THE BOTANIST IN IRELAND (1)by Robert Lloyd Praeger D.Sc.
Introduction and Preface; 1-26: Geography, Topology, Climate; 27-47: General history of the flora ;
48-69: Vegetational sub-divisions; 70-78: Human Influence and Botanical workers;
79-219: Some Interesting Plants;
The figure shows the order in which the various parts of Ireland are dealt with in the Topographical Part. The sequence is an attempt to group together areas of floristic affinity, and may be summarized as follows, the numbers indicating the sections in the letterpress devoted to each area :–
Dublin (227 – 237), Central Plain, SE portion (238 – 259), SE Coastal Region (260 – 289), South – west (290 – 332), Western Limestones (333 – 375), Central Plain, western portion (353 – 382), Galway – Mayo Metamorphic Area (384 – 415), North – west (416439), Donegal (440 – 452), Basaltic Plateau (455 – 470), NE Silurian Region (472 – 485), Central Plain, NE portion (485 – 491).
1. Ireland presents several features which give it a special interest for the botanist. It is the most westerly portion of Eurasian land, extending to 10° 29' W longitude. Portugal and N Spain come next, falling short by some 65 miles of the Kerry headlands. But this approximation in longitude does not find a parallel in the general character of its vegetation. Ireland, being separated by two sea-channels from the Continent, suffers all the botanical penalties of insularity, in the form of a flora considerably reduced when compared even with England, and more when contrasted with the Peninsula. And again, the prevalence in Ireland of W and SW winds, blowing in off the warm Atlantic, results in a climate of extraordinarily small annual range in temperature, and of high humidity (22-26), unlike the comparatively Continental climate of the greater part of Spain and Portugal. The flora more nearly akin to that which prevails over the greater part of Ireland will be found by moving eastward into N England, and thence across Holland into N Germany, though the more Continental climate encountered as one goes E introduces increasing divergence in the vegetation as compared with this country. A NE course across Scotland into Scandinavia is productive of a greater amount of diversity, though it tends to retain in the flora a number of species which are lost along the eastern line.
2. The island itself is unusual in several respects: notably in having the central portion occupied by a low plain of limestone (10, 240-5) – the largest continuous area of Carboniferous limestone in Europe – of only some 200 ft. average elevation, while mountains, of 1500 to 3000 ft. in height, occupy much of the margin, and only occasionally rise far from the sea. The surface, from sea-level to the tops of the hills, is occupied by peat-bog to an extent not found elsewhere in Europe (28, 52). The most marked characters of the Irish flora are its reduction due to insularity, and the presence, due to mildness of climate, of southern plants, which have in past times crept along the warm coast from the Mediterranean and the Peninsula, and which are not found so far north anywhere else (35, 37). A less conspicuous but more startling feature is the presence of some North American plants, which find their European headquarters in Ireland, and die out in west Scotland and the NW parts of the Continent (35, 37). All of these features, it is important to note, characterize the fauna also (37).
GEOLOGICAL HISTORY IN RELATION TO PRESENT SURFACE
4. That the present surface is so varied in its Ethology is due to several causes, the most important of which is the frequent fluctuations of level to which it has been subject. Again and again this area has sunk below the sea, and beds of mud, sand, &c. (the detritus of adjoining lands), now hardened into slate or shale, limestone or sandstone, have been laid down upon it. Again and again the surface has risen above the sea, and its burden of deposits has been exposed to the forces of denudation – rain, frost, sun, wind – and in part or whole destroyed and carried by rivers back into the ocean. An equally potent factor has been lateral earth-movements to which the crust has been subjected at intervals, causing extensive crumpling of the rocks. These foldings, by elevating certain regions, mostly in the form of ridges, have exposed them to especially severe denudation, resulting in the laying bare of underlying deposits; by depressing other areas, the beds occupying them have been protected; the great pressure produced has often melted the rocks partly or even completely, and entirely altered their character; and the weakening of the Earth's crust resulting from these movements has sometimes allowed molten material to rise from below into the arching folds, where it has solidified as granite, or to break through and pour as lava over the surface of the country. The result of the interplay of all these natural forces is that the present surface of much of Ireland resembles a patchwork quilt in which a score or more of patches, many of them frequently recurring in varying size and shape, have each a quite different tale to tell. Even in the great limestone plain, which is the largest continuous area of any one kind of rock occurring in the country, there is no point from which a walk of twenty miles in a chosen direction will not bring one to some area large or small occupied by a different geological formation. The surface rocks of Ireland, then, are extremely varied, and represent an immensely long geological history – running back, it may be, a hundred million years or more. And though this history as told by Irish rocks is very fragmentary, there are few complete gaps of any magnitude, save when we come to those comparatively late formations which are well developed, for instance, in SE England, the product of submergences which Ireland escaped.
5. The oldest rocks which now lie exposed anywhere on the surface of Ireland occupy extensive areas in the N and W, and are of late pre-Cambrian age, belonging to the series known as Dalradian. In the N they form an almost continuous mass extending from Lough Neagh and Lough Foyle westward and south-westward to the sea at Donegal Bay. To their presence is due most of the mountainous country which occupies a great deal of the counties of Londonderry, Tyrone, and Donegal. The rocks consist of gneisses, schists, greenstones, quartzites, pebble-beds, slates, and intrusive granites, of very unequal hardness. Their strongly folded character, and the heavy and long-continued denudation, both aerial and marine, which they have suffered have produced, alike inland and along the coast, highly picturesque features. In the W, the great Galway – Mayo buttress (384), which protects the limestones of the Central Plain from the fury of the Atlantic, is formed mainly of rocks of the same age and of the same types. These ancient rocks, folded and metamorphosed, uplifted and denuded, are connected with much of the finest scenery in Ireland. They support a calcifuge vegetation, of grassy or more usually heathy type. Bands of primitive limestone and other basic rocks occur, but not in quantity sufficient to modify appreciably the flora; one such band, however, is responsible for the appearance of the Maidenhair (Adiantum capillus-veneris) in its only Connemara station (to a similar cause is probably also due the presence of this fern in Achill and on Slieve League in Donegal (see 106, 193), though this has been a subject of controversy). A small area of these Dalradian rocks S of Fair Head in Antrim (460) produces a change of coastal scenery, and causes the flora to lose some of the calcicole elements which characterize the surrounding basalts.
6. The succeeding Cambrian period has left but small impress on the Ireland of to-day – merely a few patches of slates and quartzites along the SE coast, but these are of local importance, as they form those two bold sentinels of Dublin Bay, the high promontory of Howth (231) on the N, and on the S the precipitous Bray Head (267) with the Little and Great Sugarloaf (1660 ft.) behind it. The heathery upland of Howth, formerly an island but now joined to the limestone mainland by a Neolithic raised beach, is classic ground for the botanist and furnishes the most varied flora of any area of its size in Ireland.
7. Next in order of age among Irish strata come the Ordovician and Gotlandian rocks, representing the Silurian Period (in the wide sense of the term). Like the older beds just mentioned, they were long buried under subsequent deposits of various ages, but these have been removed to a considerable extent by denudation, so that the slaty rocks which are so characteristic of the Silurian system now occupy two rather large tracts in E Ireland as well as a number of smaller areas. In the N, they cover a wedge-shaped area extending from Belfast Lough S-W to the Shannon, and back to Drogheda. They form the low rolling lands of Down, and much of Louth, Armagh, Monaghan, Cavan, and Longford. Disintegration yields a light friable soil, which even on the mountains supports a grassy rather than a heathery vegetation. This is an undulating fertile area, with more tillage than is found in most parts of Ireland, and many small lakes set among the fields; it is occupied by a characteristic calcifuge vegetation (472). South of Dublin, Ordovician strata lap up against both flanks of the granite core of the Leinster Chain, attaining a considerable development on the E side, in Wicklow and Wexford; much of the most beautiful scenery of Wicklow has been carved out of these rocks (261). In the southern midlands, on many of the ridges which rise out of the Limestone Plain, rain and frost have removed the Carboniferous strata from the summits, exposing the core of Gotlandian slates and Devonian conglomerates, elsewhere buried deeply. This is the characteristic structure of all the south-central hill – massesSlieve Bloom, Devil's Bit, Keeper, Slieve Bernagh, Slieve Aughty, Slievenaman, the Galtees, Knockmealdowns, and Comeraghs. The change of rock thus produced, from limestone to sandstone and slate, is accompanied by a marked change of flora, from calcicole to calcifuge.
8. At the close of Silurian times, extensive earth-movements began, resulting in crumplings of the crust which threw the Silurian and older rocks into a series of folds running NE from Ireland towards the Arctic Circle. (Long previously, Irish rocks had been subjected to intensive folding (Huronian) with a similar trend, as may be traced in western Scotland; but in Ireland this phenomenon has been obscured by the superimposed folds of the Caledonian series, with which we are now dealing.) The results of these stresses and of subsequent uplift are seen in the uplands which characterize not only Donegal and Mayo, Wicklow, and the area around Newry, but the Highlands of Scotland and the great raised mass of Scandinavia. In Mayo, Galway, and Donegal, denudation has laid bare over wide areas the ancient pre-Cambrian strata already referred to (5), which now form a heathery mountainous surface. In the Leinster Chain, weathering has revealed a massive core of granite, which rose in a molten mass under the Silurian rocks as they became arched by lateral pressure, so that we have in Wicklow a broad heathery granite range lapped round by the denuded edges of the former overlying Ordovician dome. The uniform nature of the granite has caused it to wear down into rather similar rounded hills, now deeply peat-covered; while the slates which encircle them present a more diversified surface, with picturesque glens and a less uniform flora.
9. The Devonian Period, which was ushered in by these mountain-building movements, has left a series of thick beds of conglomerate, slate and sandstone which are a conspicuous feature of S Ireland, occupying large areas of Waterford, Cork, and Kerry. These rocks were formed not in ancient seas, like most of those with which we have been dealing, but in large lakes and deserts. Extensive “inliers” of the same rocks, resulting from denudation of upthrusted hills, have been already referred to (7) as occurring over the southern part of the great limestone area of central Ireland (to be dealt with immediately); in Tyrone Devonian rocks cover an area of several hundred square miles stretching NW from Enniskillen. The soils derived from these rocks usually take the form of red clays; their flora is distinctly calcifuge.
10. We now approach that period – the Carboniferous – which has left an impress on Ireland and its flora greater than any other of the many epochs of geological history. A large area of Europe, including that part which we now call Ireland, sank below the sea, and, remaining submerged for a very long time, received on its surface first beds of mud (now hardened into slate), then thick deposits of a very limy nature composed largely of the shells of marine animals (now limestone); next, as the sea-bottom rose, layers of sand and mud (now appearing as sandstones and shales) and finally other deposits (the well-known Coal-measures) laid down under swamp conditions, where beds of river-mud and sand alternated with layers of vegetable matter, the debris of ancient forests, now forming strata of shale, sandstone, and coal. Elevation continued, and, to premise, the “Ireland” thus produced has in the main remained a land area throughout almost the whole of the immense period intervening between Carboniferous times and our own day. As a consequence, this great series of strata has been exposed almost continually to the agents of denudation, and frost, wind, and water have played havoc with it. The first beds to go were the uppermost, the soft Coal-measures, and with them perished the possibility of Ireland's ever becoming a great manufacturing country. The largest area of these deposits remaining lies in N Kerry (330) and the adjoining part of Limerick (332), but here they do not contain coal of workable amount. Outlying patches extend northward into S Clare (340). Another relic of these once wide-spread deposits forms the Leinster coalfield (255) in Co. Kilkenny, now a plateau of about a hundred square miles, lying between the ancient granite chain of Wicklow and the newer fold of Slievc Bloom. In this area a small amount of coal is mined, but its rural character remains unaffected. The shales weather down into a heavy clayey soil, supporting a rather monotonous and uninteresting rushy calcifuge flora. Smaller areas of Coal-measures lying on the hills around Lough Allen (428) in Leitrim and at Coalisland in Tyrone need only be mentioned here.
Next, the underlying series of sandstones and shales, constituting the Millstone Grit, Yoredale and Pendleside beds, were attacked and to a great extent removed. In the areas mentioned they crop out from under the remaining Coal-measures, and extend to a considerable degree the area of the calcifuge flora.
Over the wide tracts in the Central Plain from which the Coal-measures and the beds just referred to have been removed, the Carboniferous limestone now extends. Ever since it was laid down, earth-movements have not disturbed, save to a small amount, its horizontal beds. The limestone now forms a slightly undulating plain stretching from Dublin to Galway and far to the N and S. In these latter directions its edges get more and more broken up by patches of older or newer rocks, but outlying fragments of the limestone reach Cork on the S and Donegal in the N. The most characteristic part of the plain lies between two east-and-west lines drawn respectively from Dublin to Galway Bay and from Skerries to Westport, an area of close on 4,000 square miles. Across this one may pass from the Irish Sea to the Atlantic without rising more than 300 ft. (Only occasionally, indeed, as in Sligo and N Clare, does the limestone form hills of any noticeable height.) Heavily covered with drift in the E, it is occupied there by rich pasture with many trees; in the midlands it is often marshy, with great swelling bogs; while in the W, the rock often lies quite bare (240-245).
Since the presence or absence of lime in the soil determines more than any other chemical factor the nature of the vegetation which the soil supports, this mass of limestone constitutes from a botanical standpoint the most important geological feature of the country. Its floral characters are dealt with later (41, 59).
11. After the end of Carboniferous times, an important epoch of earth-movement, known as the Hercynian, resulted, as at the close of the Silurian Period, in a general uplifting and folding of the Irish area. These foldings ran normally E and W, and in southern Ireland have given us the striking scenery of Kerry and Cork, and the bold ranges of the Galtees and Knockmeeldowns a little to the N. Further N the folding was deflected by the older Caledonian axes such as the Leinster Chain, so that Slieve Bloom and other hill-ranges formed at this time have a NE and SW trend. In Cork and Kerry, denudation of the folded Carboniferous and Devonian rocks has left a series of high ribs of slate and sandstone, while the more soluble limestone has been preserved only in the intervening valleys. In Slieve Bloom and Devil's Bit we have ridges formed of the older rocks (Devonian and Ordovician) from which the limestone covering has been denuded. In the Leinster coalfield adjoining we see how a syncline or down-fold has preserved to us the beds which lay over the limestone.
With the close of Carboniferous times both the sedimentation and the folding which have produced the Ireland of to-day had to a great degree been accomplished, and during almost the whole of the time which has intervened Ireland has remained above the sea, and at the mercy of the forces of denudation, which have worn down mountain and plain alike, exposing older and older rocks.
The Permian period, which followed the Carboniferous. has left scarcely a trace in Ireland; only a few very limited patches of limestone, etc., in the N, supply evidence of a local submergence.
12. The whole of the Secondary (Mesozoic) and Tertiary (Cainozoic) epochs, during which the SE half of the England of to-day was built up of bed after bed of marine sediments, has left an imprint only in one corner of Ireland, which, like Scotland and Wales, has remained essentially a Palæozoic area. But that corner, the NE (455), continues for us the geological history. There we still have, very locally developed, the red sandstones and gypsum-bearing marls of the Trias, the fossiliferous Rhætic, the grey shales and clays of the Lias, the glauconitic rocks and white Chalk of the Cretaceous. The Antrim Chalk differs from that of southern England in being a much harder rock, and it forms nothing approaching the English Chalk Downs, with their interesting and peculiar flora. There is reason to believe that the Chalk sea extended far over Ireland, but its deposits are now limited to Antrim and the immediate vicinity. Even there, it and the other Mesozoic strata would probably have perished completely owing to long-continued sub-aerial waste, had it not been for the intervention of volcanic outbursts, which poured lavas over the old Chalk land, and thus preserved it; in Eocene times this corner of Ireland was involved in eruptions which buried the Mesozoic rocks under hundreds of feet of dark basalt, forming a high plateau. This plateau, though broken-backed and much denuded, still persists, a peat-covered moorland with a rugged scarped edge which overhangs Belfast and extends round the coast to Lough Foyle; and from under it the buried Secondary rocks peep out. The basalt weathers into a deep heavy soil rich in calcium carbonate, and supports a varied flora which includes a number of plants elsewhere in Ireland found only on the limestone (455). The same convulsions caused depressions of the surface which produced the deep hollow, now mostly filled with clay, in which Lough Neagh (463), the largest lake in the British Isles, is situated. Near by, in the S part of the adjoining county of Down, a great mass of granite rose under the Silurian rocks, to form the Mourne Mountains (479).
13. The next event of importance (after a long interval), and one which profoundly affected the whole country, especially as regards its biological history, was the oncoming of the Pleistocene glacial period. Ice, extending from the Poles, accumulated over the area till, at the time of maximum glaciation, almost the whole country was deeply buried. Once or more than once the ice-front retreated and advanced again. When the land was at last freed, its surface was greatly altered. Hills had been lowered, valleys filled up or deepened, and a vast amount of detritus, including the pre-existing soft covering of the rocks, redistributed over the country. In some places limy gravels were left spread over areas of acid rocks, or lime-free material over calcareous rocks. The sands and gravels which had previously accumulated on the bed of the Irish Sea, full of marine shells, were gouged out by a great glacier coming down channel from Scotland, and plastered up against the basalts of the NE, the Cambrian rocks of Howth, the granites of the Dublin Mountains, and pushed over much of the low grounds. The relation between the “solid geology” and the “drift geology” was no longer direct, especially along the boundary-lines of the former, a matter of importance to the botanist in Ireland, where the leading feature of the flora is the contrast between the limy and non-limy areas: between a calcicole and a calcifuge flora. The greater part of the lowlands was left covered by a thick deposit of tough clay, mostly full of ice-scratched stones; and in the final stages of retreat the ice left behind, upon the surface of these clays, great dumps and ridges of gravel stretching especially across the middle of Ireland. The face of the country to-day bears everywhere the impress of this period of glacial erosion and deposition.
14. When the climate returned towards the conditions at present prevailing, a period rather colder and wetter than to-day favoured the growth and accumulation of peat, which attained so wide a development that it forms still one of the most conspicuous features of the surface of the country. In Neolithic times, a depression and subsequent re-elevation of the northern half of Ireland left beds of gravel (raised beaches) fringing the sea, and in the estuaries flat areas of low land formed of muds which accumulated in shallow water. The final act of the drama of our country's evolution has been something more than a mere geological episode. In the latest changes that have taken place in the Irish area civilised man has played the most important part. As a result of his agricultural and other activities native woodlands, formerly extensive, have diminished, lakes and marshes have been drained, bogs cut away for fuel, and the land tessellated by a multitude of fences and hedges, and traversed by a network of roads, canals and railways.
It will be seen that the present surface of Ireland is the result of a very long and varied history. Persistent emergence from the sea following periods of depression, and long-maintained land-surfaces, have allowed of very heavy denudation, so that a mere patchwork of rocks remains, fragments of once wide-spread formations. The ancient Caledonian and Hercynian periods of stress, which by some chance affected the margins more than the centre of our island, impressed themselves so profoundly that the sculpture of the whole country is still dominated by the foldings which they produced.
The ultimate product of the action and interaction of these forces is the Ireland of to-day.
Consult G. A. J. COLE and T. HALLISSY: “Handbook of the Geology of Ireland.” London: Murby, 1924. 8s. 6d. Also section “Geology” in the series of provincial geographies (Cambridge University Press) referred to in 3.
15. Ireland is separated from Great Britain by a channel of very variable width, the narrowest points being 134 miles between Antrim and Kintire, 234 miles between Down and Wigton, and 47 miles between Wexford and S Wales. The depth of this channel along its centre varies form 45 to 150 fathoms; the deepest and coldest water found along the coast lies off the Antrim shore. This sea-barrier may be contrasted with that which exists between England and France, which narrows to a width of 23 miles and a depth of 20-25 fathoms at the Straits of Dover, widening rapidly northwards into the broad but shallow North Sea, and more gradually southward along the deeper St. George's Channel. It is important to bear in mind that all these waters lie on a broad shelf, which surrounds Great Britain and Ireland (fig. 1). Along the coasts of Scandinavia, of Spain and Portugal, and of Africa the sea-bottom falls rapidly to oceanic depths – 1000 to 2000 fathoms. But if we trace the isobaths of 100 or 200 fathoms northward from the Peninsula, we find they project in a broad curve which passes some 40 miles westward of Ireland, to approach the Continent again on the Norwegian coast. Ireland, like Britain, is essentially a portion of European land, and a slight uplift would re-join both to the Continent.
16. Ireland is an island roughly elliptical in outline, with a longer axis (NNE and SSW) of about 300 miles, and a shorter axis (WNW and ESE) of about 185 miles, and an area of 32,524 square miles. Its limits of latitude are 51° 26' N and 55° 23' N (about the same as Holland, N Germany and Poland), and of longitude 5° 26' W and 10° 29' W. It is on the whole a very flat island, saucer-shaped owing to its marginal mountain-ranges, and devoid of any distinguishable median watershed (fig. 2). The Limestone Plain stretches across the centre from horizon to horizon, encumbered with great swelling peat-bogs and embosoming many extensive lakes – a feature unlike anything found in Great Britain. The mountain regions, on the other hand, often recall those of Wales or the Lake District or W Scotland, the most unusual Irish feature being the cliff-walled limestone plateau of the Ben Bulben region in Sligo and the basaltic plateau of Antrim; also the scarped mountains in the W, which drop into the Atlantic in grand precipices, like Brandon in Kerry, Croaghaun on Achill Island, and Slieve League in Donegal.
17. The marginal position of most of the high grounds affects the topography in many ways. In most islands a central backbone, more or less pronounced, causes the rivers to flow radially towards the sea, from near the centre to the margin. In Ireland the streams that rise on the seaward side of the mountain masses have short and mostly steep courses, while those whose sources lie on the inland side travel far before they reach an outlet. Most of the larger Irish rivers – Blackwater, Suir, Nore, Barrow, Liffey, Bann, Erne, Shannon – rise from 10 to 60 miles inland, but none of them flows towards the nearest sea; after devious courses of 100 to 200 miles, they debouch at quite other points (fig. 3). The Shannon, longest river in the British-Irish area, with a course of 214 miles, rising on the hills near Lough Allen 25 miles from Donegal Bay, flows S from that lake for 130 miles with a fall of only 51 feet, forming at intervals great lake-like expanses. Then it cuts through hills of Old Red Sandstone and Silurian rocks, and drops 97 ft. in 18 miles to reach tidal waters at Limerick. Thence it continues its course for 55 miles more through a broadening submerged valley to meet the Atlantic at Loop Head. The course of this river, and those of the Barrow, Nore, and Suir in the SE, give testimony of the great denudation to which the Carboniferous limestone of the Central Plain has been subjected, for after descending to the plain from their mountain sources they flow not towards what are now the lowest areas, but towards the southern hills of sandstone and slate, and cut through them in deep gorges which they have worn in these resistant rocks as the surface of the Limestone Plain was being lowered by denudation.
Fig. 3. – River – Basins.
18. The southern rivers, Blackwater, Lee, and Bandon, with marked W–E courses changing abruptly to S near their mouths, furnish evidence of having undergone considerable natural development since they originated as “consequent” streams. In the early stages of their history they flowed S along the slope of the uplifted peneplain in channels unrelated to the geological sub-structure. But soon after, the drainage became complicated by the development of “subsequent” tributary streams. E-and-W tributaries, working along the strike of the softer Carboniferous rocks which, in consequence of folding and subsequent denudation, alternate with the more resistant beds of Old Red Sandstone, rapidly deepened their channels and eventually became predominant. Only in their lower reaches have the streams managed to retain their original N–S channels across the anticlinal ridges. In Ulster, Lough Neagh and the present course of the Bann owe their origin to a collapse caused by the outpourings of basalt in that region. Further W, the Erne has a singularly sluggish course – a mere succession of island-filled lakes – till at Belleek it plunges over a lip of limestone and rushes down to meet the sea below Ballyshannon. In the E, the Liffey, rising on the inland side of the Leinster Chain, performs a semicircular course of 72 miles before it enters the sea at Dublin, only 14 miles from its source. Among the larger 'streams, only the Boyne, Lagan, and Foyle have courses that can be described as normal.
19. The Erne is not alone among Irish rivers in being, characterized by lake-like expansions. Owing to the soluble nature of the limestone, several of those flowing in the Central Plain show this peculiarity, the lakes being often of considerable size. Thus we have Lough Ree and Lough Derg on the Shannon, and Loughs Carra, Mask, and Corrib on the river Corrib. The quite different origin of Lough Neagh in the north has been referred to already (12). That lake covers an area of no less than 153 square miles, and has a very uniform depth of 50 ft. or less. Around its margin, especially in the SW, a broad band of freshwater clays of great depth testifies to a considerable former extension of its waters and to a long-continued subsidence of its floor.
20. The coast of Ireland in the N, W, and S is in general bold, deeply indented by drowned river-valleys and broader bays, and the influence of the Atlantic everywhere makes itself felt. In Donegal, Mayo, and Galway the ancient folded pre-Cambrian rocks produce an exceedingly broken coast-line, sometimes low, sometimes projecting in bold headlands and magnificent cliff-ranges, everywhere so exposed that the maritime flora is much reduced, though its vertical limit is greatly increased. Wherever, on the other hand, limestone prevails, the land is usually low, and the sea has penetrated deeply, as in Donegal Bay, Sligo Bay, Killala Bay, Clew Bay, and Galway Bay. In Clare the horizontal shales and sandstones of the Upper Carboniferous still withstand the onslaught of the Atlantic and form precipices which in the Cliffs of Moher rise vertically to a height of over 650 ft. In Kerry and W Cork the coast attains its boldest development. Here the great ribs of Old Red Sandstone project far into the ocean, the summits of the mountains often continuing seaward as high craggy islets. Between these Devonian buttresses the erosion of the weaker Carboniferous rocks of the troughs, coupled with submergence, has allowed the sea to flow far into the former river-valleys in deep wedge-shaped bays, producing very lovely scenery. Throughout E Cork and Waterford a less magnificent but high and broken coast-line prevails, with inlets at the river-mouths due to sunken valleys. In the E, and there alone, the coast is `usually low. The shore of Wexford and Wicklow is characterized by gravel beaches and stretches of sand, where the flora belonging to such ground attains its maximum for Ireland, and several species have their only station (270-9). In Dublin and Louth also sandy beaches prevail, broken by occasional rocky headlands. The coast of Down is low, with alternating stretches of sand and of jagged slaty rock. N of Belfast Lough (the drowned valley of the Lagan) the basaltic plateau of the NE presents to the sea a lofty and picturesque scarp.
21. As compared with the greater part of Europe, the most striking feature of Ireland (which it shares with Scotland, Wales, and Scandinavia) is the great age of the main features of the country and of the rocks which produce them. This is especially true of most of Ireland's mountain-ranges. Compared with the heathery granite domes of Wicklow and the ridges and peaks of Donegal, Connemara, and Kerry, the Mourne Mountains are happen ings of yesterday: yet the rising of the Mournes heralded the great and long-continued earth-movements from which sprang the Pyrenees, the Alps, the wide Carpathian ring, the Balkans, the snowy Caucasus, and the lordly Himalayas themselves.
22. Ireland lies in about the same latitude as Berlin, Kamtschatka, Alberta, and Labrador. A comparison with the climatic conditions prevailing in any of these areas shows the extraordinary effect of the warm seas and sea-winds which envelop Ireland. The mean annual temperature over the whole island is above 50° F. (10° C.). The annual range is singularly small. The warmest area (the centre and SE) in July (the hottest month) has a mean of barely over 60° F. (15.5° C.), and the coldest area (the centre and NE) in January (the coldest month), a little under 40° F. (4.4° C.) (fig. 4). This may be compared with the 23° F. average range prevailing in England, itself possessing an insular climate in contrast to Continental countries. Most marked are the winter temperatures in the W. The isotherms there, running parallel with the coast, show a January mean of 40° to 42° F. in Donegal, 42° to 43° F. in the Mayo – Galway projection, and over 44° F. in SW Kerry and W Cork – warmer than Bordeaux or Rome. The January isotherm of 42° F. (5.5° C.), which passes through the whole length of W Ireland from N to S, fringes the Welsh coast, cuts across Devon, runs along the W and S coasts of France, and thence through N Italy and Greece. The July isotherm of 59° F. (15° C.) runs likewise across “W Ireland from S to N; then along the Cheviots, up the backbone of Scandinavia, and across the head of the White Sea into Siberia. Frost is rare in the W, and almost unknown on some of the islands. These high winter temperatures, combined with great humidity all the year round, have a marked effect on the vegetation, particularly in the W and S.
23. But there is also a high degree of exposure. Winter on the W coast might be described as a succession of westerly gales with westerly winds between. The influence of these conditions on the vegetation is clearly seen. In exposed ground, as in S Clare, not a native tree is to be found for miles inland; and everywhere in the western district (as indeed more or less over the whole a Ireland) the bending of trees towards the E is very noticeable (Plate 1 and fig. 17). On the western islands the vegetation is often extremely wind-shorn. On Achill, Eryngium nmritimum, with spreading rhizomes an inches roots 3 to 4 ft. long, produces annual shoots from 3 to 4 inches high. On Inishkea, Daucus Carota grows quite stemless, with a sessile umbel nestling among a rosette of leaves (fig. 5 see right). See also the description of Plantago sward (63) for an example of dwarfing by exposure. On the other hand, the bitter easterly winds which retard the spring flora along the Irish Sea are scarcely felt on the W coast, and exercise no influence on the vegetation. But even in the Dublin area, Cochlearia danica and Ranunculus Lenormandi commence flowering in January the latter at 1000 ft. elevation; and on a midwinter walk, blossoms of 30 to 50 wild flowers may be seen, except after an unusual spell of frost.
24. Rainfall in Ireland increases from E to W. Along the E coast generally it corresponds to that prevailing on the British side of the Irish Sea. Along the line of the Shannon (roughly) it has risen from about 30 to about 40 inches (say 750-1000 mm.); and thence to the W coast it rises to 70 to 90 inches (1750-2250 mm.), while in the wettest spots in the western mountains (e.g., Galway, Kerry) precipitation as high as 100 inches (2500 mm.) may occur (fig. 6). The average for sixteen years at Kylemore in Connemara is given as 81.79 inches. The S end of the Upper Lake at Killarney, 75 ft. above sea-level, showed an average of 87.36 inches for 17 years. The station at 1760 ft. on Mangerton in Kerry gave an average for 15 years of 97.40 inches – minimum 63 inches in 1887, maximum 1409 inches in 1903. But it is the high humidity prevailing in the W, and large amount of showery or drizzly weather, rather than heavy downpours, which produce the almost tropical luxuriance which characterizes the vegetation of sheltered spots, especially in Kerry.
The constancy of moist atmosphere on the W coast may be judged from the fact that, on the north side of Slievemore on Achill (408) Hymenophyllum peltatum forms an abundant ingredient of the sward covering the slope, growing fully exposed and open to the sky, among Sphagnum and stunted Calluna; while Nymphacea alba grows hard by in a marsh as a terrestrial plant, with short-stalked aerial leaves. The perennial nature of the precipitation in Ireland is seen in the rarity of parched vegetation during summer, causing that intense greenness which astonishes the American or S Continental visitor. In a normal season parching is confined to wall-tops, dry gravels, and places where the rock is a very few inches below the surface. On a well-drained hill-top in Westmeath in August of an unusually dry summer, it was noticed that rank pasture on loamy soil gave no indication of the presence of foundations and slabs of stone buried six inches below the surface – suggesting that in Ireland aerial survey will not prove an efficient means of discovering buried monuments, as it has in drier countries.
Fig.6. – Rainfall.
25. The general effect of climate and soils in Ireland is to render the country specially suitable for pasture. The forests of oak which once covered much of its surface have under the influence of man given way, not to tillage, but to grass-lands, often very rich, which, apart from mountain grazing, occupy more than half of the entire area of the country (fig. 10). An important effect of the exposure is a reduction of the elevation up to which agricultural operations can be carried on. While in sheltered inland districts tillage may range as high as 1000 ft., in W Mayo and W Galway its limit does not exceed 400 ft., and is usually only about 200 ft.; on sheltered slopes in the E it may ascend to 1200 ft.
26. While the exposure has a dwarfing effect on vegetation, and on trees in particular, the mild, moist climate, and also the rich, warm soil that prevails on the limestone, produce a remarkable luxuriance of vegetation where shelter from wind is afforded. Thus, in Burren (346), Asplenium marinum may be seen with fronds 3 ft. in length; Ceterach, exuberantly crenated (fig. 16), attains a length of 1 ft.; Adiantum, nearly 2 ft. On Aran (352), Allium babingtonii attains a height of 6 ft. On the islands in Lough Erne (435), Solidago virgaurea grows 4 ft. in height, Campanula rotundifolia 3 ft., and Polygala vulgaris forms clumps with a hundred upright stems a foot high. On islands in L. Mask (371) Campanula rotundifolia grows 3 ft. 3 in. high, and Orchis maculata attains a height of 3 ft. By Lough Gill (420), Digitalis and Heracleum have been measured nearly 10 ft. high, Agrimonia odorata 7 ft., Coeloglossum viride 14 in.; and at Portlaw Polypodium vulgare on a wall with fronds over 2 ft. 3 in. long. H. C. Hart records from the banks of the Suir Apium nodiflorum 6 ft. high with leaves 3 ft. long, and Oenanthe crocata 7 ft. high. In the Killarney woods Hymenophyllum tunbridgense forms on rocks sheets of greenery up to 50 sq. ft. in area.
Another effect of the moist conditions is the abundance of epiphytes in sheltered places in the W, unexpected plants like Saxifraga spathularis (umbrosa auct.) perching on boughs 20 or 30 ft. above the ground (Blackwater near Kenmare). In more than one place in Kerry Scrophularia aquatica may be seen growing 6 to 8 ft. high on the top of 10ft. walls.
Introduction and Preface 1-26: Geography, Topology, Climate 27-47: General history of the flora
48-69: Vegetational sub-divisions 70-78: Human Influence and Botanical workers
79-219: Some Interesting Plants