Chapter XXIX: The Carboniferous Volcanoes of England (2)

In some cases there is evidence of more than one outflow of lava in the same band of toadstone. Jukes believed that each band "was the result, not of one simultaneous ejection of igneous matter, but of several, proceeding from different foci uniting together to form one band," and he found that near Buxton, two solid beds of toadstone could be seen to have proceeded from opposite quarters towards each other without overlapping.[42]

[Footnote 42: _Student's Manual of Geology_, 2d edit. (1862), p. 523.]

In Millersdale the authors of the _Geological Survey Memoir on North Derbyshire_ observed that a band of toadstone about 100 feet thick showed six distinct divisions, which they were disposed to regard as marking so many separate beds.[43] In Tideswell Dale, on the west side of the valley, immediately to the south of the old toadstone quarry, two bands of toadstone are seen to be separated by a few yards of limestone.

[Footnote 43: _Op. cit._ p. 19.]

(_b_) Lavas with Tuffs.--It will probably be found that in many, if not in most cases, the outflow of lava was preceded, accompanied or followed by fragmental discharges. As far back as 1861, Jukes noticed that a toadstone band, about 50 feet thick, near Buxton consisted of two solid beds of lava "with beds of purple and green ash, greatly decomposed into clay, both above and below each bed and between the two."[44]

[Footnote 44: _Op. cit._ p. 523.]

Fig. 183.--Section at lime-kiln, south of Viaduct, Millersdale
Station.
]

An interesting section, showing this intercalation of the two kinds of material is exposed at the lime-kilns beyond the southern end of the railway viaduct at Millersdale Station. Over a mass of solid blue limestone (1 in Fig. 183) lies a band of bright yellow and brown clay (2), varying from six inches to two feet in thickness. This may be compared with the clay found above the limestone at Peak Forest (Fig. 181). But it is probably a layer of highly decomposed tuff. It is succeeded by a thin band of greenish limestone (3) containing an admixture of fine volcanic detritus, and partially cut out by an irregular bed, four to eight feet thick, of a highly slaggy, greenish, decomposing, spheroidal and amygdaloidal diabase (4). This unmistakable lava-sheet is followed by a bed of green granular tuff (5), which in some places reaches a thickness of three feet, but rapidly dies out. Over a space several yards in breadth, the succeeding strata are concealed, and the next visible rock is a dark, compact dolerite which weathers spheroidally (6).

(_c_) Tuffs without Lavas.--Mr. Bemrose has shown that some of the bands of toadstone consist entirely of bedded tuff. In these cases, so far as the present visible outcrops allow us to judge, no outflow of lava accompanied the eruption of fragmentary materials. But that the ejection of these materials was not the result of a sudden spasmodic explosion, but of a continued series of discharges varying in duration and intensity, is indicated by the well-bedded character of the tuff and the alternation of finer and coarser layers. Large blocks of lava, two feet or more in diameter, may mark some of the more vigorous paroxysms of the vents, while the usual fine granular nature of the tuff may point to the prevailing uniformity and less violent character of the eruptions. Bands of tuff 70 feet or more in thickness, without the intercalation of any limestone or other non-volcanic intercalation, point to episodes of such continued volcanic activity that the ordinary sedimentation of the sea-bottom was interrupted, or at least masked, by the abundant fall of dust and stones.

One of the best exposures of such intercalations of bedded tuffs was pointed out to me by Mr. Bemrose, immediately to the east of the village of Litton. The matrix is crowded with the usual minutely vesicular glassy lapilli, and encloses fragments of diabase of all sizes, up to blocks more than a foot in diameter. The rock is well stratified, and the layers of coarse and fine detritus pass beneath a group of limestone beds. The actual junction is concealed under the roadway, but only two or three feet of rock cannot be seen. The lowest visible layer of limestone is nodular and contains decayed bluish fragments which may be volcanic lapilli. Immediately above the lower limestones the calcareous bands become richly fossiliferous. Some of their layers consist mainly of large bunches of coral; others are crowded with cup-corals, or are made up mainly of crinoids with abundant brachiopods, polyzoa, lamellibranchs, gasteropods and occasional fish-teeth. This remarkable profusion of marine life is interesting inasmuch as it succeeds immediately the band of volcanic ash.

Another well-marked zone of tuff, with no traceable accompaniment of lava, has already been referred to as connected with the Grangemill vents. In this case also, the limestone that lies directly upon the volcanic material is rather impure and nodular in character. The tuff itself is well bedded, perhaps from 70 to 100 feet thick and dips underneath an overlying series of marine limestones.

I did not observe thin partings of tuff and disseminated volcanic lapilli among the limestones, such as are so marked in the Lower Carboniferous formations of West Lothian, and in the Limerick basin, to be described in the following chapter. But a diligent search might discover examples of them, and thus prove that, besides the more prolonged and continuous eruptions that produced the thick bands of tuff, there were occasional feeble and intermittent explosions during the accumulation of the thick sheets of limestone. Some of the layers of "red clay" observed in shafts sunk for mining purposes may perhaps represent such spasmodic discharges of fine fragmental material.

5. THE SILLS.--No attempt has yet been made to determine whether and to what extent the toadstone bands include true intrusive sheets. My own brief examination of the ground does not warrant me in making any positive statement on this subject. I can hardly doubt, however, that some, perhaps not a few, of the toadstone bands are really sills. In the accounts of these rocks contained in the mining records a distinction, as already remarked, appears to have been generally drawn between "toadstone" and "blackstone." The latter term is applied to the black, fresh, more coarsely crystalline, and generally non-amygdaloidal rocks, which, so far as I have been able to examine them, have the general external and many of the internal characters of the Carboniferous sills of Central Scotland. At Snitterton near Matlock one of these "blackstones," as already mentioned, is said to have been found to be 240 feet thick.[45]

[Footnote 45: _North Derbyshire Memoir_, p. 23.]

It is stated that the toadstones, though subject to great variations in thickness, are never seen to cut across the limestones.[46] But I suspect that proofs of intrusion and transgression will be found when diligently sought for. It appeared to me that the dark, compact, crystalline dolerite, which was formerly quarried in the middle of Tideswell Dale, may be separated from the vesicular toadstone of that valley, which is undoubtedly a true lava-flow, and that it does not always occupy the same horizon there, being sometimes below and sometimes above the amygdaloid. Where it rests on a band of red clay the latter rock has been made columnar to a depth of nine feet.[47] Alteration of this kind is very rare among the Carboniferous bedded lavas, but is by no means infrequent in the case of sills. But the most important proof of alteration which I have myself observed occurs at Dale Farm near the village of Peak Forest, where the limestone above a coarsely crystalline dolerite has been converted into a white saccharoid marble for about two yards from the junction.

[Footnote 46: _Op. cit._ p. 123.]

[Footnote 47: J. M. Mello, _Quart. Journ. Geol. Soc._ vol. xxvi. (1871), p. 701.]

3. THE ISLE OF MAN

Rising from the middle of the Irish Sea, within sight of each of the three kingdoms, with a history and associations so distinct, yet so intimately linked with those of the rest of Britain, this interesting island presents in its geological structure features that connect it alike with England, Scotland and Ireland, while at the same time it retains a marked individuality in regard to some of the rocks that form its framework. Its great central ridge of grits and slates, which still rises 2000 feet above the sea in the summit of Snaefell, must have formed a tract of dry land in Carboniferous time, until it sank under sea-level, and was buried beneath the Carboniferous and later formations. Along the southern margin of this ancient land, a relic of the floor of the Carboniferous sea has been preserved in a small basin of Carboniferous Limestone which covers about seven or eight square miles. This remnant has a special interest in geological history, for it has preserved the records of a series of volcanic eruptions which took place contemporaneously with the deposition of the Carboniferous Limestone.

The geology of the Isle of Man was sketched in outline by J. F. Berger,[48] J. Macculloch,[49] and J. S. Henslow,[50] and was afterwards more fully illustrated by J. G. Cumming.[51] To the last-named observer we owe the recognition of true intercalated volcanic rocks among the calcareous formations of the southern end of the island. These rocks have subsequently been studied in greater detail by a number of geologists. An excellent general account of them was published in 1874 by Mr. John Horne, of the Geological Survey.[52] A few years later some further observations on them were prepared by J. Clifton Ward.[53] More recently their petrography has been studied by Messrs. E. Dickson, P. Holland and F. Rutley,[54] and in more detail by Mr. B. Hobson.[55] To some of the observations of these writers reference will be made in the succeeding pages. During the progress of the Geological Survey in the Isle of Man, the rocks in question have been mapped in detail by Mr. A. Strahan and Mr. G. W. Lamplugh, and I have had an opportunity of examining the coast-sections with the last-named geologist. The following description of these sections is taken mainly from my field note-book. The full details will appear in the official _Memoirs_.

[Footnote 48: _Trans. Geol. Soc._ 1st ser. vol. ii. (1814), p. 29.]

[Footnote 49: _Western Islands of Scotland_ (1819), vol. ii. p. 571.]

[Footnote 50: _Trans. Geol. Soc._ 1st ser. vol. v. (1821), p. 482.]

[Footnote 51: _The Isle of Man_ (1848), chap. x.]

[Footnote 52: _Trans. Geol. Soc. Edin._ ii. (1874), p. 332.]

[Footnote 53: _Geol. Mag._ 1880, p. 4.]

[Footnote 54: _Proc. Liverpool Geol. Soc._ vol. vi. (1888-89), p. 123.]

[Footnote 55: _Quart. Journ. Geol. Soc._ xlvii. (1891), p. 432. This paper was reprinted with additions and corrections in _Yn Lioar Manninagh_, Douglas, Isle of Man, vol. i. No. 10, April 1892.]

It may be remarked at the outset that the last outcrop of the plateau-lavas of the Solway basin occurs only 60 miles from the south end of the Isle of Man, at the foot of the hills of Galloway, the blue outline of which can be seen from that island. The distance from the Manx volcanoes to the nearest of the puys of Liddesdale is about 100 miles. Though the fragment which has been left of the ejections is too small to warrant any confident parallelism, there appears to be reason to believe that, alike in geological age and in manner of activity, the Manx volcanoes may be classed with the type of the puys.

The Carboniferous strata of the Isle of Man lie in a small trough at the south end of the island. The lowest members of the series consist of red conglomerates and sandstones, which pass upward into dark limestones full of the characteristic fossils of the Carboniferous Limestone. As the bottom of the basin is on the whole inclined seawards, the highest strata occur along the extreme southern coast. It is there that the volcanic rocks are displayed. They occupy a narrow strip less than two miles in length, which is almost entirely confined to the range of cliffs and the ledges of the foreshore. Yet though thus extremely limited in area, they have been so admirably dissected along the coast, that they furnish a singularly ample body of evidence bearing on the history of Carboniferous volcanic action.

Unfortunately the bottom of the volcanic group is nowhere visible. At the east or lower end of the series, exposed on the shore, an agglomerate with its dykes appears to truncate the Castletown Limestones. No trace of any tuff has been noticed among these lower limestones. We may infer that the volcanic activity began after they were deposited. The highest accessible portions of the volcanic group, as Mr. Horne showed, are clearly exposed on the coast at Poyll Vaaish, intercalated in and overlying the dark limestones of that locality (Fig. 184), which have been assigned, from their fossil contents, to the upper part of the Carboniferous Limestone series.[56] The Manx volcanoes may therefore be regarded as having probably been in eruption during the later portion of the Carboniferous Limestone period.

[Footnote 56: R. Etheridge jun., in Mr. Horne's paper above cited.]

Owing to irregularities of inclination, the thickness of the volcanic group can only be approximately estimated. It is probably not less than 200 or 300 feet. But as merely the edge of the group lies on the land, the volcanic rocks may reach a considerably greater extent and thickness under the sea.

The volcanic materials consist mainly of bedded tuffs, but include also several necks of agglomerate and a number of dykes and sills. So far as I have observed, they comprise no true lava-streams.[57] These Manx tuffs present many of the familiar features of those belonging to the puy-eruptions of Central Scotland, but with some peculiarities worthy of attention. They are on the whole distinctly bedded, and as their inclination is generally in a westerly direction, an ascending order can be traced in them from the eastern end of the section to the highest parts of the group associated with the Poyll Vaaish limestones. Their colour is the usual dull yellowish-green, varying slightly in tint with changes in the texture of the materials, the palest bands consisting of the finest dust or volcanic mud. Great differences in the size of their fragmentary constituents may be observed in successive beds, coarse and fine bands rapidly alternating, with no admixture of non-volcanic sediment, though occasional layers of fine ash or mudstone, showing distinct current-bedding, may be noticed.

[Footnote 57: The occurrence of intercalated lavas has been described in this series, but, as I shall show in the sequel, they are probably intrusive masses.]

Pauses in the succession of eruptions are marked by the intercalation of seams of limestone or groups of limestone, shale and black impure chert. Such interstratifications are sometimes curiously local and interrupted. They may be observed to die out rapidly, thereby allowing the tuff above and below them to unite into one continuous mass. They seem to have been accumulated in hollows of the tuff during somewhat prolonged intervals of volcanic quiescence, and to have been suddenly brought to an end by a renewal of the eruptions. There are some four or five such intercalated groups of calcareous strata in the thick series of tuffs, and we may regard them as marking the chief pauses in the continuity or energy of the volcanic explosions.

An attentive examination of these interpolated sedimentary deposits affords some interesting information as to the submarine conditions in which the eruptions took place. The intercalations, sometimes 12 feet or more in thickness, consist mainly of dark limestones, enclosing the usual Carboniferous Limestone fossils; black shales, sometimes showing very fragmentary and much macerated remains of ferns and other land-plants; and black impure argillaceous chert or flint, arranged in bands interposed between the other strata, and also in detached lumps and strings. The dark flaggy limestones and black shales may be paralleled lithologically with those of Castletown and Poyll Vaaish. Indeed, there seems to be little doubt that they represent the contemporaneous type of marine sediment that was gathering on the sea-floor outside the volcanic area, and which during intervals of quiescence or feeble eruptivity spread more or less continuously into that area. The thick mass of tuff must thus have been strictly contemporaneous with a group of calcareous muddy and siliceous deposits which gathered over the bottom beyond the limits of the showers of ashes.

One of the most singular features of these sedimentary intercalations is the occurrence of the black cherty material. It may generally be observed best developed at the bottom and top of each group of included strata. Looking at the lumps of this substance scattered through the adjoining tuffs, we might at first take them for ejected fragments, and such no doubt may have been the derivation of some of them. But further examination will show that, as a rule, they are of a concretionary nature, and were formed _in situ_ contemporaneously with or subsequent to the deposition of the tuffs. The accompanying section (Fig. 185) represents the manner in which the chert is distributed through two or three square yards of tuff overlying one of the calcareous groups. The material has been segregated not only into lumps, but into veins and bands, which, though on the whole parallel with the general stratification-planes of the deposits, sometimes run irregularly in tongues or strings across these planes, as shown in Fig. 186, where the dark chert band which overlies the limestones and shales sends a tongue upwards for several inches into the overlying tuff.

That these interstratified calcareous and muddy strata were laid down in water of some considerable depth may be inferred from their general lithological characters. The dark carbonaceous aspect of the limestones points to the probable intermingling of much decayed vegetation with the remains of the calcareous organisms of which these strata chiefly consist. The thin unimportant bands or partings of dark shale show that only the finest muddy sediment reached the quiet depths in which the strata were deposited, while the macerated fern-fragments suggest a long flotation and ultimate entombment of terrestrial vegetation borne seawards from some neighbouring land.

Fig. 186.--Section of intercalated dark limestone, shale and chert
in the tuff south of Poyll Vaaish Bay, Isle of Man.

1. Limestones and shales; 2. Chert; 3. Tuff.]

The cherty bands and nodules, like the flints of the chalk, bear their testimony to the quiet character of the sedimentation in rather deep water beyond the limits within which the sediment from the land was mainly accumulated on the sea-bottom. The origin of these siliceous parts of the series of deposits has still to be investigated. Whether or not they are to be referred to organic causes like chalk-flints, and the radiolarian cherts of the Lower Silurian system, they furnish a fresh example of the remarkable association of such siliceous material with volcanic phenomena, which has now been observed in many widely separated areas all over the world.

If we next turn to the stratification of the tuffs, we obtain further evidence of undisturbed conditions of deposition on the sea-floor. The bedding of these volcanic masses, though distinct, appears for the most part to be due rather to the eruption and settlement of alternately finer and coarser detritus than to any marked drifting and rearrangement of these materials by current-action into different layers. Throughout the series of tuffs, indeed, there is, on the whole, a notable absence of any structure suggestive of strong currents or of wave-action in the dispersal and reassortment of the volcanic detritus. The ashes and stones were discharged in such a way as to gather irregularly over the sea-floor into ridges and hollows. There does not seem to have been sufficient movement in the bottom water to level down these inequalities of surface, for we find that they remained long enough to allow twelve feet or more of calcareous and siliceous ooze to gather in the hollows, while the intervening ridges still stood uneffaced until buried under the next fall of ashes. At rare intervals some transient current or deeper wave may have reached the bottom and spread out the volcanic detritus lying there. Such exceptional disturbances of the still water are not improbably indicated by occasional well-defined stratification, and even by distinct false-bedding, in certain finer layers of tuff.

The materials of the tuffs are remarkably uniform in character and conspicuously volcanic in origin. With the exception of occasional blocks of limestone, which range up to masses several feet, and occasionally several yards, in diameter, the dust, lapilli and included stones consist entirely of fragmentary basic lava, so persistent in its lithological features that we may regard its slightly different varieties as merely marking different conditions of the same rock. The accumulation of pumiceous ash in this southern coast of the Isle of Man is one of the most remarkable in Britain. As Mr. Hobson has well shown, the matrix of this tuff consists of irregular lapilli, representing what may have been various conditions of solidification in one original volcanic magma. This magma he has described as an "augite-porphyrite" or olivine-basalt. Some of the lapilli, as he noted, consist of a pumice "crowded with vesicles which occupy more space than the solid part"; others show nearly as many vesicles, but the glass is made brown by the number of its fine dust-like inclusions; a third type presents the cells and cell-walls in nearly equal proportions. The same observer found that where the substance is most cellular the vesicles, fairly uniform in size, measure about a tenth of a millimetre in longest diameter.

An interesting feature of the tuffs is the abundant occurrence of loose felspar crystals throughout the whole group up to the highest visible strata. These crystals, sometimes nearly an inch in length, appear conspicuously as white spots on weathered surfaces of the rock. They are so much decayed, however, that it is difficult to extract them entire. On the most cursory inspection they are observed to enclose blebs of a greenish substance like the material that fills up the vesicles in the pumiceous fragments and in the pieces of cellular lava.

I have not ascertained the original source of these scattered felspars. In one of the dykes on the north side of the agglomerate at Scarlet Point, as was pointed out by Mr. Hobson, large crystals of plagioclase occur in the melaphyre, but the felspars in the tuffs and agglomerates differ so much from these that we cannot suppose them to have come from the explosion of such a rock. I failed to detect any other mineral in detached crystals in the tuffs, but a more diligent search might reveal such, and afford some grounds for speculating on the probable nature of the magma from the explosion of which the scattered crystals were derived. It is at least certain that this magma must have included a large proportion of plagioclase crystals.

Between the lapilli and the minute pumice-dust that constitute the matrix of this tuff much calcite may be detected. Though this mineral may have been partly derived from the decay of the felspar in the lava-fragments, I believe that it is mainly to be attributed to the intermingling of fine calcareous ooze with the ash accumulated on the sea-floor. A more remarkable association of the same kind will be described in later pages from King's County in Ireland. That abundant calcareous organisms peopled the sea in which the Manx Carboniferous volcanoes were active is shown by the contemporaneously deposited limestones. The tuffs themselves are occasionally fossiliferous. Species of _Spirifer_, _Productus_ and other brachiopods, together with broken stems of encrinites, may be found in them, and doubtless the diffused calcite, though now crystalline, as in the limestones, and showing no organic structure, owes its presence to the detritus of once living organisms.

The stones imbedded in the tuff consist almost exclusively of slightly different varieties of the same pale, always vesicular rock, and sometimes pass into a coarse slag. They vary up to six feet or more in length. In many cases, they appear to have been derived from the disruption of already solidified lava, for their vesicles are not elongated or arranged with reference to the form of the block, but have been broken across and appear in section on the outer surface. In other instances, however, the cavities are large and irregular in the centre of the block, while on the outside they are smaller and are drawn out round the rudely spherical shape of the mass, as in true volcanic bombs.

The limestone fragments enclosed in the tuff include pieces of the dark carbonaceous and of the pale encrinal varieties. In no case did I observe any sensible alteration of these fragments. They seem to have been derived from material disrupted and ejected during the opening of successive vents, and not to have been exposed for any considerable time to the metamorphic influence of volcanic heat and vapours.

Narrow though the strip of volcanic material is along the south coast of the Isle of Man, it has fortunately preserved for us some of the vents from which the tuffs were ejected. A group of these vents, three or four in number, may be traced along the shore in a general W.N.W. and E.S.E. line from Scarlet Point for rather more than a mile. Their margins are in some places exceedingly well defined. The most striking example of this feature occurs in the most westerly vent, where a neck of remarkably coarse volcanic agglomerate rises vertically through well-bedded, westerly-dipping tuff (Fig. 187). In other portions of their boundaries no sharp line can be drawn between the material filling the vent and that of the surrounding tuffs. Hence it is difficult to define precisely the form and size of the vents. I am inclined to believe from this indefiniteness of outline, and from the remarkable structure of the dykes, to which I shall afterwards refer, that the presently visible parts of these necks must lie close to the mouths of the original vents, if indeed they do not actually contain parts of the craters and of their surrounding walls.

The materials that have filled up the eruptive vents consist chiefly of agglomerate, but partly also of intrusive portions of vesicular lava. The agglomerate is composed of similar materials to the tuffs. Its matrix shows the same extraordinarily abundant fine greenish-grey basic pumiceous lapilli, with the same kind of plentiful loose felspar-crystals. The large blocks of lava, too, resemble in composition and structure those of the bedded tuffs, but greatly exceed them in size and abundance.

Besides the fragments of vesicular lava, there occur also occasional blocks of limestone. Some of these are several yards in length. Messrs. Strahan and Lamplugh have mapped a large mass of limestone at the Scarlet vent, which, so far as can be observed, lies in the agglomerate--a large cake of white limestone with pebbles of quartz, which has probably been broken off from some underlying bed and carried up in the chimney of the volcano.

As a rule the agglomerate is a tumultuous, unstratified mass. But in many places it shows lines of bedding and, as already stated, passes outward into ordinary bedded tuff, the number and size of the ejected blocks rapidly diminishing. Where this transition occurs we seem to see a remnant of the base of the actual volcanic cone. Thus, in the most westerly vent already cited, while the wall of the vent has been laid bare on the side next the sea, so that the agglomerate on the beach descends vertically through the surrounding bedded tuffs, on the western side the cliffs have preserved a portion of the material that accumulated outside the orifice (Fig. 187). In this section we observe that the coarse agglomerate which fills up the main part of the vent has been left with a hummocky, uneven surface, and that a subsequent and perhaps feebler eruption of finer material has covered over these inequalities, and has extended to the left above the fine tuffs through which the agglomerate has been drilled.

Again, in the largest of the vents, that near Scarlet Point, still clearer proof of successive eruptions and dislocations within a volcanic chimney may be noticed. At one point the accompanying section (Fig. 188) has been laid bare by the waves. The oldest accumulation is a fine green granular tuff (_a_), rudely and faintly arranged in layers inclined at high angles, like the fine materials in many of the vents of the basin of the Firth of Forth. This peculiar stratification, due not to the assortment of materials in water, but to the deposition of coarser and finer detritus by successive explosions, and to subsequent slipping or tilting, is a characteristic feature of the detritus which has filled up ancient volcanic funnels. A later explosion from some adjacent part of the same vent has given rise to the discharge of a coarse agglomerate (_b_), which with blocks sometimes six feet long, overspreads the earlier material. A third detrital accumulation in the same vent, consisting of a firm brecciated tuff (_c_) with much calcite in its matrix, has been brought down by a slip (_f_) which cuts across both of the previous deposits. A broad dyke (_d_) of vesicular diabase (augite-porphyry) traverses the vent, and is probably later than any of the other rocks in the section.

I will conclude this account of the Manx Carboniferous volcanic rocks with a brief reference to the intrusive masses which form a prominent feature of the coast-line. From the picturesque headland of Scarlet Point the broad dyke which forms that promontory may be traced for some distance westwards. Several other parallel dykes run in the same direction which, it will be observed, is also that of the chain of vents. It might be said that the vents are, as it were, strung together by a line of dykes. These eruptive masses traverse both the agglomerates and the bedded tuffs. They probably belong, therefore, to a comparatively late part of the volcanic history. That they are truly intrusive and not lava-flows is, I think, clearly shown by their vertical walls which descend through the surrounding rocks, and by the greater closeness of their texture, as well as the diminution in the size of their vesicles along the contact surfaces. But it must be admitted that in their remarkably developed vesicular structure they look more like streams of lava than ordinary dykes.

It is this structure which gives to these dykes their peculiar interest. Bands of vesicles, from an inch or less to several inches in breadth, run along the dykes parallel to the outer walls. Unlike the familiar rows of little amygdaloidal cells in ordinary basalt dykes, such as those of the Tertiary series in Scotland, these vesicles, though small and pea-like in the narrower bands towards the margins of the dykes, became so large, numerous, and irregular in the broader and more central bands, that the rock passes there into a rough slag.

While the intrusive material has for the most part risen in the form of dykes, in one part of the coast-section, a little to the west of Scarlet Point, it has been injected as a sill among the bedded tuffs.[58] A section taken at this locality gives the structure represented in Fig. 189. On the north side of the great dyke, the strata of tuff which dip under it, roll over and support an outlying sheet of the same material. The slaggy structure of parts of this sill give it some resemblance to a true lava-flow. But it is the same structure which can be seen in the dykes, while the closer grain along the contact-surface further connects it with these intrusions.

[Footnote 58: It is this sheet which has been described as a lava-stream.]

Fig. 190.--Section on south side of vesicular sill west of Scarlet
Point.
]

There is, however, a peculiarity about the development of the vesicular structure in this sill which I have not observed anywhere else. If we examine the southern side of the crag near its eastern end we observe that the successive bands of vesicles are arranged in the same direction as the surface of contact with the underlying tuffs, precisely as they are ranged in dykes parallel to the bounding walls. So far the structure is quite normal. But, moving a few yards westwards, we find that the bands begin to curve, and, instead of following the contact surface, strike it first obliquely and then at right angles, until we have the structure shown in Fig. 191. The bands here vary from less than an inch to more than a foot in breadth, and where broadest assume a slaggy texture. I sought in vain for any evidence of subsequent disturbance such as might have truncated these parallel rows of vesicles and pushed the rock bodily over the tuffs. The perfect parallelism of the bands with the surface of the tuff at the east end, and the absence of all trace of a thrust-plane at the base of the sill, seem to show that, though the rows of vesicles were undoubtedly at first arranged parallel to the surfaces between which the intrusion took place, the mass, before completely consolidating and coming to rest, was ruptured, and a portion of it was driven onwards at right angles to its previous line of movement.

A consideration of the singularly slag-like structure of the injected masses in the tuffs and agglomerates leads to the conclusion that though what we now see of these rocks did not actually flow out at the sea-bottom in streams of lava, it was intruded so close to the surface that the imprisoned vapours had opportunity to expand, as in superficial outflows.[59] This inference is in accord with that derived from an examination of the necks, wherein we find evidence of the probable survival of parts of the actual craters and volcanic cones.

[Footnote 59: As illustrative of the occurrence of the vesicular structure in superficial intrusions, I may again cite the dyke which cuts the ash of the outer crater-wall of the Puy de Pariou in Auvergne. The andesite of this dyke is in places as vesicular as the lava-stream with which it was doubtless connected, but the vesicles have been flattened and drawn out parallel to the walls of the dyke. In this instance it is quite certain that there could never have been any great depth of detrital material above the fissure into which the material of the dyke was injected (see vol. i. p. 66).]

As the records of the earliest eruptions during the Carboniferous Limestone period in the district of the Isle of Man are concealed, so also those of the last of the series lie under the sea. Where the highest visible tuffs overlie the Poyll Vaaish limestones they show no change in the nature of the materials ejected, or in the energy of eruption. They lie so abruptly on the dark calcareous deposits as to show that a considerable pause in volcanic activity was followed by a violent explosion. The same abundant grey-green pumice, the same kind of loose crystals of felspar, the same type of lava-blocks and bombs as had characterized the foregoing eruptions remained as marked at the end. But the further volcanic records cannot be perused, and we are left to speculate whether the coast-sections reveal almost the whole chronicle, or if they merely lay before us the early chapters of a great volcanic history of which the main records lie buried under the waves of the Irish Sea.

4. EAST SOMERSET

Various limited outcrops of igneous rocks have long been known to occur in the eastern part of Somerset. The largest of these lies in the midst of the Old Red Sandstone, on the crest of the axis of the Mendip Hills, between Downhead and Beacon Hill. Smaller patches occur in the Carboniferous Limestone near Wrington Warren, on the north side of Middle Hope, on Worle Hill and at Uphill. These rocks have been mapped as intrusive, though some of them have been described as conglomeratic or as volcanic breccias. While some of the masses are probably intrusive, others appear to be truly contemporaneous with the deposition of the Carboniferous Limestone. The highly vesicular basalt of Middle Hope looks much more like a superficial lava than an intrusion. Mr. Aveline gave a section showing three alternations of limestone and "igneous rock" at Middle Hope. A recent examination of that coast-line by Mr. A. Strahan shows that there are undoubted tuffs interstratified with the calcareous strata. There is thus proof that one or more small volcanic vents were in eruption on the floor of the Carboniferous Limestone sea in the neighbourhood of Weston-super-Mare.[60]

[Footnote 60: See _Geological Survey Memoir_ "On East Somerset," by H. B. Woodward, 1876, and authorities there cited. Mr. Aveline's section above referred to will be found on p. 22.]

5. DEVONSHIRE

The change from the typical Old Red Sandstone of South Wales to the Devonian system of Devonshire, to which I have already referred, is hardly more striking than the contrast between the Carboniferous formations of these two areas.[61] The well-marked threefold subdivisions of Carboniferous Limestone, Millstone Grit and Coal-measures, so persistent throughout Britain, and nowhere more typically developed than in South Wales, are replaced in a distance of less than forty miles by the peculiar "Culm-measures" of Devonshire--a series of black shales, grey sandstones and thin limestones and lenticular seams of impure coal (culm), which are not only singularly unlike in original characters to the ordinary Carboniferous formations, but have been made still more unlike by the extensive and severe cleavage to which the Palæozoic rocks of Devon and Cornwall have been subjected. That these Culm-measures are truly Carboniferous is made abundantly clear by their fossil contents, though it has not yet been possible to determine how far they include representatives of the great stratigraphical subdivisions in other parts of the country.

[Footnote 61: In the centre of England numerous outlying areas of igneous rocks are found in the Carboniferous Limestone, Millstone Grit and Coal-measures. These will be considered by themselves in Chap. xxxii.]

It is to De la Beche that geology owes the first intimation of the occurrence of interstratified igneous rocks in the Carboniferous series of Devonshire. As far back as the year 1834, in his singularly suggestive treatise, _Researches in Theoretical Geology_, this eminent geologist expressed his opinion that not only were the "trappean" bands regularly intercalated in the sedimentary series and continuously traceable with the general stratification, but that they occurred at various localities in such a manner as to raise the suspicion that these points may mark some of the centres of eruption. He particularly cited the example of Brent Tor as a remarkable volcanic-looking hill, composed in part of a conglomerate "having every appearance of volcanic cinders."[62]

[Footnote 62: _Op. Cit._ p. 384.]

In his subsequently published _Report on the Geology of Cornwall, Devonshire and West Somerset_, De la Beche dwelt in more detail on the results of his study of these rocks, which he had traced out on the ground and expressed upon the maps of the Ordnance Geological Survey.[63] Hardly any additions have since been made to our knowledge of the field-relations of the rocks. It is to the maps and Report of De la Beche that we must turn for nearly all the published information on the subject. I shall therefore give here a summary of what can be gathered from these publications.

[Footnote 63: Sheets 22, 23, 24, 25, 30, 31, 32 and 33.]

In tracing the limits of the Culm-measures, De la Beche found that no well-defined line could be drawn between these strata and the "grauwacke" or Devonian formations underneath. The Carboniferous series lies in a great trough, of which the axis runs nearly east and west, so that the lowest members of the series rise along the northern and southern margins. But De la Beche was struck with one remarkable contrast between the two opposite sides of the trough--a contrast which marks the Devonian as well as the Carboniferous formations of this region. On the south side an abundant and persistent group of intercalated bands of igneous, or as he called them, "trappean," materials can be followed along the whole line of boundary, while no such group occurs on the north side. He found these bands to be lenticular, traceable sometimes for a number of miles, then dying out and reappearing on the same or other horizons. He mapped them the whole way from Boscastle on the west to near Exeter on the east, and found that though the individual sheets might be short, the trappean zone was continuous as far as the southern margin of the Carboniferous series could be seen, except where it had been broken through by the great granitic mass of Dartmoor. He ascertained that the intercalated trappean rocks are not confined to the Culm-measures, but occur also in the contiguous portions of the "grauwacke" or Devonian system.

But further, he clearly recognized that the bands of igneous material which he mapped included both "greenstones," together with other varieties of massive eruptive rocks, and also volcanic ash or tuff, though he did not attempt to separate these out upon the maps, but contented himself with representing them all under the same colour. He admitted that some doubt might be entertained as to the age of the greenstones, for some of them might be intrusive and therefore later than the sedimentary deposits between which they lie. But he contended that there could be no uncertainty with regard to the trappean ash or tuff, which being regularly interstratified in the Carboniferous series, must be contemporaneous with it. He pointed out that many of the greenstones, as well as fragments in the conglomerates or ashes, were highly vesicular and must originally have been in the condition of pumice.

As an illustration of the centres of eruption from which these materials were ejected, De la Beche drew special attention once more to the conspicuous eminence of Brent Tor and the rocks in its neighbourhood. His remarks on this subject are well worthy of being quoted--"The idea that in the vicinity of Brent Tor a volcano has been in action, producing effects similar to those produced by active volcanoes, forcibly presents itself. That this volcano projected ashes, which, falling into adjacent water, became interstratified with the mud, silt and sand there depositing, seems probable. That greenstones and other solid trappean rocks constituted the lavas of that period and locality, here and there intermingled with the ash, appears also a reasonable hypothesis. Upon the whole there seems as good evidence as could be expected that to the north and north-west of Tavistock, ash, cinders and liquid melted rocks were ejected and became intermingled with mud, silt and sand during this ancient geological epoch, corresponding with the phenomena exhibited in connection with volcanoes of the present day, more particularly when they adjoin or are situated in the sea, or other waters where ejected ashes, cinders and lava can be intermingled with ordinary mud, silt and sand."[64]

[Footnote 64: _Op. cit._ p. 122.]

It remains for some future observer to fill up the outlines thus sketched by De la Beche, by tracing the respective areas of lavas and tuffs, distinguishing the various petrographical types, separating the intrusive from the interstratified sheets, identifying the necks and bosses that may mark centres of eruption, and expressing these various details upon maps on a sufficiently large scale.

A serious difficulty in this research arises from the effect of the profound alteration which has been produced on the igneous rocks by the cleavage of the region. Many of the "greenstones" have been so cleaved as to become slaty or almost schistose. De la Beche recognized this change and wrote of the "schistose trappean ash." A result of this metamorphism has been to impart to rocks originally massive the same fissile structure as the adjacent slates possess; and in this condition it is often hardly possible to distinguish between "greenstone" and fine-grained "ash." There can indeed be little doubt that among these Carboniferous volcanic rocks, as we have seen to be the case with those of the Devonian system in the same region, many lavas or sills have been mapped as tuffs.

The chief additions to our knowledge of the Carboniferous volcanic group of Devonshire since the time of De la Beche have been made by Mr. F. Rutley, Mr. W. A. Ussher and General M'Mahon. Mr. Rutley[65] has endeavoured to trace the respective areas occupied by the different varieties of volcanic rocks in the district around Brent Tor, near Tavistock, and to show the probable connection of the successive bands of lavas and tuffs with a central vent of discharge situated at that hill. He believes that these bands occur on four different horizons in the sedimentary series. He has studied the microscopic structure of the rocks, which in his view include "amphibolites, gabbros, basalts, pitchstones and schistose ashes, or clastic rocks of a doubtful nature."[66]

[Footnote 65: "The Eruptive Rocks of Brent Tor and its Neighbourhood," _Mem. Geol. Surv._ 1878. "On the Schistose Volcanic Rocks occurring on the west of Dartmoor, with some Notes on the Structure of the Brent Tor Volcano," _Quart. Journ. Geol. Soc._ xxxvi. (1880), p. 286.]

[Footnote 66: "The Eruptive Rocks of Brent Tor," p. 45.]

Mr. Ussher has re-mapped the tract of Culm-measures on the east side of the Dartmoor granite, besides visiting some of the other areas outside of the granite mass. While confirming the general accuracy of De la Beche's survey, he has been able to improve the mapping by inserting more detail, separating especially the tuffs from the "greenstones." The latter have been found by him to be mostly dolerites, some of which, from their parallelism the bands of tuff, may be in his opinion contemporaneous lavas, though the majority of them are evidently intrusive. The tuffs are regularly interstratified among the Culm-measures, their most important band in this district having an average breadth of about 100 yards, and being traceable for at least two miles, possibly considerably further.[67] In going over this tract with Mr. Ussher I was led to regard many of the sheets of diabase (dolerite) or gabbro as true sills and bosses. Most of them occur as short lenticular or oval patches tolerably numerous, but not traceable for more than a short distance, though a connection may often exist which cannot be detected by the scanty evidence on the surface. One sheet which has been followed by Mr. Ussher from Combe to beyond Ashton, a distance of nearly two miles, presents in the centre a somewhat coarsely crystalline texture which rapidly gives way to a much closer grain, and the rock then becomes highly vesicular. It is overlain with dark Culm-shales and bands of fine shaly tuff, passing upward into a granular tuff. Some layers of this tuff assume a finely foliated appearance by the development of pale leek-green folia, which show slickensided surfaces parallel with the bedding. The rock then presents one of the usual appearances of schalstein. This structure seems obviously due to mechanical movement along the planes of stratification.

[Footnote 67: "The British Culm-measures," _Proc. Somerset Archæol. and Nat. His. Soc._ xxxviii. (1892), p. 161.]

Bands of black chert and cherty shale are interpolated among the tuffs, which also contain here and there nodular lumps of similar black impure earthy chert--an interesting association like that alluded to as occurring in the Carboniferous volcanic series of the Isle of Man, and like the occurrence of the radiolarian cherts with the Lower Silurian volcanic series already described.[68]

[Footnote 68: Cherts containing numerous species of radiolaria have recently been found by Dr. Hinde and Mr. Howard Fox to form an important part of the Lower Culm-measures of Devonshire, _Quart. Journ. Geol. Soc._ vol. li. (1895), p. 609.]

The volcanic belt in the valley of the Teign can be followed for about two miles. It is undoubtedly interstratified among the dark Culm-measures, which are distinctly seen dipping under and overlying it.

General M'Mahon has recently shown what may be done by careful and detailed examination of the ground broadly sketched in by De la Beche. He chose for study a strip of "greenstone" shown on the Geological Survey Map to extend for about three and a half miles along the north-west margin of the Dartmoor granite. He has found that what is represented under one wash of colour on that map includes both tuffs and lavas. The tuffs, in spite of the alteration which they appear to have undergone from the proximity of the great granite mass, are found by microscopic investigation to be made up of fine volcanic dust containing minute lapilli of various lavas. Sometimes as many as six or seven different kinds of lava may be represented in the same microscopic slide. These include felsitic or rhyolitic and trachytic rocks together with fragments of dark glassy lava full of magnetite dust. With the tuffs are intercalated sheets of felsite and trachyte. In the same district coarse volcanic agglomerate occur, made up of blocks of different lavas and pieces of different sedimentary rocks.[69]

[Footnote 69: _Quart. Journ. Geol. Soc._ vol. l. (1894), p. 338.]

These observations are of special interest, inasmuch as they point to the eruption of a much more acid series of volcanic lavas and tuffs than had previously been known to exist in the Culm-measures. Until the ground has been more accurately mapped, it is impossible to say whether these rocks are older or younger than those that lie around Brent Tor, a few miles to the south-west. General M'Mahon has noted the presence of more basic eruptive rocks in the same district. He specially cites the occurrence of mica-diorite, of basaltic lavas altered into a serpentinous mass, and of a dolerite which may possibly mark the actual vent of the old Brent Tor volcano. His observations on the influence of the Dartmoor granite in inducing new mineral rearrangements in the igneous rocks of the Culm-measure series are full of interest.