i. MERIONETHSHIRE

Placing the upper limit of the Cambrian system at the top of the Tremadoc group, we pass into the records of another series of volcanic eruptions which marked various epochs during the Silurian period over the area of the British Isles. The earliest of these volcanic episodes has left its memorials in some of the most impressive scenery of North Wales. To the picturesque forms sculptured out of the lavas and ashes of that early time, we owe the noble range of cliffs and peaks that sweeps in a vast semicircle through the heights of Cader Idris, Aran Mawddwy, Arenig, and Moel Wyn. To the east other volcanic masses, perhaps in part coeval with these, rise from amidst younger formations in the groups of the Berwyn and Breidden Hills, and the long ridges of the Shelve and Corndon country. Far to the south, traces of Silurian volcanoes are met with near Builth, while still more remote are the sheets of lava and tuff interstratified among the Lower Silurian rocks of Pembrokeshire, and those which extend into Skomer Island.

The most important of these districts is unquestionably that of Merionethshire. In this area, as was pointed out in the last chapter, the eruptions certainly began before the close of the Cambrian period, for traces of them occur in the Tremadoc and Lingula Flag groups. But below these strata, in the vast pile of grits and conglomerates of the Harlech anticline, there does not appear to be any trace of contemporaneous volcanic action.

At the time when the Geological Survey maps of this region were prepared, the Cambrian and Lower Silurian rocks had not been subdivided into the various palæontological groups which are now recognized. Nor had any attempt been made to separate the various kinds of contemporaneous igneous masses from each other and from the tuffs in so extensive and complicated a mountain-region. The task undertaken by the Survey was beset with difficulties, some of which geologists, furnished with the advantages of a later time, can hardly perhaps realize. The imperfections of the mapping were long ago recognized by the original surveyors, and various corrections of them were made from time to time. First of all, the volcanic rocks, which originally had been all massed under one colour, were traced out separately on the ground, according to their structure and mode of origin, and were distinguished from each other on the maps.[132] Subsequently divisional lines were followed out between some of the larger stratigraphical groups, the maps and sections were still further modified, and the results were summed up in the volume on the _Geology of North Wales_.[133] But short of actually resurveying the whole of that rugged tract, it was impossible to bring the maps abreast of the onward march of science. They consequently remain, as a whole, very much as they were some thirty or forty years ago.

[Footnote 132: _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 95, note.]

[Footnote 133: Some of the modifications introduced are, I think, to be regretted, for the earlier editions of the maps and sections are in certain respects more accurate than the later. On this point I concur with the criticism made by Messrs. Cole and Jennings, _Quart. Journ. Geol. Soc._ vol. xlv. (1889), p. 436.]

Sir Andrew Ramsay, in his great Monograph on the geology of North Wales, has described the Merionethshire volcanic district in considerable detail. He seems finally to have come to the conclusion that the eruptions of that area were included within the Arenig period.[134] He shows, indeed, that on Rhobell Fawr the ejected materials lie directly on disturbed Lingula Flags without the intervention of the Tremadoc group, which is nevertheless present in full development in the near neighbourhood.[135] And in trying to account for this remarkable fact he evidently had in his mind the possibility that volcanic eruptions had taken place long before as well as after the beginning of the deposition of the Arenig grit and slates.[136] He seems eventually, however, to have looked on the Rhobell Fawr sections as exceptional and possibly to be accounted for by some local disturbance and intrusion of eruptive rock.[137] He clearly recognized that there were two great epochs of volcanic activity during the Silurian period in Wales, one belonging to the time of the Arenig, the other to that of the Bala rocks, and he pointed out that the records of these two periods are separated by a thick accumulation of sedimentary strata which, being free from interstratifications of contemporaneous igneous rocks, may be taken to indicate a long interval of quiescence among the subterranean forces.[138]

[Footnote 134: _Mem. Geol. Survey_, vol. iii. 2nd ed., p. 96.]

[Footnote 135: The ashes and agglomerates of Rhobell Fawr can be seen in various places to rest on the highest members of the Lingula Flags. See Messrs. Cole and Holland, _Geol. Mag._ (1890), p. 451.]

[Footnote 136: _Op. cit._ p. 72.]

[Footnote 137: He was disposed to regard Rhobell Fawr as one of the great centres of eruption of the district. See _Memoir of A. C. Ramsay_, p. 81, and _Geology of North Wales_, 2nd edit. p. 98.]

[Footnote 138: _Op. cit._ pp. 71, 96, 105.]

The lower limit of the Arenig rocks has been fixed at a band or bands of grit or conglomerate (Garth grit) which can be followed with some slight interruptions all round the great dome of Cambrian strata from Llanegrin on the south to the shore at Criccieth on the north. The volcanic group doubtless lies, generally speaking, above that basement platform. But, besides the sections at Rhobell Fawr just referred to, where the volcanic materials lie on the Lingula Flags, the same relation may, I think, be observed on the north flank of Cader Idris. Messrs. Cole, Jennings, and Holland have come to the conclusion that the eruptions began at a rather earlier date than that assigned to them in the _Survey Memoirs_, and my own examination of the ground led me to accept their conclusion.[139] I inferred that the earliest discharges in the southern part of the region took place in Cambrian time, at or possibly before the close of the deposition of the Lingula Flags, and that intermittent outbursts occurred at many intervals during the time when the Tremadoc and Arenig rocks were deposited.

[Footnote 139: _Quart. Journ. Geol. Soc._ vol. xlv. (1889), p. 436; _Geol. Mag._ (1890), p. 447. _Pres. Address Geol. Soc._ 1890, p. 107.]

Important confirmation of this view of the Cambrian age of the earlier volcanic eruptions of the Cader Idris region has recently been obtained by Messrs. P. Lake and S. H. Reynolds who, in the ground intervening between the lower slopes of Cader Idris and Dolgelly, have ascertained the existence of a marked band of andesitic lava traceable for some distance in the upper Lingula Flags. They have also observed a higher volcanic group reposing upon the Tremadoc strata at the top of the Cambrian system, and consisting of rhyolite with rhyolite-tuffs.[140]

[Footnote 140: _Quart. Journ. Geol. Soc._ vol. lii. (1896), p. 511.]

Some of the most stupendous memorials of the earlier eruptions are to be seen in the huge mountain mass of Rhobell Fawr (2403 feet). They consist mainly of agglomerates and tuffs, one of the most remarkable varieties of which is distinguished by its abundant scattered crystals of hornblende and of augite. The fragments of rock included in these rocks are scoriæ and lumps of various lavas, especially basaltic and trachytic andesites. The tuffs become finer towards the top of the mountain where they are interleaved with grits. Among the pyroclastic materials occasional lavas (basaltic andesites) occur which may be contemporaneous streams, but most of the lava-form rocks appear to be intrusive. They include dolerites (augite-aphanites), basaltic andesites, and trachytic andesites.[141]

[Footnote 141: Prof. Cole, _Geol. Mag._ (1893), p. 337.]

L L, Lingula flags; _t_, tuffs and ashy slates; _s_, slates and grits; F F, Arenig volcanic series; D, dolerite.]

[Footnote 142: After Messrs. Cole and Holland, _Geol. Mag._ (1890), p. 450.]

The materials from the Rhobell Fawr volcano are clearly distinguishable from those of the Arenig volcanoes in the neighbourhood. The latter begin to make their appearance among the black slates at the base of the northern declivities of Cader Idris, and extend upward through that mountain into the country beyond.

An upper limit to this volcanic group is not easily traceable; partly, no doubt, from the gradual cessation of the eruptions and partly from the want of any marked and persistent stratigraphical horizon near the top of the group. Sir Andrew Ramsay, indeed, refers to the well-known band of pisolitic iron-ore as lying at or near to the top of the Arenig rocks.[143] There can be no doubt, however, that the volcanic intercalations continue far above that horizon in the southern part of the district.

[Footnote 143: _Mem. Geol. Survey_, vol. iii. 2nd edit. pp. 249, 250.]

In spite of the extent to which the volcanic masses of the Arenig period have been covered by later Palæozoic formations, it is still possible to fix approximately the northern, western, and southern limits of the district over which the ashes and lavas were distributed. These materials die out as they are traced southwards from Cader Idris and north-westwards from Tremadoc.[144] The greatest diameter of ground across which they are now continuously traceable is about twenty-eight miles. They attain their greatest thickness, upwards of 5000 feet, in Aran Mawddwy, which rises from their most easterly escarpment. We may therefore infer that the main vent or vents lay somewhere in that direction. The noble range of precipices facing westwards shows how greatly the limits of the volcanic rocks have been reduced by denudation. There can be little doubt that at least the finer tuffs extended westwards as far as a line drawn from Tremadoc to Llanegrin--that is, some fifteen miles or more beyond the cliffs of Aran Mawddwy, thus stretching across much of the site of what is now the great Harlech anticline.

[Footnote 144: _Op. cit._ p. 96.]

This compact, well-defined volcanic area, in spite of the faults which traverse it and the disturbed positions into which its rocks have been thrown, is, in many respects, one of the simplest and most easily studied among the Palæozoic formations of this country. Its main features have been delineated on the maps of the Geological Survey and have been described in Sir Andrew Ramsay's monograph. But these publications cannot be regarded as more than a first broad, though masterly, outline of the whole subject. There is an ample field for further and more minute research wherein, with the larger and better Ordnance maps now available, and with the advantage of the numerous modern petrographical aids, a more exhaustive account may be given of the district. The whole volcanic succession from base to summit is laid bare in innumerable magnificent natural sections along ranges of hills for a distance of some forty miles, and a careful study and re-mapping of it could not fail to add greatly to our knowledge of the early history of volcanic action.[145]

[Footnote 145: The excellent papers of Professor Cole, Mr. Jennings, Mr. Holland, Mr. G. J. Williams, Mr. P. Lake and Mr. S. H. Reynolds are illustrations of how the published work of the Geological Survey may be modified and elaborated.]

According to the observations of the Geological Survey, the Arenig volcanic rocks of Merionethshire naturally arrange themselves in three great bands, each of which is described as tolerably persistent throughout the whole district:--1st, a lower series of ashes and conglomerates, sometimes 3300 feet thick (Aran Mawddwy); 2nd, a middle group of "felstones" and "porphyries," consisting partly of true contemporaneous lava-streams and partly of intrusive sheets, and reaching a thickness of 1500 feet; 3rd, an upper series of fragmental deposits like that beneath, the extreme thickness of which is 800 feet (Arenig mountain). A re-mapping of the ground on the six-inch maps would, no doubt, show many local departures from this general scheme.

The pyroclastic members of this volcanic series present many features of interest both to the field-geologist and the petrographer; but they have as yet been only partially studied. At the southern end of the district it is remarkable to what a large extent the earliest eruptions must have been mere gaseous explosions, with the discharge of comparatively little volcanic material. Many of the tuffs that are interstratified with black slates (? Lingula Flags) at the foot of the long northern slope of Cader Idris, consist mainly of black-slate fragments like the slate underneath, with a variable proportion of grey volcanic dust.

The accompanying section (Fig. 47) represents the arrangement of the rocks exposed at the Slate Quarry of Penrhyn Gwyn. About 50 feet of black slate (_a_) are there seen, the bedding in which dips S. at 20°, while the cleavage is inclined towards S.W. at a slightly higher angle. The next 20 feet of slate (_b_) are distinguished by many intercalations of slate-tuff or breccia, varying from less than an inch to three feet in thickness. An intrusive sheet of andesite (_c_), which varies from two or three to ten feet in thickness, and is strongly cellular in the centre, interrupts the slates and hardens them. Above this sill the indurated slate and tuff (_d_), containing abundant felspar crystals, pass under a flinty porphyritic felsite (_e_) or exceedingly fine tuff, enclosing a band of granular tuff. Beyond this band the black slates with their seams of tuff continue up the hill and include a sheet of slaggy felsitic lava 8 or 10 feet thick.

This section, affording as it does the first glimpse of the volcanic history of Cader Idris, indicates a continued series of feeble gaseous discharges, probably from one or more small vents, whereby the black clay on the sea-floor was blown out, the fragments falling back again to be covered up under a gradual accumulation of similar dark mud. By degrees, as the vigour of eruption increased, lava-dust and detached felspar crystals were ejected, and eventually lava rose to the surface and flowed over the sea-bottom in thin sheets.

But elsewhere, and likewise at a later period in this same southern part of the district, the fragmental discharges consisted mainly of volcanic material. Sir Andrew Ramsay has described the coarse conglomerates composed of subangular and rounded blocks of different "porphyries," sometimes 20 inches in diameter, embedded in a fine matrix of similar materials. The true nature of the component fragments in these rocks has still to be worked out.

Messrs. Cole and Jennings have noticed that the grey volcanic dust of the older slate-tuff of Cader Idris is seen under the microscope "to abound in particles of scoriaceous andesite-glass, now converted into a green palagonite."[146] Their investigations show that while the same kinds of volcanic rocks continue to be met with from the bottom to the top, nevertheless there is an increase in the acid character of the lapilli as the section is traced upwards. Some of the fragments consist of colourless devitrified glass, with pieces of pumice, as if derived from the breaking up of previously-formed tuffs. Others resemble quartz-andesites, rhyolites, or trachytes, while in at least one instance, somewhat low down in the section, quartz-grains with intruded material point to the existence of some fairly acid and vitreous lava.[147] On the south side of Llyn Cau, that is towards the top of the volcanic group, I found a coarse agglomerate with blocks of felsitic lavas, sometimes three feet across (see Fig. 48). This gradual increase of acidity in the lapilli of the tuffs finds an interesting confirmation in the contemporaneous lava-sheets to which I shall afterwards allude.

[Footnote 146: _Quart. Journ. Geol. Soc._, vol. xlv. (1889), p. 424; _Geol. Mag._ (1890), p. 447.]

[Footnote 147: _Op. cit._ p. 429. A tuff lying below the ironstone near Cross Foxes, east of Dolgelly, likewise contains fragments of trachytic lavas.]

One of the most noticeable features in the tuffs of this volcanic group is the great abundance of entire and broken crystals dispersed through them. These crystals have certainly not been formed _in situ_, but were discharged from the vents as part of the volcanic dust. They usually consist of felspar which, at least in the southern portion of the district, appears generally to be plagioclase. Frequent reference to these crystals as evidence of volcanic explosions may be found in the publications of the Survey. Nowhere can they be better seen than in the black slate-tuffs of Cader Idris. They are there white, more or less kaolinized, and as they lie dispersed through the black base, they give the rock a deceptive resemblance to some dark porphyry. The large crystals of hornblende and augite abundantly scattered through much of the tuff of Rhobell Fawr have been already referred to.

In the central parts of the district thick bands of ashes were mapped by the Survey, and described as consisting almost wholly of volcanic materials, but containing occasional thin bands of slate which suffice to mark pauses in the eruptions, when ordinary sediment was strewn over the sea-bottom. In the Cader Idris ground, on the other hand, interstratifications of non-volcanic material are of such frequent recurrence as to show that there, instead of constant and vigorous discharges accumulating a vast pile of ashes, the eruptions followed each other after intervals of sufficient duration to allow of the usual dark sediment spreading for a depth of many feet over the sea-bottom.

One of the most interesting deposits of these interludes of quiescence is that of the pisolitic ironstone and its accompanying strata on the north front of Cader Idris (_i_ in Fig. 48). A coarse pumiceous conglomerate with large slag-like blocks of andesite and other rocks, seen near Llyn-y-Gadr, passes upward into a fine bluish grit and shale, among which lies the bed of pisolitic (or rather oolitic) ironstone which is so widely diffused over North Wales. The finely-oolitic structure of this band is obviously original, but the substance was probably deposited as carbonate of lime under quiet conditions of precipitation. The presence of numerous small _Lingulæ_ in the rock shows that molluscan life flourished on the spot at the time. The iron exists in the ore mainly as magnetite, the original calcite or aragonite having been first replaced by carbonate of iron, which was subsequently broken up so as to leave a residue of minute cubes of magnetite.[148]

[Footnote 148: Messrs. Cole and Jennings, _op. cit._ p. 426.]

Above the ironstone some more blue and black shale and grit pass under a coarse volcanic conglomerate like that below, lying at the base of the high precipice of Cader Idris. Hence this intercalated group of sedimentary strata marks a pause in the discharge of ashes and lavas, during which the peculiar conditions of sedimentation indicated by the ironstone spread over at least the southern part of the volcanic area. Some few miles to the east, where the ironstone has been excavated near Cross Foxes, the band is again found lying among tuffs and grits full of volcanic lapilli.

Between a lower and an upper band of tuff in the Arenig volcanic group the Maps and Memoirs of the Geological Survey distinguish a central zone of "felspathic porphyry," which attains a maximum thickness of 1500 feet (see Fig. 48). From Sir Andrew Ramsay's descriptions, it is clear that he recognized in this zone both intrusive and extrusive sheets, and that the latter, where thickest, were not to be regarded as one mighty lava-flow, but rather as the result of successive outpourings, with occasional intervals marked by the intercalation of bands of slate or of tuff. To a certain extent the intruded sheets are separated on the map from the contemporaneous lavas; but this has been done only in a broad and sketchy way. One of the most important, and at the same time most difficult, tasks yet to be accomplished in this district is the separation of the rocks which were probably poured out at the surface from those that were injected underneath it. My own traverses of the ground have convinced me that good evidence of superficial outflows may be found in tracts which have been mapped as entirely intrusive; while, on the other hand, some of the so-called "lavas" may more probably be of the nature of sills.

The petrography of the rocks, moreover, still requires much study. Among the so-called "felspathic porphyries" of the Survey maps a considerable variety of texture, structure and composition will doubtless be detected. In the _Descriptive Catalogue of Rock-Specimens in the Museum of Practical Geology_ (3rd edit., 1862) the rocks that form the "lava-streams of Llandeilo age," in Merionethshire, are named "felstone," "felspar-porphyry," "felstone-porphyry," "felspathic-porphyry," and "calcareous amygdaloid."

The most interesting feature which my own slight personal acquaintance with the region has brought before me is the clear evidence of a succession from comparatively basic lavas in the lower part of the group to much more acid masses in the higher part. In the Survey map numerous sheets of intrusive "greenstone" are shown traversing the Lingula Flags, Tremadoc slates, and lower part of the volcanic group along the northern slopes of Cader Idris. The true intrusive nature of much of this material is clearly established by transgressive lines of junction and by contact-metamorphism, as well as by the distinctive crystalline texture of the rocks themselves. But the surveyors were evidently puzzled by some parts of the ground. Sir Andrew Ramsay speaks of "the great mass of problematical vesicular and sometimes calcareous rock which is in places almost ashy-looking." After several oscillations of opinion, he seems to have come finally to the conclusion that this vesicular material, which occurs also in the upper part of the mountain, passes into, and cannot be separated from, the undoubted intrusive "greenstones."[149]

[Footnote 149: _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 36; see also pp. 31, 32.]

The true solution of the difficulty will be found, I believe, in the recognition of a group of scoriaceous lavas among these greenstones. The presence of a cellular structure might not be sufficient to demonstrate that the rocks in which it appears are true lava-beds, for such a structure is far from unknown both among dykes and sills. But in the present case there is other corroborative testimony that some of these Cader Idris amygdaloids were really poured out at the surface. Below Llyn-y-Gadr--the dark tarn at the foot of the vast wall of Cader Idris--the beds of coarse volcanic conglomerate (_b_ in Fig. 48), to which I have already alluded, are largely composed of blocks of the vesicular "greenstones" on which they lie. These "greenstones," moreover, have many of the most striking characteristics of true lavas (_a_ in Fig. 48). They are extraordinarily cellular; their upper surfaces sometimes present a mass of bomb-like slags with flow-structure, and the vesicles are not infrequently arranged in rows and bands along the dip-planes.

A microscopic examination of two slides cut from these rocks shows them to be of a trachytic or andesitic type, with porphyritic crystals of a kaolinized felspar embedded in a microlitic groundmass. The rocks are much impregnated with calcite, which fills their vesicles and ramifies through their mass.

A few miles to the east some remarkable felsitic rocks take the place of these vesicular lavas immediately below the pisolitic iron ore. I have not determined satisfactorily their relations to the surrounding rocks, and in particular am uncertain whether they are interbedded lavas or intrusive sheets. Dr. F. H. Hatch found that their microscopic characters show a close resemblance to the soda-felsites described by him from the Bala series of the south-east of Ireland.

The slopes of Cader Idris are partly obscured with debris, from above which rises the great precipitous face formed by the escarpment of "porphyry," here intrusively interposed among the Arenig volcanic rocks. This enormous sill will be referred to a little further on in connection with the other intrusive sheets of the region.

The remarkably cellular rock which forms the peak of Cader Idris is coloured on the Survey map as an intrusive sill of "greenstone," which in the Memoir is said to alter the contiguous slates and to appear to cut across them diagonally. I am disposed, however, to think that these appearances of intrusion are deceptive. On the southern declivity of the mountain this rock presents one of the most curious structures to be seen in the whole district. Its surface displays a mass of spheroidal or pillow-shaped blocks aggregated together, each having a tendency to divide internally into prisms which diverge from the outside towards the centre.[150] Some portions are extremely slaggy, and round these more solid portions finely crystalline parts are drawn, suggestive rather of free motion at the surface than of the conditions under which a subterranean sill must be formed. The idea occurred to me on the ground that while the band of rock marked as "greenstone" on the map is probably, in the main, an interstratified lava, there may nevertheless be basic intrusions along its course, as in the lower part of the mountain. The minute structure of this amygdaloid, as revealed by the microscope, shows it to be an epidiorite wherein the hornblende, paramorphic after augite, has been again partially altered along the margins into chlorite.

[Footnote 150: This peculiar structure of the more basic Arenig lavas, where the rock looks as if built up of irregularly-spheroidal, sack-like or pillow-shaped blocks, will be again referred to in connection with the Arenig (and Llandeilo) lavas of Scotland and Ireland. It appears to be widely distributed, and especially in connection with the occurrence of radiolarian cherts. The black slate above the Cader Idris amygdaloid would, in a similar position in Scotland, be associated with such cherts, but these have not yet been noticed at this locality. With the spheroidal internally-radiating prismatic structure of the Cader Idris rock, compare that of the lava at Acicastello already noticed on p. 26.]

The highest lavas of Cader Idris, forming the ridge to the south of Llyn Cau, are separated from the amygdaloid just described by a thick zone of black slate with thin ashy intercalations, beyond which comes the coarse volcanic agglomerate already referred to as containing blocks of felsite a yard or more in diameter. These lavas are true felsites, sometimes beautifully spherulitic and exhibiting abundant flow-structure, like some of the felsites of the next or Bala volcanic period.[151] The petrography of these rocks still remains to be worked out.

[Footnote 151: Messrs. Cole and Jennings, _Quart. Journ. Geol. Soc._ vol. xlv. (1889), p. 430. From the examination of slices prepared from a few of the felsites of the Dolgelly district, Dr. Hatch observed a "striking difference between their characters and those of the Cambrian felsites of Caernarvonshire. The porphyritic constituent is now no longer quartz, but felspar (plagioclase), and the rocks belong, not to the rhyolitic, but rather to the less acid trachytes, perhaps even to the andesites."]

The volcanic series of Cader Idris sweeps northward through the chain of Aran and Arenig, and then curves westward through the group of Manod and Moelwyn, beyond which it rapidly dies out. In its course of about 45 miles it undergoes considerable variation, as may be seen by comparing a section through Moelwyn with that through Cader Idris already given. According to the researches of Mr. Jennings and Mr. Williams,[152] the main mass of volcanic material in the northern part of the region consists of fragmentary rocks varying in texture from agglomerates into fine tuffs, but showing some differences in the succession of beds in different localities.

[Footnote 152: _Quart. Journ. Geol. Soc._ xlvii. (1891), p. 368.]

The Tremadoc group of strata clearly underlies the volcanic series of these more northerly tracts. But it contains, so far as appears, no intercalation of volcanic material. The inference may thus be drawn that the eruptions began in the Cader Idris district, and did not extend into that of Manod and Moelwyn until after the beginning of the Arenig period. Above the Tremadoc group lies the well-marked and persistent band, about 13 feet thick, known as the Garth grit, which has been already referred to as a convenient base-line to the Arenig group.

1, Tremadoc Group; 2, Garth or Arenig grit (base of Arenig group); 3, Arenig slates, etc.; 3^1, Lower slate band; 3^2, Middle slate band; 3^3, Upper slate band; 4^1, Lower agglomerate; 4^2, Middle agglomerate; 4^3, Upper agglomerate; 5, Llandeilo group; G, Granite boss of Moel tan y Grisiau. ]

[Footnote 153: After Messrs. Jennings and Williams, _Quart. Journ. Geol. Soc._ vol. xlvii. (1891), p. 371, and Horizont. Sect. Geol. Surv. Sheet 28.]

In this northern district, among the sediments which overlie the Garth grit, layers of fine tuff begin to make their appearance, which north of Cwm Orthin thicken out into a considerable mass between the grit and the lowest of the great agglomerates. These tuffs, which mark the beginning of the volcanic eruptions of the district, are followed by a band of slate which in some places has yielded a _Lingula_, _Orthis Carausii_, and a _Tetragraptus_, and points to an interval of quiescence in the volcanic history. We now enter upon an enormous thickness of agglomerates and tuffs separated by several bands of slate. Taking advantage of the slaty intercalations, Messrs. Jennings and Williams have divided this great accumulation of fragmentary volcanic material into three beds (Fig. 49). The matrix of the agglomerates is compact and pale, so as to resemble and to have been called "felstone," but showing its fragmentary nature on weathered surfaces. The blocks imbedded in this paste range up to sometimes as much as 11 feet in length by 4 feet in width. Their minute petrographical characters have not been studied, but the blocks are stated to consist for the most part of "slaty and schistose fragments mixed with rounded pebbles of fine-grained 'felstone.'" They are heaped together as in true agglomerates. In the upper agglomerate, fragments of cleaved slate containing _Lingula_ have been observed.

The name of "felstone" is restricted by Messrs. Jennings and Williams to certain fine-grained varieties of rock, of which a thin band lies at the base of the lower agglomerate, while another of considerably greater importance occurs in the middle of the upper agglomerate. These bands consist of a fine compact greenish base, and weather with a dull white crust; sometimes, as in the thicker sheet, a columnar structure shows itself. Whether these rocks are to be regarded as lavas or sills, or even as finer varieties of tuff, is a question that awaits further inquiry. But it is clear, from the investigation of the two observers just cited, that the pyroclastic constituents must vastly preponderate in the volcanic series over the northern part of the region. All these rocks, whether coarse or fine-grained, appear to be rather acid in composition, and no evidence has yet been obtained of a sequence among them from a more basic to a more acid series, as in Cader Idris.

The highest agglomerate bed of the Manod and Moelwyn area is covered by slates which contain Llandeilo graptolites. In this way, by means of palæontological evidence, the upward and downward limits of the Arenig volcanic series in this part of Wales are definitely fixed.

Hardly any information has yet been obtained as to the situation and character of the vents from which the lavas and ashes of Merionethshire were discharged. In the course of the mapping of the ground, the Geological Survey recognized that, as the greatest bulk of erupted material lies in the eastern and south-eastern parts of the region, the chief centres of emission were to be looked for in that quarter, and that possibly some of the intrusive masses which break through the rocks west of the great escarpment may mark the site of vents, such as Tyddyn-rhiw, Gelli-llwyd-fawr, Y-Foel-ddu, Rhobell Fawr, and certain bosses near Arenig.[154] The distribution of the volcanic materials indicates that there were certainly more than one active crater. While the southward thickening of the whole volcanic group points to some specially vigorous volcano in that quarter, the notable thinning away of the upper tuffs southward and their great depth about Arenig suggest their having come from some vent in this neighbourhood. On the other hand, the lower tuffs are absent at Arenig, while on Aran Mawddwy, only nine miles to the south, they reach a depth of 3000 feet. Still farther to the south these volcanic ejections become more and more divided by intercalated bands of ordinary sediment. One of the most important volcanoes of the region evidently rose somewhere in the neighbourhood of what is now Aran Mawddwy. There seems reason to surmise that the sites of the chief vents now lie to the east and south of the great escarpment, buried under the thick sedimentary formations which cover all that region.

[Footnote 154: _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 98; see also pp. 44, 54, 58, 71.]

If we are justified, on stratigraphical and petrographical grounds, in connecting the lowest volcanic rocks of the Berwyn range with those of Merionethshire, we may speculate on the existence of a group of submarine vents, coming into eruption at successive intervals, from some epoch during the period of the Lingula Flags up to that of the Bala rocks, and covering with lavas and ashes a space of sea-bottom at least forty miles from east to west by more than twenty miles from north to south, or roughly, an area of some 800 square miles.[155]

[Footnote 155: The Berwyn Hills, however, will be described in later pages as a distinct volcanic district.]

Besides the materials ejected to the surface, the ancient volcanic region of Merionethshire was marked by the intrusion of a vast amount of igneous rock between and across the bedding-planes of the strata deep underground. One of the most prominent features of the Geological Survey map is the great number of sills represented as running with the general strike of the strata, especially between the top of the Harlech grits and the base of the volcanic series. On the north side of the valley of the Mawddach, between Barmouth and Rhaiadr Mawddach, in a distance of twelve miles the Survey mapped "more than 150 intrusions varying from a few yards to nearly a mile in length."[156] This zone of sills is equally marked on the south side of the valley. It may be traced all round the Harlech anticline until it dies out, as the bedded masses also do, towards Towyn on the south and about Tremadoc on the north.

[Footnote 156: _Mem. Geol. Surv._ vol. iii. p. 26.]

The presence of such a zone of intrusive sheets at the base of an ancient volcanic series is a characteristic feature in the geology of Britain. It is met with again and again among the Palæozoic systems, and appears on a striking scale in association with the Tertiary basaltic plateaux of Antrim and the Inner Hebrides. But nowhere, perhaps, is it more strongly developed than beneath the Arenig group of lavas and tuffs in North Wales. Abundant as are the protrusions marked on the Geological Survey map, they fall short of the actual number to be met with on the ground. Indeed, to represent them as they really are would require laborious surveying and the use of maps on a far larger scale than one inch to a mile.

The vast majority of these sills are basic rocks, or, in the old and convenient terminology, "greenstones." Those of the Cader Idris district have been examined by Messrs. Cole and Jennings, who found that, notwithstanding the considerable alteration everywhere shown by the abundant epidote and calcite, the coarser varieties may be recognized as having originally been dolerites approaching gabbro, with a well-developed ophitic character, the general range of structure being from dolerites without olivine and aphanites to andesitic rocks with an originally glassy matrix.[157] Dr. Hatch confirmed this diagnosis from slides prepared from my specimens. The ophitic structure is usually characteristic and well preserved, in spite of the alteration indicated by epidote, chlorite, uralite, and leucoxene.

[Footnote 157: _Quart. Journ. Geol. Soc._ vol. xlv. (1889), p. 432.]

That this zone of "greenstone" sills belongs to the period of the Merionethshire volcanoes may be reasonably concluded. The way in which they follow the line of the great escarpment, their almost entire absence from the Cambrian dome to the west, their cessation as the overlying lavas and tuffs die out laterally, and their scarcity above the lower part of the volcanic group, seem to indicate their close relationship to that group. Moreover, that they must have been as a whole later than the main part of the lavas and tuffs may be inferred from their position. The molten material of which they were formed could hardly have forced its way between and across the strata unless egress to the surface had been impeded by some thick overlying mass. The "greenstones" may therefore be regarded as lateral emanations from funnels of more basic lava towards the close of the volcanic period. Possibly some at least of the highly slaggy and vesicular bands to which I have referred may represent portions of this material, which actually flowed out as streams of lava at the surface.

But there is likewise evidence of extensive intrusion of more siliceous rocks. On the Geological Survey map, besides the numerous "greenstones," various sheets of "felspathic porphyry" are represented as running with the general strike of the region, but here and there breaking across it. One of the most remarkable of these acid sills is that which, in the noble precipice of Cader Idris, has a thickness of about 1500 feet and a length of three or four miles. It is shown on the map to be transgressive across other rocks, and, as seen on the ground, it maintains the uniformity of texture which is characteristic rather of sheets that have solidified underneath than of those which have congealed with comparative rapidity at the surface. On a fresh fracture the rock presents a pale bluish-grey tint, becoming yellowish or brownish as the result of weathering. Its texture is finely granular, with occasional disseminated felspars. Under the microscope a section of it was found by Dr. Hatch to exhibit the characteristic structure of a microgranite, a confused holocrystalline aggregate of quartz and felspar, with a few porphyritic felspars. Messrs. Cole and Jennings have proposed to revive for this rock Daubuisson's name "Eurite."[158]

[Footnote 158: Mr. Harker speaks of the rock as a granophyre.]

A similar rock occurs at a lower horizon among the Lingula Flags at Gelli-llwyd-fawr, two miles south-west of Dolgelly,[159] and much microgranite has been injected along the slopes above Tyddyn-mawr.

[Footnote 159: Messrs. Cole and Jennings, _op. cit._ p. 435.]

The chronological relation of these acid sheets and bosses to the more basic intrusions has not yet been definitely determined. That some of them may have solidified in vents and may have been directly connected with the protrusion of the later or more highly siliceous lavas is not at all improbable. Others again would seem to belong to a much later geological period than the Arenig volcanoes. In this late series the well-known boss of Tan-y-grisiau near Festiniog should probably be included. This mass of eruptive material was mapped by the Geological Survey as "intrusive syenite." It has been more recently examined and described by Messrs. Jennings and Williams as a granitite.[160] These observers have noticed not only that it intrusively traverses and alters the Tremadoc group, but that its intrusion appears to have taken place subsequent to the cleavage which affects the Llandeilo as well as older formations. This granitic boss has thus probably no connection with the Arenig volcanoes, but belongs to a later period in the volcanic history of the Principality.

[Footnote 160: _Quart. Journ. Geol. Soc._ vol. xlvii. (1891), p. 379.]

The remarkable scarcity of dykes in the volcanic districts of Wales has been noticed by more than one observer. Among the intrusive "greenstones" of Merionethshire some occasionally assume the dyke form, and through the agglomerates and tuffs of Rhobell Fawr dykes of olivine-diabase have worked their way. In the Festiniog district various altered andesitic dykes have been noted. But there has been no widespread fissuring of the ground and uprise of lava in the rents, such as may be seen in the Archæan gneiss, and in the later Palæozoic, but still more in the Tertiary volcanic regions. This feature becomes all the more notable when it is viewed in connection with the great development of sills, and the evidence thereby afforded of widespread and extremely vigorous subterranean volcanic action.

In the Merionethshire region there certainly was a long period of quiescence between the close of the Arenig and the beginning of the Bala eruptions. Moreover, no evidence has yet been found that active vents ever again appeared in that district, the subterranean energy at its next outburst having broken out farther to the east and north. In Anglesey, however, where, as I shall point out, there is proof of contemporaneous tuffs among the Arenig rocks, it is possible that a continuous record of volcanic action may yet be traced from Arenig well onward into Bala time.