The Physical Geology and Geography of Ireland. ... With ... maps and ... engravings

Front matter

London- Edward Stanford. 264 27 Cockspnr S£. Charing Cross. S.W THE PHYSICAL GEOLOGY & GEOGRAPHY OP IKELAND BY EDWAED HULL, M.A., LL.D., F.E.S. LATE DUIECTOR OF THE GEOLOGICAL SURVEY OP IRELAND AND PROFESSOR OF GEOLOGY IX THE ROYAL COLLEGE OF SCIENCE, DUBLIN EXAMINER IN GEOLOaV TO THE UNIVERSITY OF LONDON SECOND EDITION, REVISED WITH TWO COLOURED MAPS AND TWENTY-NINE ILLUSTRATIONS LONDON: EDWARD STANFORD 26 & 27 COCKSPUR STREET, CHARING CROSS, S.W DUBLIN: HODGES, FIGGIS, & CO. 1891 All rights reserved

Preface

TO THE SECOND EDITION By a happy coincidence I am able to bring out a new edition of this treatise concurrently with the completion of the Geological Survey of the country with which it deals, and thus to present the reader with the results of the most recent researches regarding its geological structure. While engaged in revising or adding to these pages, I have been often surprised to find how much there has been to modify or add, owing to the progress of our knowledge during the past twelve years, consequent on the fuller investigation of several important phenomena connected with the physical structure of the country carried on during the progress of the A Preface IV National Survey with which I had the honour of being connected. The more important of these additions to our knowledge may be summed up under the following heads : 1.

The determination of the occurrence of Archæan rocks in certain districts of the west and north of Ireland. 2. The determination of the peculiar relations subsisting between the Lower Devonian (or Devono- Silurian) strata and the Upper Old Red Sandstone and Carboniferous series of the southern districts. 3. Additional evidence regarding the relative ages of the trachytic and basaltic lavas of Antrim. 4. Evidence of the invasion of Ulster by a great ice-sheet from the Grampian Mountains of Scotland, during the earliest stage of the Glacial Period. The above are the principal points connected with the geology of Ireland upon which fresh evidence has come to light since the publication of this treatise ; and they will probably be recognised as of great interest, not only to Irish naturalists, but to all students of the geology of the British Isles.

For we cannot forget that Ireland is an island amongst a group of islands, and that its Preface V geological history is closely bound up with that of the other members of the group ; so that, in order to a full comprehension of its physical history, the subject must be studied in connection with that of the sister islands. This is a point of view which I have endeavoured to keep before my own mind when preparing this treatise, and to place before the mind of the reader. E. H. INTEODUCTOEY Ireland has been designated by a distinguished German naturalist ' The Land of the Giant Stag and the Giant's Causeway,' ' thus graphically naming the natural peculiarities for which this country is widely known amongst foreigners and students of Natural History. For, although the remains of the Megaceros are by no means exclusively restricted to Ireland, but occur at intervals over England and Western Europe, they have been found in such pro fusion in some parts of this country that the specific name Hibernicus 2 has been proposed by Professor Owen as pointing to the region of the ' Giant Stag's ' 1 Dr. Ferdinand Roemer commences a short sketch of his tour throughout Ireland in 1876 thus : — ' Ich war in diesem Herbst im Lande des Riesendamms und der Riesenhirsche.

Schon lange hatte ich gewunscht, die griine Insel kennen zu lernen.' Neu. Jahrbuch fiir Min., Geol. u. Palaon. (1877.) 1 Palæontology, 2nd edit. p. 405. Introductory VIII favourite haunts, and where it was free to roam unmolested by many of the fierce carnivores which infested the lands on the opposite side of St. George's Channel. Those, however, who have studied the physical history of this country are well aware that it is full of interest to the student of Nature, and that it offers many special problems for solution. The relative ages and mode of formation of its mountains, the origin of its Central Plain, of its river valleys, and of its numerous lakes ; its volcanic phenomena, the evidences of extensive glaciation at a former period exhibited by its rock-surfaces, its eskers and extensive moorlands — all these will repay careful study, and will form the subject of the fol lowing pages.

In order to lay a solid foundation for the more recent phenomena to be treated of, it will be advis able first to present a brief sketch of the structural geology of the country. Fortunately, geological science is becoming so generally a subject of study that it is likely to be ere long the property of the many, rather than, as at present, of the few ; and along with the extension of knowledge of its general Introductory IX principles there is being diffused sounder views regarding the mode in which the outward configuration of the earth's surface has been brought about. Thanks to the labours of the modern school of geologists, the worthy disciples of Playfair and Hutton (if not of still older philosophers), it is generally recognised that the features of the landscape are due to the action of water, under its varied forms and modes of working, whether as rain, rivers, torrents, ocean-waves, snow and ice, operating on rocks of varying composition, degrees of hardness, and variety of structure.

We now know that the form of the loftiest mountain equally with that of the slightest eminence, the extent of the widest plain equally with that of the narrowest gorge, the rugged coastline with its bold headlands and deep indentations, or the featureless shore which descends almost imperceptibly into the sea — all owe their existence to the great sculptor, Water ; — slowly, almost imperceptibly, working on the rocks which have been placed within its reach, and where they have been elevated into dry land by the action of those terrestrial forces which, from the earliest times in the world's history, have been converting land into sea, and sea into land. x Introductory Upon such principles I shall endeavour to place before the reader in a connected form the origin of those features of the landscape which have always made Ireland so attractive to the traveller from other lands, and have endeared her soil to her own people. PAGE CONTENTS

Part I Geological Formations Of Ireland Chapter

I palæozoic formations Comparative Table of Formations of England and Wales and of Ireland — Irish geologists — Archæan Rocks — Cambrian Rooks — Lower Silurian Rocks — Two types of Lower Silurian strata — Unaltered and Metamorphosed — West Galway district — Donegal and Derry district— Geological Age of the Donegal Rocks — Fossils — Upper Silurian Beds — Devono- Silurian and Lower Devonian Beds of Dingle — The Reeks and Killarney Mountains — Upper Old Red Sandstone and Kiltorcan Beds — Carboniferous series — Lower Carboniferous volcanic rocks — Yoredale Beds and Millstone Grit — Gannister Beds — Middle Coal-Measures — The Tyrone and Leinster coal-series — Terrestrial movements at the close of the Carboniferous Period — Permian Beds — Boulder Beds of Armagh 1

Chapter II Mesozoic Formations

New Red Sandstone and Marl — Rhætic and Liassic Beds — Cretaceous Beds — The Chalk Contents XII

Chapter III Tertiary, Or Cainozoic, Formations Page

Miocene Volcanic Rocks — The Basaltic Plateau of Antrim — Trachyte Porphyry — Pisolitic iron-ore and plant-beds — Latest stage of volcanic period — Volcanic Necks and Basaltic Dykes — Relative ages of the Trachyte and plateau basalt — Pliocene Clays of Lough Neagh .... 81

Chapter III Details Of Movement Of The Great Ice-sheet

Invasion of Ulster by ice from the Grampians — District north of the Central Snow-field — District south of the Central Snow-field — Ice-movement in the Shannon Valley — In the Eastern districts — In Wicklow — About Carlingford and the Mourne Mountains — Over Antrim— In Kerry — Southern limit of the Ice-sheet 268 CHAPTER TV DEPTH OF THE ICE-SHEET . . . Contents XVI

Chapter VI Extinct Animals Of Ireland

The Mammoth — Hippopotamus — Ancient Ox — Red Deer — Reindeer — Great Irish Deer — Wolf — Bear — List of Prehistoric Mammalia of Ireland 301 MAPS Sketch Geological Map or Ireland . . . Frontispiece A Map or the General Glaciation or Ireland To face p. PAET I GEOLOGICAL FORMATIONS OF IRELAND

Chapter I

PALpEOZOIC formations The formations present in Ireland belong chiefly to the oldest and newest periods of geological time as represented in the British Isles. The Cambrian, Silurian, the Old Red Sandstone,1 and the Carboni ferous formations are largely developed, as are also the Cretaceous, Miocene, Pliocene, and Post-pliocene in the north-eastern portion of the country; but between these two groups there occurs a wide gap in the series of formations — the Permian, Triassic, and Liassic strata being only sparingly represented, and the whole of the Jurassic (or Oolitic) series being without any representatives whatever. This absence 1 The Old Red Sandstone is the freshwater, or lacustrine, representative of the Devonian marine formation. In Ireland the formation occurs under the former type.

B Ireland 2 of so large a portion of the Mesozoic ' strata, which are to be found occupying an important position in the geological structure of England, is probably due to absence of deposition rather than to denudation, by which term is meant the sweeping away of strata which had once been formed. We may, therefore, suppose that at the close of the Carboniferous period the Irish area, with the exception of the north-east (and that only to a small extent), was elevated into dry land, during which time the sea, under whose waters the Mesozoic strata were deposited, overspread the central portions of Eng land. In order to bring the representative series of formations on both sides of St. George's Channel more clearly before the mind of the reader, and to in dicate their analogies and differences, the following table of strata has been drawn up.

This may also be useful for reference in future portions of this work in case the reader's acquaintance with geological terms should happen to be in any degree defective : a contingency which, I trust, is exceedingly improbable : Or ' Secondary.' Table of Formations 3 TABLE OF FORMATIONS. England and Wales. Ireland. Mesozoic Series. Oajnozoic. Post-pliocene, or Drift deposits. Post-pliocene, or Drift de posits. Pliocene. Pliocene Clays of L. Neagb. Miocene (Bovey Tracey beds). Miocene (volcanic). Eocene beds. (Absent.) Upper Cretaceous beds. Upper Cretaceous beds Lower ,, „ \ Neocomian beds. Wealden beds. Purbeck beds. L (Absent.) Oolitic or Jurassic group. ( Upper. Liassic group -j Middle. / (Lower and Rhætic. Lower and Rhætic (in part i. Trin«ip e-ronn J KeuPer Marls. Keuper Marls (in N.E. only). j.nabbic group | Bunter Sandstone.

Buuter Sandstone „ / Permian beds. Permian beds (sparingly). (Upper. Upper(inpait). Carboniferous group -j Middle. Carboniferous i Middle. J [ Lower. ( Lower. (Upper (Up. O. R. Sandst. Old Red Sandstone or Devonian Middle. gw ,Rted I (Middle absent.) I £ower Sandstone j Lower, or Dingle V "I beds (in part i. / Devono-Silurian (passage beds). Devono.Silurian ' Dingle beds ■ vlJ » (in part). / Upper, resting unconformably ( Upper, unconform 1 Silurian j Loweri with -Arenig beds ' at Silurian s Lower,°Caradoc and [ base. I. Llandeilo beds.' ( Upper, Lingnla, Tremadoc, &c. ) ,TT , Cambrian J Lower. Barmouth and Llan- '- ' upper absent. ) ( beris beds. 1 Lmver- Howth alld B™? Heai1- Archæan (Gneissose rocks, Anglesea, &c.) ' Gneissose rocks ; Galway. Tyrone. <fcc. Palæozoic Serils. Lower. Upper.

With the above table before him, the reader will not have much difficulty in following me in the attempt to give a concise account of the formations, and their distribution in Ireland ; and having thus become acquainted with their petrographical characters, he will more easily understand the action of those causes which have operated in moulding their ' Though the existence of representatives of the ' Fundamental Gneiss,' or so-called Laurentian beds, is strongly advocated by Dr. Hicks and supported by others, their views are not as yet generally adopted. r Irelana 4 physical features and determining their mutual relations over the surface of the country. We shall, therefore, commence with the Archæan Rocks, which are the oldest, and follow the course of the geological record downwards ; noting those pages which are most fully recorded, as well as those which we can only fill up by having recourse to the geological details of other countries.

We shall then proceed to that part of our subject, which is really the more important from our present standpoint, and discuss the origin of the mountains, plains, river-valleys, and lakes. All these subjects, it is almost unnecessary to observe, have been discussed, in greater or less detail, by previous authors ; a list of whom, with references to their works, will be found at the end of this volume. To these I must refer the reader for fuller information than can be admitted into these pages. Irish geologists, though perhaps not so industrious with either the pen or the hammer as the materials which their country furnishes seem to demand, have by no means been wanting in efforts to elucidate the physical structure of their country. The pages of the Transactions and Journal of the Geological Society of London, and of the Geological Society of Dublin (now the Eoyal Geological Society of Ireland), bear witness to the labours of Berger, Portlock, Bryce, Oldham, Baily, Irish Geologists 5 Jukes, Haughton, Kinahan, Close, Harkness, and others ; while the explanatory memoirs which accom pany the maps of the Geological Survey, written by its officers, afford ample details of the geology of special districts.

But amongst the corps of ' knights of the hammer ' of which Ireland can boast, there is one whom all will allow to be facile princeps ; who, though amongst the early explorers of the geological treasures of his country, and the first to reduce to order and system its tangled skein of strata, has only passed away from amongst us within the last few years. Sir Richard Griffith will always be remembered amongst men of science as the first to construct a geological map of Ireland ; doing for this country what William Smith accomplished for England, and Macculloch and Boue for Scotland. Griffith's map,1 constructed with the help of the late Mr. Kelly and others, but chiefly by his own personal exertions, must be considered as remarkable for the accuracy with which the boundaries of the geological formations are delineated, the broad views it evinces in the grouping of the formations, and the effectiveness of its colouring.

It is a work which only a man of uncommon industry, of 1 Published by Hodges, Foster & Co., Dublin (1855). Jukes's ' Geological Map,' on a smaller scale (1867), is taken partly from Griffith's map, and from the maps, when published, of the Govern ment Survey. A new edition was issued in 1878. Irclana 6 great physical powers, and of high intellectual attainments could have accomplished. The author of the first geological map of Ireland has thus established his claim to be known amongst geolo gists in all time to come as ' the Eather of Irish geology.' » Archæan Rocks. — Recent investigations have conclusively established the existence, in several de tached areas, of representatives of these the most ancient of known rock-groups. The tracts alluded to are, as might be inferred, situated in the western and northern parts of the country where the crystalline and metamorphosed strata predominate.

The most important tract is probably that which lies along the northern shore of Galway Bay, and is bounded along the north by the quartzite range called the ' Twelve Bins of Connemara.' This wild tract of country, indented by numerous arms of the sea, full of lochlets, with rocky masses rising at intervals from beneath thick folds of peat and morass, bears a strong resemblance to some tracts in Sutherlandshire formed of rocks of the same age. In both districts, also, they are glaciated and boulder strewn, and are formed of similar materials consisting of beds of gneiss, composed of red orthoclase, 1 As William Smith, who constructed the first geological map of ngland, is generally known as ' the Father of English geology.' Archæan Rocks 7 often porphyritically developed, greenish or yellowish oligoclase, quartz, black, green, or white mica with accessory minerals.

These gneissose rocks pass into hornblendic and micaceous schists with veins of pegmatite ; and the whole series is bounded on the north by the newer quartzites, schists, and lime stones, presumed to be of Lower Silurian age, but under highly metamorphosed conditions.1 Other districts where Archæan rocks presumably occur are the Slieve Gamph and Ox Mountains on the borders of Mayo and Sligo ; the western and northern parts of Erris Head, and Belmullet, and the extreme western coast of Achill Island. In all these places the rocks are of similar character, consisting chiefly of gneiss with masses of schist, and in Achill overlain uncomformably by the newer schists with a conglomerate base. Finally, through the investigations of the Geological Survey it has been determined that the great group of metamorphosed sedimentary strata, forming nearly the whole area of Donegal and Derry, rests with its southern base on the flanks of a partially developed ridge of 1 The Archæan age of these rocks was originally pointed out by the Author in his paper, ' On the Laurentian Eocks of Donegal.' Scient.

Trans. Roy. Dublin Soe. Feb. 1882 ; and this view has been confirmed by Dr. A. Geikie in his paper on ' Recent Researches into the Origin and Age of the Highlands of Scotland and Ireland,' Proc. Roy. Inst. p. 528, 1889. Archæan gneissose rocks stretching from the banks of the Erne at Ballyshannon in a north-easterly direc tion towards Pomeroy. This mass is principally formed of hornblendic rocks and gneiss, and is overlain in its eastern part by a volcanic series con sisting of amygdaloidal basic lavas, tuffs, and coarse volcanic agglomerates, which gives place in turn to the chloritic schists forming the mountainous tracts of Derry and South Donegal, and which are presum ably Lower Silurian beds under altered conditions.1 Cambrian Rocks. — The beds which have been referred to this formation are only found along the eastern coast, and in detached localities ; namely, Howth Hill, on the north shore of Dublin Bay ; Bray Head, and the coast nearly as far as the town of Wick low ; and along the coast of Co. Wexford, from Cahore Point southwards.

They consist of green and purple grits, quartzites, and rough slates, which occur generally in a highly disturbed and broken position, and are of great, but unknown, thickness. Fine 1 Part of this area in Tyrone has been referred to by Dr. Hicks as one of his Pre-Cambrian areas, Proc. Geol. Assoc. vol. vii. No. i. p. 28. hee Expl. Mem. sheet 34 of the Geol. Survey, by J. Nolan (1878); also Dr. Geikie's paper above referred to, p. 544. At the time I wrote the paper on 'The Laurentian Rocks of Donegal,' I had arrived at the conclusion that the granitic and gneissic rocks of North-west Donegal were of Laurentian (or Archæan) age. This view, for which there is much conflicting evidence, I have been obliged to abandon. Cambrian Rocks 9 sections in these beds are laid open along the Dublin and Wicklow Railway, where it has been carried along the face of the steep and lofty cliffs of Bray Head, sometimes spanning chasms descending below the restless waters, at other times cutting deep into the projecting rocks, or traversing them in tunnels.

To the north of Dublin Bay, the old rocks are covered over on the land side by beds of Carbonifer ous limestone, which appear to have been deposited Fig. 1. w. Howth Hill. E. A, Cambrian quartzite and slate. B, Conglomerate at base of Carboniferous limestone (shore beds), c, Carb. limestone. D, Drift deposits. against their flanks, at that point forming a steeply shelving coast, or possibly sunken rocks iu the sea of the Carboniferous period (see fig. 1) ; but to the southwards, in the districts of Wicklow and Wex ford, they are overlaid unconformably by the Lower Silurian beds, which fold round to the eastward and occupy the coast-line southwards from Wicklow. The organic forms yielded by these beds are few, and of low types of animal life : they consist of the tracks of marine worms, and of a peculiar genus of zoophyte called after its discoverer,1 Oldhamia.

There are two species, 0. antiqua and 0. radiata. (Forbes.) The surface features of the Cambrian Rocks of Wicklow are generally bold and varied. The beds of quartzite run in rugged and high ridges, or form conical hills, such as the Sugar Loaf Mountains, which are essential features in the landscape of that beautiful county. Seen from the north — as, for example, from Killiney Hill — the quartzite ridge appears to trend from east to west, culminating in the stately cone of the Great Sugar Loaf, at an elevation of 1,659 feet, and abruptly terminated along the bold bluffs of Bray Head. From the base of this ridge the wide and richly wooded valley of the river Dargle slopes gently down to the shore of Killiney Bay, which sweeps inwards in a graceful curve, and terminates in low cliffs of gravel and clay belonging to the Post-pliocene period, with which the floor of the valley is thickly covered.

The view from this point has been compared to that of the 1 Dr. Oldham, F.R.S., late Superintendent of the Geological Survey of India. Prof. Ferdinand Roemer has expressed his opinion, fortified by microscopic study, that both species are in organic. Neu. Jahrb. Heft I. 1877. Professor Sollas has come to a similar conclusion, but Irish geologists will not readily admit this view of their origin. Lower Silurian Rocks 11 Bay of Naples, in which the cone of the Sugar Loaf represents that of Vesuvius. But the comparison is illusory or fanciful ; each possesses features peculiar to itself, and the comparison has probably had its origin in the popular notion that all conical moun tains are volcanic.1 Lower Silurian Rocks. — In Wales the Lower Cambrian beds, whose representatives in Ireland have just been described, are overlaid by a series of fossiliferous strata now generally denominated Upper Cambrian, including the ' Lingula Flags ' 2 and ' Tremadoc slates,' with a peculiar fauna consisting mainly of Brachiopods and Trilobites.

These beds, however, have not been discovered in Ireland, and there is every reason to suppose that they are unconformably overlapped, or concealed, along the margin of the Lower Cambrian beds, or have not been deposited over the Irish ai-ea.3 The Lower Silurian beds of this country are, therefore, the re- 1 Professor Geikie has shown, in his Scenery and Geology of Scotland, how frequently in Scotland formations of quartzite as sume the conical form — as in the case of Schehallion ; other ex amples will occur when we come to speak of the western districts. 2 The Lingula beds, with their peculiar forms of Trilobites and a few Brachiopods, were considered by Barrande and Murchison pro perly to form the base (zone primordiale) of the great Silurian series. Some authorities assume the overlying ' Arenig beds ' as the base ; it is possible these are represented in Ireland. 3 Prof. Jukes, Expl.

Mem., sheets 121 and 136, p. 11, and Ibid. sheets 102 and 112, pp. 22 and 42 {note), 2nd edit. 1875. presentatives of those in the British series known as the ' Landeilo ' and ' Caradoc ' (or Bala) beds, and they rest discordantly on the Lower Cambrians of the south-eastern coast, and then stretch away northwards, westwards, and southwards, forming the general floor upon which the Carboniferous and Devonian formations have been laid down. Two Types. — The Lower Silurian beds of Ireland occur under two types, petrologically different, though generally representative ; and as this is an essential feature in the structure of the country, and must always take an important place in any sketch of its physical history, it will be necessary to dwell upon it for a few moments, and to consider the formation under its two aspects.

If the reader will look at a map of the British Islands (a geological map is preferable), and draw a line from Galway Bay on the south-west to Belfast Lough on the north-east, and then continue it in the same direction through the entrance to the Firth of Clyde, and right across the central valley of Scotland to the Firth of Forth, he will find that (generally speaking) the mountainous regions of both countries lying to the north and west of this line consist of crystalline strata, while the less elevated tracts to the south and east of the line consist of Silurian strata in their ordinary condition, Types of Lower Silurian Beds 13 and which are frequently stored with organic remains. In the case of Ireland, the former class of rocks (or the crystalline) form the mountainous districts of West Galway, North Mayo, the Highlands of Donegal and Derry ; and in that of Scotland, the central Highlands from the flanks of the Grampians northwards ; while the latter class (the ordinary fossiliferous beds) in the south of Ireland rise into groups of isolated mountains such as Slieve Boughta, the Devil's Bit, Galtymore, and Slieve Bloom ; and further north into the rugged uplands of Monaghan, Armagh and Down, represented in the south of Scotland by the Southern Uplands.

The phenomena of both countries are strictly analogous, and admit of a similar explanation. The geological age of the beds south and east of the line I have indicated has been clearly established, both by their fossil contents and geological position : it is recognised that they belong to the great Lower Silurian series of Murchison. But on passing north and west of the line, which is generally obscured by Carboniferous strata, they undergo a singular transformation. In Scotland these beds are concealed under those of Car boniferous age of the central valley, and in Ireland they also pass under a tract of Carboniferous limestone and other beds newer than themselves ; but on emerging in both countries, and rising into the mountainous regions which characterise the northern tracts of the British Isles, they are found to occur in the form of crystalline schist, quartzite, gneiss, and granitic rocks, all of which are included by geologists under the general term of ' Metamorphic' ' (1) Unaltered Silurian Beds. — The Lower Silurian rocks in their normal condition, to the south of the line where metamorphism sets in, consist in the lower part of dark schists with Graptolites, cleaved and interstratified with lenticular bands of grit, felstone, porphyry, and volcanic ash, the whole representing the ' Landeilo Beds' of North Wales; and in the upper, of grey and greenish slates, grits, and fine conglomerates, in some places calcareous and fossiliferous, and in a few localities containing beds of limestone, as on the coast near Donabate, and at the Chair of Kildare.

These are the oldest fossiliferous limestones in Ireland, and may be con sidered as the representatives of the celebrated ' Bala Limestone ' group of North Wales. 1 The metamorphism of strata is doubtless due to the action of intense heat, in the presence of aqueous vapour or superheated steam, accompanied by pressure and shearing. It has taken place at considerable depths below the surface ; and thus shales and slates have been converted into mica schist, hornblende schist, &c. ; grits and shales into schist and gneiss ; sandstone into quartz schist and quartzite ; and beds of augitic lava into diorite and epi diorite. These are largely developed in Donegal and. Mayo. See paper by Dr. J. S. Hyland on ' Epidiorites in N.W. Ireland,' Soien. Proc. Roy. llubl. Soe. Feb. 1890. Unaltered Silurian Beds 15 Throughout the north-eastern district from Lonoford to Down these strata are thrown into numerous flexures, and often reversed foldings, the axes of which run in E.N.E. and W.S.W. directions, corresponding to the trend of these beds in the Southern Uplands of Scotland.1 In the districts north of Dundalk Bay, and flanking the shores of Carlingford Lough, they have been penetrated by granitic and plutonic rocks in great variety, forming the Mourne and Carlingford Mountains, while along a line of country extending from Slieve Gullion to Slieve Croob they have been invaded by granite and subjected to intense metamorphism.

It is also through these rocks that the granite of the Dublin and Wicklow Mountains has been in truded, accompanied by eruptive outbursts, giving origin to dykes and granitic protrusions into the surrounding schists. All along the margin of the granite, the Silurian rocks are highly altered, being converted into micaceous schists, with chiastolite and other minerals which are frequently found under similar conditions. To the question of the 1 Some of these beds in Co. Down — e.g. Coalpit Bay, near Donaghadee, Tullygarvan, Carnalea, Ballygrot, and Craigavad — have yielded a large number of genera and species of Graptolites and other fossils, collected and described by Mr. W. Swanston, F.G.S., and Prof. C. Lapworth, F.G.S., in Proc. Belfast Nat. Field Club. Appendix, 1876-77. geological age and origin of the mountain ranges here described we shall return in a future page.

Another point of analogy between the Lower Silurian beds of Ireland and those of Wales is the occurrence of extensive sheets of felspathic trap, ashes, and agglomerates, which were ejected from submarine volcanoes at intervals during the deposition of the sedimentary strata. These beds of volcanic materials are to be found in the districts of Waterford, Wexford, Wicklow, and Louth. They consist of felstones, diorites, lampophyres, and el van ites,1 which are interbedded with the grits and slates of the formation ; and it is probable that the vents from which they were poured out were in active eruption at the time when the Old Silurian volcanoes of North Wales were evolving those great sheets of trap which rise into the grand ridges of Cader Idris, Aran Mowddwy, and the flanks of Snowdon. But while the original materials out of which the physical features of North Wales have been sculp tured are somewhat similar in both countries, the scenery of the unaltered Silurian districts of Ireland is seldom so striking and bold as in the former country.