnish an exhaustive narrative of the whole volcanic history of Derbyshire. Meanwhile no adequate account of the area can be given. But I will here state all the essential facts which up to the present time have been ascertained.
[28] Quart. Journ. Geol. Soc. vol. l. (1894), p. 603.
1. THE ROCKS ERUPTED.—Mr. Allport has described the microscopic character of some of the toadstones,[29] and further details have been supplied by Mr. Teall.[30] The fullest account of the subject, however, is that given by Mr. Bemrose in the paper above referred to. This observer distinguishes the lava-form from the fragmental rocks, and gives the minute characters of each series. He does not, however, separate true interstratified lavas from injected sills, nor the bedded tuffs from the coarse agglomerates which fill up the vents. These distinctions are obviously required in order that the true nature and sequence of the materials in the volcanic eruptions may be traced, and that the phenomena exhibited in Derbyshire may be brought into comparison with those found in other Carboniferous districts. But to establish them satisfactorily the whole region must be carefully re-examined and even to some extent re-mapped.
The lavas (including, in the meantime, sheets which there can be little doubt are sills) show three main types of minute structure and composition, which are discriminated by Mr. Bemrose as—(a) Olivine-dolerites; these, the most abundant of the series, consist of augite in grains, olivine in idiomorphic crystals, plagioclase giving lath-shaped and tabular sections, and magnetite or ilmenite in rods and grains; (b) Ophitic olivine-dolerites, consisting of augite in ophitic plates forming the groundmass, in which are imbedded idiomorphic olivine, plagioclase (often giving large lath-shaped sections and magnetite or ilmenite); (c) Olivine-basalts; these rocks are distinguished by containing crystals of augite and olivine in a groundmass of small felspar-laths, granular augite and magnetite or ilmenite, with very little interstitial matter. They have been noticed only in two of the outcrops of toadstone.
The fragmental rocks have been shown by Mr. Bemrose to cover a much more extensive space than had been previously supposed. He has found them to be distinguished by an abundance of lapilli varying from minute fragments up to pieces about the size of a pea, and composed of a material that differs in structure from the dolerites and basalts with which the tuffs are associated. These lapilli consist largely of a glassy base more or less altered, which is generally finely vesicular and encloses abundant skeleton crystals and crystallites. The tuffs thus very closely resemble some of the Carboniferous basic tuffs of Fife, already referred to (vol. i. p. 422), and like these they include abundant blocks of dolerite and basalt.
2. GEOLOGICAL STRUCTURE OF THE TOADSTONE DISTRICT.—As the volcanic rocks of Derbyshire lie among the Carboniferous Limestones of a broad anticlinal dome, they are only exposed where these limestones have been sufficiently denuded, and as the base of the limestones is nowhere laid bare, the lowest parts of the volcanic series may be concealed. Over the tract where the toadstones can be examined they appear as bands regularly intercalated with the limestones, but varying in thickness in the course of their outcrops. As they are prone to decay, they usually form smooth grassy slopes between the limestone scarps, though isolated blocks of the dull brown igneous rocks may often be seen protruding from the surface. Now and then a harder bed of toadstone caps a hill, and thus forms a prominent feature in the landscape, but as a rule these igneous bands play no distinguishing part in the scenery, and are indeed less conspicuous than the white escarpments of limestone which overlie them.
It was the opinion of the older geologists that three distinct platforms of toadstone extend without break throughout the district, and subdivide the limestones into four portions. But this opinion does not seem to have been based on good evidence either of sequence or of continuity. Various facts were brought forward by the officers of the Geological Survey to show that the supposed persistence of the three platforms of toadstone did not really exist, but that these sheets of igneous material are found at different spots on very different horizons, and are of limited horizontal range.[31] So far as my own limited observations go, they entirely corroborate this view. There can be little doubt, I think, that the identity of certain outcrops of toadstone has been assumed, and the assumption has been carried throughout the district. The truth is that the number of successive platforms on which igneous materials appear will never be satisfactorily determined until the stratigraphy of the Derbyshire Carboniferous Limestone is worked out in detail. When the successive members of this great calcareous formation have been identified by lithological and palæontological characters over the district, it will be easy to allocate each outcrop of toadstone to its true geological horizon. When this labour has been completed, it will probably be found that instead of three, there have been many discharges of volcanic material during the deposition of the limestone series; that these have proceeded from numerous small vents, and that they are all of comparatively restricted horizontal extent. Such a detailed examination will also determine how far the toadstones include veritable sills, and on what horizons these intrusive sheets have been injected.
[31] Geol. Surv. Mem. on North Derbyshire, by Messrs. Green and Strahan (1887), p. 104.
In the meantime, we know that the lowest visible bands of toadstone are underlain by several hundred feet of limestone, thus proving that the earliest known volcanic explosions took place over the floor of the Carboniferous Limestone sea, after at least 700 or 800 feet of calcareous sediment had accumulated there. The latest traces of volcanic activity are found in a part of the Yoredale group of shales and limestones which form the uppermost member of the Carboniferous Limestone of this region. But it is not quite clear whether the vesicular diabase found there is interstratified or intrusive. Certainly no contemporaneous tuffs have yet been found among the Yoredale rocks, nor in any higher subdivision of the Carboniferous system, though coarse agglomerates marking the position of vents do traverse the Yoredale group at Kniveton.
It may be remarked that in the district over which the toadstones can be seen, two areas are recognizable, in each of which the exposures of the igneous rocks are numerous, while between them lies an intervening tract wherein there is hardly any visible outcrop of these rocks. The northern and much the more extensive area stretches from Castleton to Sheldon, while the southern spreads from Winster to Kniveton. This distribution not improbably points to the original position of the vents, and indicates a northern more numerous group of volcanic orifices, and a southern tract where the vents were fewer, or at least spread their discharges over a more limited space.
3. THE VENTS.—It had always appeared to me singular that, in ground so deeply trenched by valleys as the toadstone district of Derbyshire, no trace had been recognized of any bosses or necks from which these volcanic sheets might have been erupted. It is true that in mining operations masses of toadstone had been penetrated to a considerable depth without their bottom being reached, and the suggestion had been made that in such cases a shaft may actually have been sunk on one of the vents through which the toadstone came up.[32] One instance in particular was cited where, at Black Hillock, on Tideswell Moor, close to Peak Forest Village, a mass of toadstone was not cut through, though pierced to a depth of 100 fathoms. In that neighbourhood, however, several of the sheets of eruptive material are probably sills, and the shaft at Black Hillock may have been sunk upon the pipe or vein that supplied one or more of these intrusive sheets.