CHAPTER XXXII

PERMIAN VOLCANOES OF ENGLAND

The Devonshire Centre--Eruptive Rocks of the Midland Coal-fields.

From the south of Scotland we need to pass to the extreme south-west of England before we again encounter a group of volcanic rocks which may be referred with some confidence to the Permian period. An interesting group of lavas and tuffs has been preserved in some of the valleys over a limited area in the east of Devonshire. The Midland coal-fields, however, are traversed by a series of basic eruptive rocks which are younger than the Coal-measures, and may possibly be Permian. Their mode of occurrence, and the arguments regarding their geological age, will be given in the present chapter.

1. DEVONSHIRE

The counties of Devon and Cornwall furnish one of the most striking examples to be met with in Britain of the persistence of volcanic action over a limited area through a long succession of geological periods. The extensive eruptions in Devonian time were followed after a long interval by a diminished series in the Carboniferous period. But the subterranean energy was not then wholly exhausted, for it showed itself on a feeble scale in at least one limited tract of the same region during the Permian period. Thus throughout the later half of Palæozoic time the extreme south-west of England continued to be a theatre of volcanic action.

The geological age of the igneous rocks now to be referred to depends upon the particular place in the geological record to which we assign the remarkable breccias and sandstones with which they are associated. By many geologists who have been unable to recognize any true break in the red rocks from their base up to the bottom of the Lias, these strata have been grouped as one great series referable to the "New Red Sandstone" or Trias. This is the classification adopted on the one-inch maps of the Geological Survey. On the other hand, various able observers have pointed out the close resemblance of the coarse and fine breccias at the bottom of the series to recognized Permian deposits in the centre of England and to parts of the typical Rothliegende of Germany. I need only refer to the strongly expressed views of Murchison, in which, as he stated in his _Siluria_, he "entirely agreed with Conybeare and Buckland, who, after a journey in Germany in 1816, distinctly identified the Heavytree conglomerate, near Exeter, with the Rothliegende of the Germans."[104] In the absence of any fossil evidence, we have only lithological characters and sequence to guide us, and though the known facts hardly warrant a very positive opinion, my inclination is to regard these red Devonshire breccias as probably Permian, and to follow Murchison in looking upon their associated igneous masses as furnishing additional reason for assigning them to that particular geological platform.[105]

[Footnote 104: _Siluria_, 4th edit. (1867), p. 333. See also Berger, _Trans. Geol. Soc._ vol. i. (1811), pp. 98-102; Conybeare and Phillips, _Geology of England and Wales_, p. 313, footnote; De la Beche, _Report on the Geology of Cornwall, Devon and West Somerset_ (1839), chap. vii. p. 193. Messrs. Hull and Irving (_Quart. Journ. Geol. Soc._ vol. xlviii. 1892, pp. 60, 68) have more recently discussed the subject, and follow the view of Murchison.]

[Footnote 105: Murchison cogently argued that as no signs of volcanic activity were known in the Trias, but were abundant in the Permian system, the Devonshire rocks might be regarded as appertaining to the older series, _op. cit._ Triassic volcanic rocks, however, are now well known on the Continent.]

No proper account has yet been written of the volcanic group which I now propose to describe.[106] De la Beche was, I think, the first to recognize the true volcanic nature of the rocks and their contemporaneous interstratification in the red sandstone series.[107] As traced by him on the Geological Survey maps, these rocks lie at or near the base of the red sedimentary deposits, resting sometimes directly on the Culm-measures, sometimes on an intervening layer of red strata. He found them in three separate districts in the neighbourhood of Exeter, the most northerly lying near Tiverton, the central extending from Kellerton for a few miles up the Yeo Valley, beyond Crediton, and the third stretching from the City of Exeter to Pen Hill, about five miles to the south-west. He recognized the amygdaloids as slaggy lavas, and saw that the volcanic breccias and tuffs are interleaved with the sandstones. With regard to the probable vents from which these materials were ejected, he thought that the chief centre of activity lay at Kellerton Park, while in other localities he believed the bosses of igneous rock "to descend in mass downwards, as if filling up some crater or fissure through which these rocks had been vomited."[108] He speaks also of "quartziferous porphyries" occurring among them, a statement which, if petrographically accurate, would suggest the uprise of a later more acid lava in some of the vents.

[Footnote 106: An outline of some of their characters will be found in a paper by Mr. W. Vicary in _Trans. Devonshire Assoc._ 1865, vol. i. part iv. p. 43.]

[Footnote 107: See his "Report" cited in the note above. De la Beche quotes J. J. Conybeare as pointing out the intimate connection of these igneous and stratified rocks (_Annals of Philosophy_, 2nd series, vol. ii. (1821) p. 165); but this author wrote at the time of the Plutonist and Neptunist controversy, and does not commit himself to any distinct expression of opinion on the subject.]

[Footnote 108: Report, p. 201.]

More recently the ground has been revised by Mr. W. A. E. Ussher of the Geological Survey, who has ascertained that the volcanic rocks appear in many more places than those where they were noted on the older maps, and likewise extend for some miles further to the north and west.

It now appears that in the central and chief district the lavas can be followed westward from Spray Down near Kellerton to Greenslade near North Tawton, a distance of about twenty-one miles. Their most northerly outcrop is at Thorn above Loxbere in the Tiverton district, and their most southerly visible portion passes under the Cretaceous rocks of Pen Hill. The distance between these extreme points is likewise about twenty-one miles. The whole display of volcanic phenomena is comprised within an area of less than 400 square miles.

One of the most obvious features in this volcanic tract is the way in which the erupted materials lie along the lines of hollow or valley in which the red rocks were deposited. This is most distinctly exhibited in the central district. Here a belt of breccias and sandstones, varying from one to three and a half miles in breadth, runs for about five and twenty miles westward in a depression of the Culm-measures. At intervals, the lavas which lie near the base of the red rocks crop out along the margin of the belt throughout most of its extent. But they do not spread out over the older rocks, and they have evidently been erupted from orifices situated along the line of the valley. It is another example of the relation between the trend of hollows and the outbreak of volcanic vents, which I have referred to as so strikingly displayed in the distribution of the Permian volcanic rocks of south-western Scotland.

The volcanic materials of the Devonshire Permian district consist mainly of lavas, but include also red sandy and gravelly tuffs. The whole volcanic group is remarkably thin, never attaining even the limited development of the Ayrshire series. No adequate petrographical investigation of these rocks has yet been made. Externally, as seen in the quarries and lanes, the lavas present the closest resemblance to those of the Permian basins of Ayrshire and Nithsdale. They show considerable differences of texture even within the same mass, some portions being dull, fine-grained purplish-red rocks, with scattered pseudomorphs of hæmatite and a few porphyritic felspars, other parts passing into an exceedingly coarse amygdaloid or slaggy pumice. Dr. Hatch, after a microscopical examination of a small collection of specimens, found that while most are olivine-basalts, containing ferruginous pseudomorphs after olivine (Raddon Court, Pocombe, and near Budlake), others are true andesites (Ide, Kellerton Park) and even mica-trachytes (Copplestone, near Knowle Hill).[109] As already remarked, some of the older writers mention the existence of quartz-porphyries.[110]

[Footnote 109: _Geol. Mag._ 1892, p. 250. The rocks have been more recently described by Mr. B. Hobson, _Quart. Jour. Geol. Soc._ vol. xlviii. (1892), p. 502. The rock of Kellerton Park is called by Mr. Hobson "mica-augite-andesite," and he gives a chemical analysis of it by Mr. E. Haworth, _op. cit._ p. 507. Mr. Watts has lately found one of the orthoclase rocks to be rich in olivine.]

[Footnote 110: See De la Beche, _Report_, pp. 203, 204. My colleague, Mr. Ussher, found close to the Thurlestone outlier of conglomerate near Kingsbridge, Devonshire, a small boss of quartz-porphyry which rises through and alters the Devonian rocks. The actual junction of this mass with the conglomerate is not seen, nor have any fragments of the porphyry been noticed among the pebbles.

Mr. Ussher informs me that in the quarry the visible exposure of the acid rock is surrounded an covered by mica-porphyrite, probably andesite.]

The geographical conditions in which the red rocks of Devonshire accumulated were those so characteristic of the Permian and Trias formations throughout Britain. The red sandstones and sandy marls gathered in inland basins, where the water seems to have become too saline and bitter to support animal life. The strata are consequently singularly devoid of organic remains. The climate was probably arid, and the absence or scarcity of traces of terrestrial vegetation indicates that the land around the water-basins stretched in wide sandy and rocky wastes. In the dry atmosphere and under the influence of rapid radiation the cliffs and crags of Culm-measures would disintegrate into angular rubbish, and this material, slipping into the lakes or washed down by occasional rain-storms, forms now the breccias that constitute so typical a feature in the Permian system.

It was while this geographical type continued in the South-west of England that the volcanic eruptions took place which we are now considering. De la Beche correctly referred these eruptions to the early part of the red sandstone series. A brief examination of the ground suffices to show that although, as he pointed out, the volcanic rocks lie towards the base of that series, as shown in Fig. 228, they do not all occupy the same platform. That in some cases the lavas lie directly on the Culm-measures, while in others they are separated from these strata by 100 feet or more of red sandstones and breccias (Fig. 229), would not in itself be proof of any difference of age or stratigraphical position in the igneous rocks, for the floor on which the Permian formations were here laid down can be shown to have been singularly uneven. Prominent hills of Culm grit, several hundred feet high, rose above the basins in which the earliest Permian sediments were deposited, and these eminences were gradually submerged and buried under the detritus.

But that the volcanic zone includes in some places more than one outflow of lava with layers of sandstone, breccia and tuff between the successive sheets may be proved in different parts of the district. Thus the two conspicuous hills at Kellerton are composed of several sheets of highly slaggy lava, separated by breccia, and a third much thinner sheet lies above these, intercalated in a mass of breccia, sandstone and sandy tuff (Fig. 230). Again, at Budlake the sandstones and fine breccias include a thin band of vesicular lava, while farther to the east they are interrupted by a higher and thicker zone of similar material.

These igneous sheets can be shown by many interesting sections to have been poured out contemporaneously with the deposit of the sedimentary material among which they occur. At Crabtree, for instance, near Kellerton, the uppermost lava is a thin sheet of highly slaggy texture, which rests immediately on the gravelly red sandstone and catches up parts of it, while the pebbles include fragments of some of the andesites below. The dark lavas are occasionally traversed by veins of fine hard sandstone, which descending from above, like those in the Old Red Sandstone and Permian lavas of Scotland, have been produced by the silting or drifting of fine sand into cracks in the lava, before the igneous material was entirely buried. These features are well exposed in the high ridge of the Belvedere near Exeter (Fig. 228), where, over a thin and inconstant band of red breccia and marl which rests on the upturned ends of the Culm-measures, a band of dull-red andesite may be seen. This rock, partly compact and partly highly amygdaloidal, is in some portions full of irregular fissures and cavities filled with sandstone.

Nowhere among the Palæozoic volcanic rocks of Britain are more remarkable examples of the slaggy structure to be found than in these Devonshire lavas of probably Permian age. I would especially cite the rock of Knowle Farm, a few miles to the west of Crediton, as in part a mere spongy pumice, blocks of which would originally have floated in water.

One of the best sections in the district for the exemplification of the internal structures of these lavas is that in the large quarry at the top of Posbury Hill. On the west side of this quarry the rock is tolerably compact, but contains vesicles and irregular steam-holes. On the east side it passes upward and laterally into a coarse agglomerate of its own fragments, and in its mass it encloses similar agglomerate. No sharp passage can be traced between the two rocks. So far as I could judge, it seemed to me that the lava had broken up as it moved along, possibly shattered by coming in contact with water. The agglomerate is overlain by some reddish ashy sandstone, which fills up the interstices between the slags, and is immediately covered by a bed of lilac andesite, marking another distinct outflow.

As in Ayrshire, the lavas of Devonshire are not accompanied by any thick accumulation of tuff. The fragmentary discharges consisted in both areas of fine dust and gravelly detritus of small lapilli, which were not ejected in such quantities as entirely to conceal the ordinary non-volcanic sediment of the water-basin. The dust and cinders mingled with the red sand and angular scree-material, so that we now see a group of red, somewhat ashy sandstones and breccias. Among the component fragments of the breccias, a considerable variety of igneous material may be observed. While the most of the non-volcanic stones may have been derived by ordinary processes of weathering from rocks exposed at the surface, it is by no means improbable that some of them, including even pieces of Culm grit, killas and baked slate, may have been ejected from volcanic vents.[111]

[Footnote 111: On the composition of the Devonshire breccias see Mr. R. N. Worth, _Quart. Journ. Geol. Soc._ vol. xlvi. (1890), p. 69. This author has adopted the view that the granite of Dartmoor represents the neck of a great volcano from which these later volcanic materials were ejected. But all the evidence seems to me in favour of numerous small vents situated not far from the outcrops of the lavas, as stated in the text. See Mr. B. Hobson, _Quart. Journ. Geol. Soc._ vol. xlviii. (1892), p. 498. The Dartmoor granite is later than the surrounding Carboniferous rocks, but no good evidence has been obtained to connect it with the Permian volcanic phenomena of Devonshire.]

Taking the volcanic rocks of this district as a whole, I regard them as the mere edges of sheets that have flowed from vents which not improbably lie concealed somewhere along the centres of these old Permian valleys. No visible necks have been described from any part of the area, and though I have not examined the whole of it, nothing of that nature was detected by me either in the Crediton Valley or between Silverton and the Exeter neighbourhood. The Tiverton district, which has not yet been searched, appears to be the only tract where any chance remains of finding some of the vents.

No indication of any sills has been met with among the Devonshire Permian rocks. None of the lavas which I have seen have the internal characters of true sills, while in the field their association with the sandstones and breccias in no observed case points to intrusion.

Though much remains to be done in this region before an adequate account can be given of the interesting series of eruptions which concludes the long volcanic history of the South-west of England, enough is known to indicate the general character of the phenomena. The eruptions were on even a feebler scale than those of the Permian period in Scotland, but they seem to have resembled them in their general character. Small puy-like vents were opened, from which dark scoriaceous lavas and showers of gravelly tuff and stones were discharged over the floor of the inland sea or lake-basin in which the red sandstones and breccias were accumulated. These outflows and explosions took place too, as in Scotland, towards the beginning of the deposition of the red strata, and entirely ceased long before that deposition came to an end. In each area the eruptions mark the close of Palæozoic volcanic activity in Britain. The varied and recurrent volcanic episodes which distinguished each successive geological period from the Archæan onwards now definitely terminate, not to be resumed until after the passing of the whole of the vast cycle of Mesozoic ages.

2. ERUPTIVE ROCKS IN THE MIDLAND COAL-FIELDS

Between the thick and thoroughly marine development of the Carboniferous Limestone in Derbyshire and in South Wales, there lies the region, already referred to, wherein both the Carboniferous Limestone and Millstone Grit die out against what must have been a ridge of land or group of islands that stretched in a general east and west direction from the high grounds of Wales through Shropshire, Staffordshire and Leicestershire. On the slopes of this ridge the limestone is gradually overlapped by the Millstone Grit, and both are in turn overlapped by the Coal-measures, which are then found lying immediately on the more ancient rocks of the region--Cambrian or pre-Cambrian, Silurian and Old Red Sandstone. The gradual subsidence that led to the deposit of several thousand feet of Carboniferous strata over the regions to north and south, before the beginning of the Coal-measure period, does not seem to have sensibly affected the persistence of this old terrestrial surface, which probably lay on an axis of upward movement, so that, amidst the surrounding depression, its position above water was on the whole maintained. But there are indications that the inequality of movement in this part of the earth's crust was of much older date than the Carboniferous period. The Old Red Sandstone is conformably continuous below the base of the Carboniferous system, and in Wales is estimated to be some 10,000 feet thick. No break has yet been detected in this vast accumulation of sedimentary material, though it is highly probable that some such unconformability must exist in it as that between the Scottish Lower Old Red Sandstone, which passes down into the Upper Silurian shales, and Upper Old Red Sandstone, which graduates upward into the base of the Carboniferous formations. But even if such a break should be discovered, it will not account for the position of the Coal-measures on Cambrian or even perhaps older rocks. It is hardly conceivable that, had these rocks been covered with a full development of Old Red Sandstone, they could have been stripped of it by denudation before the deposition of the Coal-measures. It seems much more probable that the discrepancy in the terrestrial movements had commenced in Old Red Sandstone time, and that these ridges of ancient Palæozoic rocks never sank below the waters in which the vast thickness of red sandstones, marls and conglomerates was laid down.[112]

[Footnote 112: See a discussion of this subject in Jukes' Preface to his _South Staffordshire Coal-field_.]

But apart from the question of its antiquity, this tract of persistent land has a special interest in the history of volcanic action in Britain, for it was the scene of some remarkable protrusions of eruptive material which took place after a part, and possibly after the whole, of the Coal-measures were accumulated. The date of these protrusions cannot be fixed with greater precision; but there can be no doubt that they belong to one of the later volcanic periods in the geological history of Britain, and the account of them is therefore included in the present Chapter of this work.

In the English Midlands south of Stafford, over a tract of country about 700 square miles in extent, stretching from Birmingham on the east, across the vale of the Severn, to the uplands of Shropshire on the west, the Coal-measures, partly isolated into outliers by denudation and partly separated by overlying younger formations, are pierced by masses of intrusive igneous rocks. Many of these masses have long been familiar to geologists. Those, for example, of the Clee Hills of Shropshire, and the Rowley, Barrow and Pouk Hills of Staffordshire and Worcestershire, have been frequently described, their relations to the surrounding strata have been minutely sought out, their composition has been chemically determined, and their microscopic structure has been investigated. But they have been studied rather as individual masses of local importance. No attempt has yet been made to ascertain how far they are capable of being grouped together as one connected series, linked with each other in chemical and mineralogical characters, and containing a definite record in the volcanic history of the country. This is a task which, it is to be hoped, some competent inquirer will before long undertake.

In the meantime it is only possible to review here the already published information, and to gather from it what may at present be surmised to have been the history of these later eruptions of the Midlands.

The areas where the igneous rocks now to be described are exhibited may be conveniently placed in the following five groups:--1st, Titterstone Clee Hill; 2nd, Brown Clee Hill; 3rd, The Forest of Wyre Coal-field; 4th, The Coalbrookdale Coal-field; and 5th, The South Staffordshire Coal-field.

1. _The Titterstone Clee Hill_ forms a ridge about seven miles long and a mile and a quarter broad, running in a north-easterly direction over the Old Red Sandstone uplands of the south of Shropshire. The ground rises gradually towards the south-west, until it reaches there a height of 1754 feet (Fig. 232). On the north-western side of the ridge, the last vanishing representative of the Carboniferous Limestone can be seen to be overlapped the Millstone Grit, which, as it is traced towards the south-west, is in turn overlapped by the Coal-measures, and these, about 400 feet thick, then rest immediately on the Old Red Sandstone. Two sheets of columnar olivine-dolerite, possibly originally connected, lie as cakes on the summit and eastern slope of the ridge, and cover in all a space of about a square mile and a half. The larger sheet, which varies from 60 to 180 feet in thickness, overlies the Coal-measures, and the coals of the Cornbrook coal-field have been worked underneath it. The smaller mass, which may be 300 feet in thickness, forms the summit of the ridge. On its eastern side it reposes on Coal-measures, which are there much disturbed; but on the west side, where it forms a bold capping to the escarpment, it is underlain at once by the Old Red Sandstone. There cannot be any doubt that these masses of eruptive material are sills, which have been injected into the Carboniferous strata, and partly between these strata and the Old Red Sandstone. One or more dykes of eruptive rock have been met with in mining, and the coal on approaching them undergoes alteration.[113]

[Footnote 113: See J. R. Wright, _Trans. Geol. Soc._ (2nd ser.) iii. (1832), p. 487. Titterstone Clee Hill is shown on Sheet 55 N.E. and N.W. of the Geological Survey, and in Horizontal Sections, Sheets 33 and 36, from which Fig. 232 is reduced. The microscopic structure of the dolerite has been described by Mr. Allport, _Geol. Mag._ 1870, p. 159; _Quart. Journ. Geol. Soc._ xxx. (1874), p. 550.]

2. _Brown Clee Hill_ consists of two outliers of Coal-measures, each about a mile long, placed on the summit of a broad ridge of Old Red Sandstone, and rising to a height of 1800 feet above the sea. Both of the outliers is capped with a cake of dolerite, and a third smaller patch of the same material lies on the southern outlier between the cappings. Neither at this locality nor around Titterstone Clee have any eruptive rocks been observed rising through the older strata. It is evident that in both cases the orifices or fissures up which the molten material rose lie concealed under the surviving cakes of dolerite.[114]

[Footnote 114: Brown Clee Hill is mapped in Sheet 61 S.W. of the Geological Survey, and its structure is shown in Sheet 36 of the Horizontal Sections.]

3. _Forest of Wyre Coal-field._--On both sides of this extensive tract of Coal-measures, the strata near the base of the series are traversed by sills or dykes of olivine-dolerite like that of the Clee Hills. The sandstones in contact with the eruptive rock have been indurated. In this district, also, the evidence shows that the sheets are intrusive, and later than the portion of the Coal-measures there visible.[115]

[Footnote 115: This district is represented in Sheets 55 N.E. and 61 S.E. of the Geological Survey. The microscopic structure of the larger mass on the west side of the coal-field, and the variations in the minute structure of the intrusion which forms a long ridge on the east side, are described by Mr. Allport, _Quart. Journ. Geol. Soc._ xxx. pp. 550, 551.]

4. _Coalbrookdale Coal-field._--In this interesting district a sill of rather finely crystalline olivine-dolerite, which is estimated to be nearly 200 feet thick, is traceable from near Little Wenlock for three miles to the north, intercalated between the Carboniferous Limestone and the Silurian rocks underneath. It appears to underlie the western part of the Coal-field, for it is exposed by denudation in several valleys between Little Wenlock and Great Dawley. Owing to the thinning out of the Carboniferous Limestone in an easterly direction, the sill gradually comes to have the Millstone Grit on its upper surface, and at one point is represented on the Geological Survey map as even intruded into the Coal-measures. Here again we have an intrusive sheet of later date than at least the earlier part of the Coal-measures, and no evidence of any superficial outflow of volcanic material.[116]

[Footnote 116: The Coalbrookdale coal-field has been described by Sir Joseph Prestwich, _Trans. Geol. Soc._ (2) v. p. 428; and Prof. E. Hull, _Quart. Jour. Geol. Soc._ xxxiii. (1877), p. 629. The minute structure of the sill at Little Wenlock is referred to by Mr. Allport, _op. cit._ p. 550. The ground is mapped on Sheet 61 N.E. of the Geological Survey, and its structure is shown on Sheet 54 of the Horizontal Sections.]

5. _South Staffordshire Coal-field._--This district, in respect to its igneous intercalations, has been much more fully examined and described than any of the others. It forms the subject of an exceedingly able memoir by Jukes, who carefully studied its geology and delineated it on the maps and sections of the Geological Survey. Since his time the rocks have been studied microscopically, but no material facts regarding the stratigraphy have been obtained in addition to those which he patiently collected and generalized upon.[117]

[Footnote 117: Jukes, "South Staffordshire Coal-field," _Mem. Geol. Surv._ 2nd edit. (1859). The area is embraced in Sheet 62 N.W. and S.W. of the Geological Survey, and is illustrated in Sheets 23, 24 and 25 of the Horizontal Sections.]

This coal-field is above 20 miles long and 5 miles broad. Its strata rest unconformably on Upper Silurian strata, which, as part of the ancient ridge or island already referred to, project here and there from amidst the Coal-measures. The boundaries of the field on the east and west sides are chiefly made by faults which bring down Permian and Triassic formations against the Carboniferous strata.

Throughout this coal-field sheets of igneous rock are abundant. In the detailed account of them given by Jukes in his admirable essay on the South Staffordshire Coal-field,[118] he distinguished two kinds of igneous material--"basalt," which comes out at the surface, and sometimes overlies the Coal-measures in large cakes like that of the Rowley Hills, which extends for two miles in one direction and more than a mile in another; and "greenstone," which burrows among the coal-bearing strata, and gives off dykes and veins of "white rock-trap." There does not appear, however, to be any essential difference in composition, age or origin between these contrasted kinds of igneous material. They not improbably all belong to one series of extrusions, their distinctions being due rather to the conditions under which they were erupted, and in particular to their comparative thickness, and the influence of adjacent coals and carbonaceous shales upon them.

[Footnote 118: _Op. cit._ p. 117.]

The igneous rocks seen at the surface in this district form a series of well-marked eminences. Of these the largest extends as a ridge from Dudley to beyond Rowley Regis, a distance of more than two miles. To the west of this tract, a number of small patches of the same material crop out at the surface, the most important forming Barrow Hill. Six miles farther north another group of similar patches may be seen. Of these the largest occurs at Wednesfield, but the most noted forms the Pouk Hill, which has long been noted for the beauty of its columnar structure.

The sheets of "greenstone" met with in the coal-field are more numerous and extensive than the detached areas of more compact rock visible above ground, a single sheet being sometimes traceable in the coal-workings for two miles in one direction.

The eruptive rocks of this district, when examined in their freshest form, consist of well-preserved olivine-dolerite. An examination of the "greenstone" and the "white rock-trap," which runs in fingers and threads through the coal, shows that these are really the same dolerite which has undergone alteration, the ferruginous silicates having especially been decomposed.[119]

[Footnote 119: Allport, _Quart. Journ. Geol. Soc._ xxx. (1874), p. 547. Chemical analysis also shows the identity of the rocks and the nature of the alteration of the "white rock." See Jukes, "South Staffordshire Coal-field," pp. 117, 118.]

The sills of greenish decomposed material that have been injected amongst and alter the coals, vary from 15 feet to 80 or 90 feet in thickness. The largest of the dolerite cakes on the surface, that of the Rowley Hills, is somewhat irregular in its thickness, but may reach as much as 100 feet.

That nearly the whole of the igneous material is intrusive is admitted by all observers who have studied the ground. The manner in which the "basalts" and "greenstones" send out veins into the Coal-measures shows conclusively that they have been injected into the strata. The only rock about which some doubt has been expressed is that of the Rowley Hills, which Jukes was disposed, though not without some hesitation, to consider as part of an actual lava-stream. He based this inference chiefly on the occurrence, immediately under the dolerite, of what he looked upon as a "trappean breccia or brecciated ash, containing rounded and angular fragments of igneous rock lying in a brown rather ferruginous paste, that looks like the debris of a basaltic rock."[120] This breccia he regarded as belonging to and passing into the Coal-measures, and he was thus inclined to regard the dolerite as a lava of Coal-measure age.

[Footnote 120: _Op. cit._ p. 119.]

It is possible, however, that the "trappean breccia" may be of the same nature as the "uncompressed balls of basalt bedded in a mass of decomposed basalt or basaltic 'wacke' or clay"[121]--that is, a decayed contact layer of the eruptive rock. But if it be regarded as the fragmental accompaniment of a lava-stream, it can hardly belong to the Coal-measures. If the dolerite had been a lava of that age, it ought to be found lying conformably on the Coal-measures. But this it does not appear to do. Making every allowance for the way in which an advancing current of lava might plough up soft sediment on the bottom of the sea or of a lake, we can hardly thus account for the very uneven surface of Coal-measures on which the sheet of igneous rock rests. If the Rowley rock be looked upon as a lava which flowed out at the surface, it must, I think, be assigned to a time subsequent to that of the Coal-measures, when these strata had been upraised and had suffered some amount of denudation. I confess, however, that the petrographical characters of the rock, the alteration of the coals which have been worked underneath it, and the abundant veins of "white rock" which there traverse the seams, induce me to regard this rock as forming no exception to the general rule in the Midlands, but as having been intruded as a sill, now laid bare by denudation. Its fresher condition may arise from its thickness, or from some other circumstance which has not been ascertained.

[Footnote 121: _Op. cit._ p. 126.]

We have now to consider the probable geological date of the various intrusions of basic igneous material which can be traced over so wide an area in the centre of England. In discussing the subject, Jukes pointed out that in the surrounding district "no igneous rocks of any kind are found in any formation newer than the Coal-measures."[122] This statement is, with the exception of one locality, undoubtedly true.[123] But on any view there must have been a long interval of time between the formation of the highest strata of the South Staffordshire coal-field and that of the lowest Permian deposits of the district. It is quite conceivable, though at present incapable of proof, that the extravasation of eruptive material took place after the close of the Carboniferous period and during the earlier part of the Permian period.

[Footnote 122: _Op. cit._ p. 131.]

[Footnote 123: See note on next page.]

Jukes further shows that "at whatever period these igneous rocks were produced, they were all existent before the production of the faults and dislocations that have traversed the Coal-measures, and before any great denudation had been effected on the country." This argument may be readily granted. But, so far as we know, many, if not most, of the faults traverse also the surrounding Permian and Triassic rocks, so that igneous masses protruded during those periods would be affected by the same dislocations.

When we consider the history of Palæozoic time in this country, and especially the proof, obtainable everywhere else in Britain, that volcanic energy became quiescent during the accumulation of the Coal-measures, we may well demand better evidence than has hitherto been forthcoming that any portion of the dolerites of the Midlands is of Carboniferous age. It is important to notice that though the dolerite sills and veins are so abundant in the South Staffordshire coal-field, coming even in many places up to the present surface of the ground, no single case has been observed where they rise into the Permian rocks that overlie the Coal-measures unconformably. It is difficult to believe that, had these intrusions taken place after the deposition of the younger formation, they should not be found penetrating it.[124] It seems almost certain that they must be of an age intermediate between the Coal-measures of South Staffordshire and the surrounding breccias and sandstones of the Permian series. And as there is clear evidence of contemporaneous volcanic action in the lowest part of the Permian system to the north in Scotland and to the south in Devonshire, the inference seems not unreasonable that these intrusive basalts of the Midlands are most probably of Permian age.

[Footnote 124: Only one instance is known where in Staffordshire any igneous rock has been intruded into rocks younger than the Coal-measures (Allport, _Quart. Journ. Geol. Soc._ vol. xxx. p. 551; Sheet 72 S. W. of the Geological Survey, and Horizontal Sections, Sheet 57). It forms a dyke which has been traced near Norton Bridge, Swinnerton and Butterton, running for 8 miles in a N.N.W. direction, and rising through Permian, Bunter and Keuper strata. It is a highly basic olivine-basalt, and is unquestionably a dyke. Mr. J. Kirkby, who has recently mapped and described it (_Trans. North Staffordshire Naturalists' Field-Club_, xxviii. (1894), p. 129), suggests that it may be connected with the igneous rocks of the South Staffordshire coal-field. But of this idea there is no evidence. The last point to which the dyke has been traced is some five-and-twenty miles from the nearest known portion of the dolerites of the coal-field. I have little doubt that this dyke is really an outlying member of the great system of Tertiary dykes described in Book VIII. of the present work.]

No trace of vents has been met with in the Coal-measures of the Midland district or among the surrounding older rocks, nor any proof that the abundant sills and veins were connected with the eruption of volcanic materials at the surface. Nevertheless, from the analogy of the structure of these intrusive sheets to that of the sills in such volcanic districts as the southern half of Scotland, we may well believe that they were connected here and there with eruptive vents, and thus that besides the northern and southern districts of Permian volcanoes, there rose a central group among the lagoons of the heart of England. Though no vestige of any such group has been detected, we must remember that a large portion of the Midlands is overspread with Permian and Triassic deposits, and that much more igneous rock may be concealed than appears at the surface. Possibly there may be buried under these younger sheets of red sandstone and marl, lavas and tuffs with their connected vents, such as may be seen where the Permian volcanic series has been laid bare by denudation in Ayrshire and Devonshire. In this respect it would be interesting to make a thorough examination of the Permian breccias of the district, with the view of discovering whether, though the volcanic rocks _in situ_ may still lie covered up, fragments of them may not be found in these deposits.