Chapter 25

Fig. 49.—Section across the Moelwyn Range.[153]1, Tremadoc Group; 2, Garth or Arenig grit (base of Arenig group); 3, Arenig slates, etc.; 31, Lower slate band; 32, Middle slate band; 33, Upper slate band; 41, Lower agglomerate; 42, Middle agglomerate; 43, Upper agglomerate; 5, Llandeilo group; G, Granite boss of Moel tan y Grisiau.

[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 aLingula,Orthis Carausii, and aTetragraptus, 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 theupper agglomerate, fragments of cleaved slate containingLingulahave 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.

[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 ofMerionethshire, 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]

[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.

[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.

[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 lavasand 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]

[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.

[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 whichaffects 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.

[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.

About 35 miles to the south-east of the great volcanic range of Merionethshire a small tract of Arenig rocks rises from amidst younger formations, and forms the picturesque country between Church Stoke and Pontesbury. Murchison in his excellent account of this district clearly recognized the presence of both intrusive and interstratified igneous rocks.[161]The ground has in recent years been more carefully worked over by Mr. G. H. Morton[162]and Professor Lapworth.[163]

[161]Silurian System(1839), chap. xix.;Siluria, 4th edit. (1867), pp. 26, 49.[162]Proc. Liverpool Geol. Soc.x. (1854), p. 62.[163]Geol. Mag.(1887), p. 78.

[161]Silurian System(1839), chap. xix.;Siluria, 4th edit. (1867), pp. 26, 49.

[162]Proc. Liverpool Geol. Soc.x. (1854), p. 62.

[163]Geol. Mag.(1887), p. 78.

At the top of the Arenig group of this district lies a zone of well-stratified andesitic tuff and breccia (Stapeley Ash), with frequent intercalations of shales, and occasionally fossiliferous.[164]There is thus satisfactory proof of contemporaneous eruptions at intervals during the accumulation of the later Arenig sediments. That there were also outflows of lava is shown by the presence of sheets of augite- and hypersthene-andesite. These volcanic intercalations form marked ridges, having a general northerly trend. They are folded over the broad laccolitic ridge of Corndon, on the east side of which they are thrown into a synclinal trough, so that successive parallel outcrops of them are exposed. According to the mapping ofthe Geological Survey they are thickest towards the west, and become more split up with intercalated sediments as they range eastward.

[164]Prof. Lapworth and Mr. W. W. Watts,Proc. Geol. Assoc.xiii. (1894), pp. 317, 337.

Volcanic eruptions in this Shropshire region continued from the Arenig into the Bala period. They are marked among the Llandeilo strata by occasional tuffs and by two massive beds of "volcanic grit," described by Murchison,[165]but they appear to have been rather less vigorous in the interval represented by this subdivision of the Silurian system. Those of Bala time gave forth abundant discharges of ash, of which the lowest accumulation, locally known as the Hagley Ash, consists of andesitic detritus. Occasional layers of tuff are intercalated in the overlying Hagley Shales, above which comes an important band called the Whittery Ash, "consisting of andesitic and rhyolitic breccias and conglomerates, fine ashes with curious spherulitic or pisolitic structures, and bands of shale often fossiliferous."[166]It is evident that the eruptions of the Shelve district came from independent vents in that neighbourhood, and never reached the importance of the great volcanoes of Arenig age in Montgomeryshire or of Bala age in Caernarvonshire.

[165]Silurian System, p. 229.[166]Messrs. Lapworth and Watts,op. cit.p. 318.

[165]Silurian System, p. 229.

[166]Messrs. Lapworth and Watts,op. cit.p. 318.

Fig. 50.—Section across the anticline of Corndon.[167]A, Arenig flags and shales; B, andesites and tuffs; C, intrusive dolerite.

[167]After Prof. Lapworth and Mr. Watts,op. cit.p. 342.

Numerous dykes and sills traverse the rocks of this district. They consist chiefly of hypersthene-dolerite. They appear to belong to a much later period than the interstratified volcanic series; at least some of them are found altering the Pentamerus limestones, and these must be later than the Llandovery rocks.[168]The most important sill is that which forms Corndon, the central igneous mass of the district. This body of dolerite was ascertained by Mr. Watts not to be a boss but a laccolite, which wedges out both towards the north-west and south-east, as shown inFig. 50.

[168]Op. cit.p. 339.

Six miles to the north of the Shelve and Corndon district the Breidden Hills rise on the border of Shropshire and Montgomeryshire, and include a mass of volcanic material belonging to a distinct area of eruption. In the ridge that extends for about three and a half miles through Moel-y-golfa and Middletown Hill, a synclinal trough of volcanic rocks lies upon shales, which from their fossils have been placed in the Bala group. The volcanic series appears to exceed 1000 feet in thickness. The lowest part of it on Moel-y-golfa consists of andesitic lavas about 400 feet thick, followed by tuffs and volcanic conglomerates. The lavas resemble some of the "porphyrites"of the Old Red Sandstone, and contain two forms of pyroxene—one rhombic, probably enstatite, and the other monoclinic augite. There are likewise considerable masses of intrusive rock, which are varieties of diabase or dolerite.[169]

[169]See Mr. W. W. Watts on the Igneous and Associated Rocks of the Breidden Hills,Quart. Journ. Geol. Soc.vol. xli. (1885), p. 532.

From the centre of England we must in imagination transport ourselves into the Southern Uplands of Scotland, where a widely distributed series of Silurian volcanic rocks has been preserved. It was, until recently, supposed that the Silurian system north of the Tweed contains no contemporaneously erupted volcanic rocks. Yet, as far back as the year 1860, I pointed to the abundant existence of volcanic detritus in these strata throughout the southern counties as a probable indication of volcanic activity at the time and in the area within which the strata were deposited.[170]Some years later, when the microscope had been introduced as an aid to field-geology, I sliced some of the Silurian sediments of that region and found them, particularly certain shales and grits of Moffatdale, to contain a large admixture of perfectly fresh unworn felspar crystals, which I felt tolerably certain had been supplied by volcanic explosions. As no trace, however, had then been detected of an intercalated volcanic group in any part of the Silurian series of the south of Scotland, I used at that time to speculate on the possibility of the volcanic detritus having been wind-borne from the volcanoes of the Lake District. I had at that time no suspicion that its source was rather to be sought under my feet. The presence of volcanic rocks underneath the uplands of the south of Scotland would have been a welcome explanation of the frequent felspathic composition of many of the Silurian greywackes and shales of that region, and particularly the abundance of andesitic and felsitic fragments in them.

[170]Trans. Roy. Soc. Edin.xxii. (1860), p. 636.

It had been long known that the Scottish Silurian formations, besides having undergone extensive plication, have also been injected by protrusions of igneous material of various kinds. The intrusive character of many of these is so obvious that a similar origin was attributed even to those bosses which could not be proved to be intrusive. Recent work of the Geological Survey, however, and more especially the numerous and careful traverses of my friend and colleague Mr. Peach, have revealed the unlooked-for and important fact that a large number of these supposed intrusions are really portions of a volcanic group brought up on the crests of anticlinal folds, and laid bare by denudation. This group can be traced for at least 100 miles from north-east to south-west over a belt of country sometimes 30 miles broad. Its original limits cannot be ascertained, but they obviously exceeded those within which the rocks can now be seen. Nevertheless the present boundaries embrace an area of nearly 2000 square miles. This Palæozoic volcanic region is thus one of the most extensive in the British Isles.Owing, however, to the constant plication of the strata, and the wide space which the overlying sedimentary deposits are thus made to cover, the volcanic group only comes occasionally into view, and thus occupies but a mere fraction of the superficial extent of the region over which its scattered outcrops appear. These exposures, sometimes only a few square yards in extent, may always be looked for where the anticlinal folds bring up a sufficiently low portion of the Silurian system; they prove that a vast volcanic floor underlies the visible Lower Silurian grits and shales over the length and breadth of the Southern Uplands of Scotland.

Without anticipating details which will properly appear in the officialMemoirsof the Geological Survey, I may briefly indicate the visible boundaries of the volcanic group, and refer to some of the localities where it may best be seen. The most easterly points where it has been recognized by Mr. Peach stand on the crests of some sharp anticlinal folds near St. Mary's Loch and near Leadburn and Winkstone in Peeblesshire. Farther westwards it appears at many places along the northern border of the Silurian territory, as at Romanno Bridge, Wrae, Kilbucho, Culter Water and Abington, the length and breadth of each exposure depending partly on the breadth of the anticline and partly on the depth to which it has been cut down by denudation. Near Sanquhar the volcanic series opens out for a breadth of more than a mile, and is seen at intervals across the wild moorlands of Carrick, until from the Stinchar valley it widens out seaward and occupies much of the coast-line of Ayrshire between Girvan and the mouth of Loch Ryan. It probably rises again along a fold near Portpatrick, and it is seen at various points along the southern borders of the Silurian uplands, as near Castle-Douglas, at Glenkiln, Bell Craig near Moffat, and the head of Ettrickdale.

The best sections are those exposed along the coast to the north and south of Ballantrae. When that ground was first examined by the Geological Survey, the hypothetical views in regard to metamorphism already referred to were in full ascendant, and the rocks were mapped on the same general principles as those which had been followed in Wales. Professor Bonney, however, a few years later recognized the true igneous nature of many of the rocks. He found among them porphyrite lavas and agglomerates which he regarded as of Old Red Sandstone age, likewise intrusive serpentines and gabbros.[171]

[171]Quart. Journ. Geol. Soc.vol. xxxiv. (1878), p. 769.

The volcanic rocks of this wide district include both lavas and their pyroclastic accompaniments, as well as intrusive sills and bosses of various materials. They have recently been studied by Mr. J. J. H. Teall, and full descriptions of them by him will appear in a forthcoming volume of theMemoirsof the Geological Survey. He has ascertained that though generally more or less decomposed, the lavas would be classed by German petrographers as diabases and diabase-porphyrites. The former are compact dark-green non-porphyritic rocks, often containing numerous small spherical amygdales; while the latter are markedly porphyritic, enclosinglarge phenocrysts of more or less altered plagioclase, often measuring half an inch across. These two groups of rock are connected by transitional varieties. They were probably, in the first instance, composed of plagioclase, augite, iron-ores, and a variable quantity of imperfectly crystallized interstitial matter.

Some of these rocks closely resemble in outward appearance the andesites ("porphyrites") of the Old Red Sandstone of the district not many miles to the north, that is, fine purplish-red rocks with a compact base through which porphyritic felspars are abundantly scattered. Occasionally they are markedly slaggy, and show even a ropy surface, while the breccias associated with them contain blocks of similar slag.

Fig. 51.—Structure in finely-amygdaloidal diabase lava, south of mouth of Stinchar River, Ayrshire. The fine dots and circles mark the lines of amygdales.

But the most characteristic external feature of these lavas is their tendency to assume irregularly-elliptical, sack-like or pillow-shaped forms. On a weathered face they sometimes look like a pile of partially-filled sacks heaped on each other, the prominences of one projecting into corresponding hollows in the next. The general aspect of this structure is shown inFig. 12, which represents a face of rock about eight feet high and six feet broad. The rocks exhibiting this peculiarity are usually finely amygdaloidal, and it may be observed that the vesicles are grouped in lines parallel to the outer surface of the pillow-like block in which they occur. The diagram inFig. 51represents in ground-plan a surface about twelve feet square on the shore immediately to the south of the mouth of the River Stinchar. In the heart of the spheroids enclosed fragments of other lavas are sometimes observable.

This singular structure has already (p. 184) been referred to as strikingly displayed in a rock at the top of Cader Idris. It is found in dark basic lavas probably of Arenig age, which will be afterwards referred to as occurring along the southern flanks of the Scottish Highlands and also in the north of Ireland. It has been observed by Mr. Teall among the rocks of the Lizard, and has been described as occurring in Saxony and California.[172]In these different localities it is associated with jaspers and cherts, some of which contain abundant Radiolaria. The same structure has been found among the variolitic diabases of Mont Genèvre,[173]and likewise in some modern lavas, as in that of Acicastello already referred to (ante,p. 26).

[172]Mr. J. J. H. Teall,Roy. Geol. Soc. Cornwall, 1894, p. 3. Mr. L. Ransome,Bull. Depart. Geol. University of California, vol. i. p. 106.[173]Messrs. Cole and Gregory,Quart. Journ. Geol. Soc.vol. xlvi. (1890), p. 311.

[172]Mr. J. J. H. Teall,Roy. Geol. Soc. Cornwall, 1894, p. 3. Mr. L. Ransome,Bull. Depart. Geol. University of California, vol. i. p. 106.

[173]Messrs. Cole and Gregory,Quart. Journ. Geol. Soc.vol. xlvi. (1890), p. 311.

Fig. 52.—View of Knockdolian Hill from the east.

The volcanic agglomerates and breccias, in the south-west of Ayrshire, attain a great development in several centres probably at or near the original volcanic vents. They present several distinct petrographical types. The remarkable neck-like hill of Knockdolian in the Stinchar Valley is made of a coarse breccia composed mainly of angular pieces of dull greyish-green fine-grained diabase. The breccias and agglomerates of Bennane Head in some parts consist largely of broken-up shales, flinty mudstone, black radiolarian flint or chert, and abundant fragments of andesites and felsites. In other parts the volcanic material predominates, including angular and subangular fragments of various somewhat basic lavas, lumps of vesicular slag and pieces of pumice. Here and there much calcite is diffused through the matrix in strings, veins and patches, which enclose the lapilli. The agglomerate north of Lendalfoot possesses a greenish, somewhat serpentinous matrix, through which immense numbers of tabular felspar crystals are scattered. Similar crystals also occur abundantly in embedded blocks ofone of the purplish diabase-porphyrites, which occurs in mass on the shore and inland, and closely resembles the rock of Carnethy in the Old Red Sandstone volcanic series of the Pentland Hills.

Yet another and very distinct type of agglomerate is to be seen on the Mains Hill south-east of Ballantrae. It is a coarse rock, enclosing blocks up to a yard or more in diameter, of a fine compact purplish porphyrite, with large crystals of plagioclase and smaller ones of augite. In some places immense numbers of the small lapilli in the matrix consist of an extremely fine vesicular pumice. Small perfect and larger broken crystals of augite are likewise abundant in some of the greenish, more basic parts of the mass. These greenish serpentinous parts and the numerous augite crystals point to the explosion of some tolerably basic pyroxenic lava. A similar dark green, almost black, rock, with augite crystals, which sometimes measure a quarter of an inch in diameter, occurs near Sanquhar in Nithsdale. It presents a close resemblance to the agglomerate of Rhobell Fawr, already alluded to. So far as these Scottish agglomerates have yet been microscopically examined, they have been found to be composed of crystals, crystal-fragments, and lapilli derived partly from lavas similar to those above described, and partly from felsitic and other rocks which have not yet been observed here in the form of lavas.

The finer tuffs show likewise a considerable range of composition. According to Mr. Peach's observations along the south-eastern parts of the volcanic area, the ejected materials have consisted largely of fine dust (probably in great measure felsitic), which towards the north-east is gradually interleaved with ordinary sediment till the ashy character disappears. As I have already remarked, there is reason to believe that the overlying greywackes and shales derived part of their material either directly from volcanic explosions or from the attrition of banks of lavas and tuffs exposed to denudation.

But besides the interstratified lavas and fragmental rocks there occur numerous intrusive masses which are so intimately associated with the volcanic series that they may with little hesitation be regarded as forming part of it. They consist of various gabbros and serpentines, which are especially developed where the volcanic series comes out in greatest force in the south-west of Ayrshire. They also include more acid intrusions which, as in the case of the rock of Byne Hill, near Girvan, even assume the characters of granite.

The dying out of the volcanic material towards the north-east probably indicates that the vents of the period lay rather in the central or south-western parts of the district. Unfortunately, the limited extent of the exposures of the rocks makes it a hopeless task to search for traces of these vents over by far the largest part of the area. There are two localities, however, where the search may be made with better prospect of success. One of these is a tract to the north of Sanquhar in Nithsdale, which still requires to be studied in detail with reference to the sequence and structure of its volcanic rocks. The other area is that south-westernpart of Ayrshire which has been already cited as displaying so large a development of the volcanic series. Here the coast-sections reveal the intercalation of fossiliferous bands which show the true stratigraphical horizons of the lavas and tuffs. Under Bennane Head, Professor Lapworth some years ago found, in certain hardened black shales, a group of graptolites which mark an undoubted Arenig platform.[174]Recently the ground has been carefully re-examined by Messrs Peach and Horne, who have detected a number of other fossiliferous zones which confirm and extend previous observations. They have also been able to unravel the complicated structure of the volcanic series, and to represent it on the 6-inch maps of the Geological Survey, of which a reduction on the scale of 1 inch to a mile is now in course of preparation. The following tabular summary, taken partly from notes made by myself during a series of traverses of the ground with Mr. Peach when the revision was begun, and partly from memoranda supplied by that geologist himself, may suffice as a general outline of the volcanic history of this exceedingly interesting and important region.

[174]Geol. Mag.1889, p. 22.

It will be noticed from this table that the bottom of the volcanic series is not reached, so that no estimate can be formed of its full thickness, nor on what geological platform it begins. Possibly its visible portions represent merely the closing scenes of a long volcanic history, which, over the area of the south of Scotland, extended into Cambrian time, like the contemporary series of Cader Idris.

Among the lowest lavas there are interstratified courses of fine tuffs,flinty shales and thin limestones, which sometimes fill in the hollows between the pillow-like blocks above referred to. Among the characteristic Lower Arenig graptolites of these intercalated layers areTetragraptus bryonoides,T. fruticosus,T. quadribrachiatus, andT. Headitogether withCaryocaris Wrightii. Considerable variation is to be seen in the development of the upper part of the volcanic series. In some places the lavas ascend almost to the top; in others, thick masses of breccia or agglomerate take their place. These fragmentary materials are locally developed round particular centres, which probably lie near the sites of active vents whence large quantities of pyroclastic material were discharged. One of the volcanic centres must have been situated close to the position of Knockdolian Hill already referred to. The exceedingly coarse breccia of that eminence is rudely stratified in alternations of coarser and finer material, which was probably to some extent assorted under water around the cinder-cone that discharged it. The date of the explosions of this hill has been ascertained by Mr. Peach from the intercalation of black shales containing Arenig graptolites among the breccias. Another vent lay somewhere in the immediate neighbourhood of the Mains Hill agglomerate, if not actually on part of the site of that rock. Though probably not more than a mile from the Knockdolian volcano, and belonging to the same epoch of eruption, this vent, to judge from the peculiarities of its ejected material, must have been quite distinct in its source. A third vent lay somewhere in the immediate vicinity of Bennane Head, and threw out the extraordinary masses of agglomerate and the sheets of lava seen on the coast at that locality. A fourth may be traced by its separate group of fine tuffs on the coast three miles south of Ballantrae.

Fig. 53.—Section across the Lower Silurian volcanic series in the south of Ayrshire (B. N. Peach).B, Interstratified lavas in Arenig group;t, tuffs;r, radolarian cherts;b2, Llandeilo group;b3Caradoc group. Σ, Serpentine. G, Gabbro.

A feature of singular interest in the material erupted from these various centres of activity consists in the evidence that the explosions occurred at intervals during the deposition of the Lower Silurian formations, and that these formations were successively disrupted by submarine explosions. Mr. Peach has found, for example, abundant pieces of the peculiar and easily recognized radiolarian cherts imbedded in the volcanic series. That these cherts were deposited contemporaneously with the volcanic eruptions is proved by their intercalation among the breccias. Yet among these very breccias lie abundant fragments of chert which must have already solidified before disruption. It is thus evident that this siliceous ooze not only accumulated but set into solid stone on the sea-floor, between periods ofvolcanic outburst, and that such an occurrence took place several times in succession over the same area.

These facts derive further interest from the organic origin of the chert. It is now some years since Mr. Peach and his colleagues observed that between the Glenkiln Shale with its Upper Llandeilo graptolites and the top of the volcanic group in the central part of the Silurian uplands, alternations of green, grey or red shaly mudstones and flinty greywackes are interleaved with fine tuffs, and are specially marked by the occurrence in them of nodules and bands of black, grey and reddish chert. This latter substance, on being submitted to Dr. Hinde, was found by him to yield twenty-three new species of Radiolaria belonging to twelve genera, of which half are new. It thus appears that during the volcanic activity there must have been intervals of such quiescence, and such slow, tranquil sedimentation in clear, perhaps moderately deep water, that a true radiolarian ooze gathered over the sea-bottom.[175]

[175]Ann. Mag. Nat. Hist.(1890), 6th ser. vi. p. 40.

That the deposition of this ooze probably occupied a prolonged lapse of time seems clearly indicated by the evidence of the fossils that occur below and above the cherts. The graptolites underneath indicate a horizon in the Middle Arenig group, those overlying the cherts are unmistakably Upper Llandeilo. Thus the great depth of strata which elsewhere constitute the Upper Arenig and Lower and Middle Llandeilo subdivisions is here represented by only some 60 or 70 feet of radiolarian cherts. These fine siliceous, organic sediments probably accumulated with extreme slowness in a sea of some depth and over a part of the sea-floor which lay outside the area of the transport and deposit of the land-derived sediment of the time.[176]

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