Lakes

The lakes (called loughs--pronounced _lochs_) of Ireland are

innumerable, and (apart from their formation) are almost all contained

in two great regions, (1) The central plain by its nature abounds in

loughs--dark, peat-stained pools with low shores. The principal of

these lie in county Westmeath, such as Loughs Ennel, Owel and

Derravaragh, famed for their trout-fishing in the May-fly season. (2)

The Shannon, itself forming several large loughs, as Allen, Ree and

Derg; and the Erne, whose course lies almost wholly through

loughs--Gowna, Oughter and the Loughs Erne, irregular of outline and

studded with islands--separate this region from the principal

lake-region of Ireland, coincident with the province of Connaught. In

the north lie Loughs Melvin, close above Donegal Bay, and Gill near

Sligo, Lough Gara, draining to the Shannon, and Lough Conn near

Ballina (county Mayo), and in the south, the great expanses of Loughs

Mask and Corrib, joined by a subterranean channel. To the west of

these last, the mountains of Connemara and, to a more marked degree,

the narrow plain of bog-land between them and Galway Bay, are sown

with small lakes, nearly every hollow of this wild district being

filled with water. Apart from these two regions the loughs of Ireland

are few but noteworthy. In the south-west the lakes of Killarney are

widely famed for their exquisite scenic setting; in the north-east

Lough Neagh has no such claim, but is the largest lake in the British

Isles, while in the south-east there are small loughs in some of the

picturesque glens of county Wicklow.

_Climate._--The climate of Ireland is more equable than that of Great

Britain as regards both temperature and rainfall. No district in Ireland

has a rainfall so heavy as that of large portions of the Highlands of

Scotland, or so light as that of several large districts in the east of

Great Britain. In January the mean temperature scarcely falls below 40

deg. F. in any part of Ireland, whereas over the larger part of the

eastern slope of Great Britain it is some 3 deg. lower; and in July the

extremes in Ireland are 59 deg. in the north and 62 deg. in Kilkenny.

The range from north to south of Great Britain in the same month is some

10 deg., but the greater extent of latitude accounts only for a part of

this difference, which is mainly occasioned by the physical

configuration of the surface of Ireland in its relations to the

prevailing moist W.S.W. winds. Ireland presents to these winds no

unbroken mountain ridge running north and south, which would result in

two climates as distinct as those of the east and west of Ross-shire;

but it presents instead only a series of isolated groups, with the

result that it is only a few limited districts which enjoy climates

approaching in dryness the climates of the whole of the eastern side of

Great Britain. (O. J. R. H.)

_Geology._--Ireland, rising from shallow seas on the margin of the

submarine plateau of western Europe, records in its structure the

successive changes that the continent itself has undergone. The first

broad view of the country shows us a basin-shaped island consisting of

a central limestone plain surrounded by mountains; but the diverse

modes of origin of these mountains, and the differences in their

trend, suggest at once that they represent successive epochs of

disturbance. The north-west highlands of Donegal and the Ox Mountains,

with their axes of folding running north-east and south-west, invite

comparison with the great chain of Leinster, but also with the

Grampians and the backbone of Scandinavia. The ranges from Kerry to

Waterford, on the other hand, truncated by the sea at either end, are

clearly parts of an east and west system, the continuation of which

may be looked for in South Wales and Belgium. The hills of the

north-east are mainly the crests of lava-plateaux, which carry the

mind towards Skye and the volcanic province of the Faeroe Islands. The

two most important points of contrast between the geology of Ireland

and that of England are, firstly, the great exposure of Carboniferous

rocks in Ireland, Mesozoic strata being almost absent; and, secondly,

the presence of volcanic rocks in place of the marine Eocene of

England.

The fact that no Cambrian strata have been established by

palaeontological evidence in the west of Ireland has made it equally

difficult to establish any pre-Cambrian system. The great difference

in character, however, between the Silurian strata at Pomeroy in

county Tyrone and the adjacent metamorphic series makes it highly

probable that the latter masses are truly Archean. They form an

interesting and bleak moorland between Cookstown and Omagh, extending

north-eastward into Slieve Gallion in county Londonderry, and consist

fundamentally of mica-schist and gneiss, affected by earth-pressures,

and invaded by granite near Lough Fee. The axis along which they have

been elevated runs north-east and south-west, and on either flank a

series of "green rocks" appears, consisting of altered amygdaloidal

andesitic lavas, intrusive dolerites, coarse gabbros and diorites, and

at Beagh-beg and Creggan in central Tyrone ancient rhyolitic tuffs.

Red and grey cherts, which have not so far yielded undoubted organic

remains occur in this series, and it has in consequence been compared

with the Arenig rocks of southern Scotland. The granite invades this

"green-rock" series at Slieve Gallion and elsewhere, but is itself

pre-Devonian. Even if the volcanic and intrusive basic rocks prove to

be Ordovician (Lower Silurian), which is very doubtful, the

metamorphic series of the core is clearly distinct, and appears to be

"fundamental" so far as Ireland is concerned.

The other metamorphic areas of the north present even greater

difficulties, owing to the absence of any overlying strata older than

the Old Red Sandstone. Their rocks have been variously held to be

Archean, Cambrian and Silurian, and their general trend has

undoubtedly been determined by post-Silurian earth-movements. Hence it

is useful to speak of them merely as "Dalradian," a convenient term

invented by Sir A. Geikie for the metamorphic series of the old

kingdom of Dalriada. They come out as mica-schists under the

Carboniferous sandstones of northern Antrim, and disappear southward

under the basaltic plateaux. The red gneisses near Torr Head probably

represent intrusive granite; and this small north-eastern exposure is

representative of the Dalradian series which covers so wide a field

from central Londonderry to the coast of Donegal. The oldest rocks in

this large area are a stratified series of mica-schists, limestones

and quartzites, with numerous intrusive sheets of diorite, the whole

having been metamorphosed by pressure, with frequent overfolding.

Extensive subsequent metamorphism has been produced by the invasion of

great masses of granite. Similar rocks come up along the Ox Mountain

axis, and occupy the wild west of Mayo and Connemara. The quartzites

here form bare white cones and ridges, notably in Errigal and Aghla

Mt. in county Donegal, and in the group of the Twelve Bens in county

Galway.

Following on these rocks of unknown but obviously high antiquity, we

find fossiliferous Ordovician (Lower Silurian) strata near Killary

harbour on the west, graduating upwards into a complete Gotlandian

(Upper Silurian) system. Massive conglomerates occur in these series,

which are unconformable on the Dalradian rocks of Connemara. In the

Wenlock beds of the west of the Dingle promontory there are

contemporaneous tuffs and lavas. Here the Ludlow strata are followed

by a thick series of barren beds (the Dingle Beds), which have been

variously claimed as Upper Silurian and Lower Devonian. No certain

representative of the Dingle Beds has been traced elsewhere throughout

the south of Ireland, where the Old Red Sandstone succeeds the

uptilted Silurian strata with striking unconformity. The Silurian

rocks were indeed greatly folded before the Old Red Sandstone was laid

down, the general trend of the folds being from south-west to

north-east. The best example of these folds is the axis of Leinster,

its core being occupied by granite which is now exposed continuously

for 70 m., forming a moorland from Dublin to New Ross. On either flank

the Silurian shales, slates and sandstones, which are very rarely

fossiliferous, rise with steep dips. They are often contorted, and

near the contact with the granite pass into mica-schists and

quartzites. The foothills and lowlands throughout southern Wicklow and

almost the whole of Wexford, and the corresponding country of western

Wicklow and eastern Kildare, are thus formed of Silurian beds, in

which numerous contemporaneous and also intrusive igneous rocks are

intercalated, striking like the chain N.E. and S.W. In south-eastern

Wexford, in northern Wicklow (from Ashford to Bray), and in the

promontory of Howth on Dublin Bay, an apparently earlier series of

green and red slates and quartzites forms an important feature. The

quartzites, like those of the Dalradian series, weather out in cones,

such as the two Sugarloaves south of Bray, or in knob-set ridges, such

as the crest of Howth or Carrick Mt. in county Wicklow. The radial or

fan-shaped markings known as _Oldhamia_ were first detected in this

series, but are now known from Cambrian beds in other countries; in

default of other satisfactory fossils, the series of Bray and Howth

has long been held to be Cambrian.

All across Ireland, from the Ballyhoura Hills on the Cork border to

the southern shore of Belfast Lough, slaty and sandy Silurian beds

appear in the axes of the anticlinal folds, surrounded by Old Red

Sandstone scarps or Carboniferous Limestone lowlands. These Silurian

areas give rise to hummocky regions, where small hills abound, without

much relation to the trend of the axis of elevation. The most

important area appears north of the town of Longford, and extends

thence to the coast of Down. In Slieve Glah it reaches a height of

1057 ft. above the sea. Granite is exposed along its axis from near

Newry to Slieve Croob, and again appears at Crossdoney in county

Cavan. These occurrences of granite, with that of Leinster, in

connexion with the folding of the Silurian strata, make it highly

probable that many of the granites of the Dalradian areas, which have

a similar trend and which have invaded the schists so intimately as to

form with them a composite gneiss, date also from a post-Silurian

epoch of earth-movement. Certain western and northern granites are

however older, since granite boulders occur in Silurian conglomerates

derived from the Dalradian complex.

This group of N.E. and S.W. ridges and hollows, so conspicuous in the

present conformation of Donegal, Sligo and Mayo, in the axis of Newry,

and in the yet bolder Leinster Chain, was impressed upon the Irish

region at the close of Silurian times, and is clearly a part of the

"Caledonian" system of folds, which gave to Europe the guiding lines

of the Scottish Highlands and of Scandinavia.

[Illustration: Map of Ireland.]

On the land-surface thus formed the Devonian lakes gathered, while the

rivers poured into them enormous deposits of sand and conglomerate. A

large exposure of this Old Red Sandstone stretches from Enniskillen

to the Silurian beds at Pomeroy, and some contemporaneous andesites

are included, reminding us of the volcanic activity at the same epoch

in Scotland. The numerous "felstone" dikes, often lamprophyric,

occurring in the north and west of Ireland, are probably also of

Devonian age. The conglomerates appear at intervals through the

limestone covering of central Ireland, and usually weather out as

conspicuous scarps or "hog's-backs." The Slieve Bloom Mountains are

thus formed of a dome of Old Red Sandstone folded on a core of

unconformable Silurian strata; while in several cases the domes are

worn through, leaving rings of Old Red Sandstone hills, scarping

inwards towards broad exposures of Silurian shales. The Old Red

Sandstone is most fully manifest in the rocky or heather-clad ridges

that run from the west of Kerry to central Waterford, rising to 3414

ft. in Carrantuohill in Macgillicuddy's Reeks, and 3015 ft. in

Galtymore. In the Dingle Promontory the conglomerates of this period

rest with striking unconformity on the Dingle Beds and Upper Silurian

series. Here there may be a local break between Lower and Upper

Devonian strata. The highest beds of Old Red Sandstone type pass up

conformably in the south of Ireland into the Lower Carboniferous,

through the "Yellow Sandstone Series" and the "Coomhola Grits" above

it. The Yellow Sandstone contains _Archanodon_, the oldest known

fresh-water mollusc, and plant-remains; the Coomhola Grits are marine,

and are sometimes regarded as Carboniferous, sometimes as uppermost

Devonian.

[Illustration: Geological Map of Ireland.]

In the south, the Carboniferous deposits open with the Carboniferous

Slate, in the base of which the Coomhola Grits occur. Its lower part

represents the Lower Carboniferous Shales and Sandstones of the

central and northern areas, while its upper part corresponds with a

portion of the Carboniferous Limestone. The Carboniferous Limestone,

laid down in a sea which covered nearly the whole Irish area, appears

in the synclinal folds at Cork city and Kenmare, and is the prevalent

rock from the north side of the Knockmealdown Mountains to Enniskillen

and Donegal Bay. On the east it spreads to Drogheda and Dublin, and on

the west to the heart of Mayo and of Clare. Loughs Mask and Corrib are

thus bounded on the west by rugged Silurian and Dalradian highlands,

and on the east appear as mere water-filled hollows in the great

limestone plain.

The Lower Carboniferous Sandstones are conspicuous in the region from

Milltown near Inver Bay in southern Donegal to Ballycastle in county

Antrim. In the latter place they contain workable coal-seams. The

Carboniferous Limestone often contains black flint (chert), and at

some horizons conglomerates occur, the pebbles being derived from the

unconformable ridges of the "Caledonian" land. A black and often shaly

type called "calp" contains much clay derived from the same

land-surface. While the limestone has been mainly worn down to a

lowland, it forms fine scarps and table-lands in county Sligo and

other western regions. Subterranean rivers and water-worn caves

provide a special type of scenery below the surface. Contemporaneous

volcanic action is recorded by tuffs and lavas south-east of Limerick

and north of Philipstown. The beds above the limestone are shales and

sandstones, sometimes reaching the true Coal-Measures, but rarely

younger than the English Millstone Grit. They are well seen in the

high ground about Lough Allen, where the Shannon rises on them, round

the Castlecomer and Killenaule coalfields, and in a broad area from

the north of Clare to Killarney. Some coals occur in the Millstone

Grit horizons. The Upper Coal-Measures, as a rule, have been lost by

denudation, much of which occurred before Triassic times. South of the

line between Galway and Dublin the coal is anthracitic, while north of

this line it is bituminous. The northern coalfields are the L.

Carboniferous one at Ballycastle, the high outliers of Millstone Grit

and Coal-Measures round Lough Allen, and the Dungannon and Coalisland

field in county Tyrone. The last named is in part concealed by

Triassic strata. The only important occurrences of coal in the south

are in eastern Tipperary, near Killenaule, and in the Leinster

coalfield (counties Kilkenny and Carlow and Queen's County), where

there is a high synclinal field, including Lower and Middle

Coal-Measures, and resembling in structure the Forest of Dean area in

England.

The "Hercynian" earth-movements, which so profoundly affected

north-west and north-central Europe at the close of Carboniferous

times, gave rise to a series of east and west folds in the Irish

region. The Upper Carboniferous beds were thus lifted within easy

reach of denuding forces, while the Old Red Sandstone, and the

underlying "Caledonian" land-surface, were brought up from below in

the cores of domes and anticlines. In the south, even the

Carboniferous Limestone has been so far removed that it is found only

in the floors of the synclinals. The effect of the structure of these

folds on the courses of rivers in the south of Ireland is discussed in

the paragraphs dealing with the geology of county Cork. The present

central plain itself may be regarded as a vast shallow synclinal,

including a multitude of smaller folds. The earth-wrinkles of this

epoch were turned into a north-easterly direction by the pre-existing

Leinster Chain, and the trend of the anticlinal from Limerick to the

Slieve Bloom Mountains, and that of the synclinal of Millstone Grit

and Coal-Measures from Cashel through the Leinster coalfield, bear

witness to the resistance of this granite mass. The Triassic beds rest

on the various Carboniferous series in turn, indicating, as in

England, the amount of denudation that followed on the uplift of the

Hercynian land. Little encouragement can therefore be given in Ireland

to the popular belief in vast hidden coalfields.

The Permian sea has left traces at Holywood on Belfast Lough and near

Stewartstown in county Tyrone. Certain conglomeratic beds on which

Armagh is built are also believed to be of Permian age. The Triassic

sandstones and marls, with marine Rhaetic beds above, are preserved

mainly round the basaltic plateaus of the north-east, and extend for

some distance into county Down. An elongated outlier south of

Carrickmacross indicates their former presence over a much wider area.

Rock-salt occurs in these beds north of Carrickfergus.

The Jurassic system is represented in Ireland by the Lower Lias alone,

and it is probable that no marine beds higher than the Upper Lias were

deposited during this period. From Permian times onward, in fact, the

Irish area lay on the western margin of the seas that played so large

a part in determining the geology of Europe. The Lower Lias appears at

intervals under the scarp of the basaltic plateaus, and contributes,

as in Dorsetshire and Devonshire, to the formation of landslips along

the coast. The alteration of the fossiliferous Lias by dolerite at

Portrush into a flinty rock that looked like basalt served at one time

as a prop for the "Neptunist" theory of the origin of igneous rocks.

Denudation, consequent on the renewed uplift of the country, affected

the Jurassic beds until the middle of Cretaceous times. The sea then

returned, in the north-east at any rate, and the first Cretaceous

deposits indicate the nearness of a shore-line. Dark "green-sands,"

very rich in glauconite, are followed by yellow sandstones with some

flint. These two stages represent the Upper Greensand, or the sandy

type of the English Gault. Further sands represent the Cenomanian. The

Turonian is also sandy, but in most areas was not deposited, or has

been denuded away during a local uplift that preceded Senonian times.

The Senonian limestone itself, which rests in the extreme north on

Trias or even on the schists, is often conglomeratic and glauconitic

at the base, the pebbles being worn from the old metamorphic series.

The term "Hibernian Greensand" was used by Tate for all the beds below

the Senonian; the quarrymen know the conglomeratic Senonian as

"Mulatto-stone." The Senonian chalk, or "White Limestone," is hard,

with numerous bands of flint, and suffered from denudation in early

Eocene times. Probably its original thickness was not more than 150

ft., while now only from 40 to 100 ft. remain. This chalk appears to

underlie nearly the whole basaltic plateaus, appearing as a fringe

round them, and also in an inlier at Templepatrick. The western limit

was probably found in the edge of the old continental land in Donegal.

Chalk flints occur frequently in the surface-deposits of the south of

Ireland, associated with rocks brought from the north during the

glacial epoch, and probably also of northern origin. It is just

possible, however, that here and there the Cretaceous sea that spread

over Devonshire may have penetrated the Irish area.

After the Irish chalk had been worn into rolling downs, on which

flint-gravels gathered, the great epoch of volcanic activity opened,

which was destined to change the character of the whole north-west

European area. The critical time had arrived when the sea was to be

driven away eastward, while the immense ridges due to the "Alpine"

movements were about to emerge as the backbones of new continental

lands. Fissure after fissure, running with remarkable constancy N.W.

and S.E., broke through the region now occupied by the British Isles,

and basalt was pressed up along these cracks, forming thousands of

dikes, from the coast of Down to the Dalradian ridges of Donegal. One

of these on the north side of Lough Erne is 15 m. long. The more

deep-seated type of these rocks is seen in the olivine-gabbro mass of

Carlingford Mountain; but most of the igneous region became covered

with sheets of basaltic lava, which filled up the hollows of the

downs, baked the gravels into a layer of red flints, and built up,

pile upon pile, the great plateaus of the north. There was little

explosive action, and few of the volcanic vents can now be traced.

After a time, a quiet interval allowed of the formation of lakes, in

which red iron-ores were laid down. The plant-remains associated with

these beds form the only clue to the post-Cretaceous period in which

the volcanic epoch opened, and they have been placed by Mr Starkie

Gardner in recent years as early Eocene. During this time of

comparative rest, rhyolites were extruded locally in county Antrim;

and there is very strong evidence that the granite of the Mourne

Mountains, and that which cuts the Carlingford gabbro, were added at

the same time to the crust. The basalt again broke out, through dikes

that cut even the Mourne granite, and some of the best-known columnar

masses of lava overlie the red deposits of iron-ore and mark this

second basaltic epoch. The volcanic plateaus clearly at one time

extended far west and south of their present limits, and the

denudation of the lava-flows has allowed a large area of Mesozoic

strata also to disappear.

Volcanic activity may have extended into Miocene times; but the only

fossiliferous relics of Cainozoic periods later than the Eocene are

the pale clays and silicified lignites on the south shore of Lough

Neagh, and the shelly gravels of pre-glacial age in county Wexford.

Both these deposits may be Pliocene. Probably before this period the

movements of subsidence had set in which faulted the basalt plateaus,

lowered them to form the basin of Lough Neagh, and broke up the

continuity of the volcanic land of the North Atlantic area. As the

Atlantic spread into the valleys on the west of Ireland, forming the

well-known marine inlets, Europe grew, under the influence of the

"Alpine" movements, upon the east; and Ireland was caught in, as it

were, on the western edge of the new continent. It seems likely that

it was separated from the British region shortly before the glacial

epoch, and that some of the ice which then abutted on the country

travelled across shallow seas. The glacial deposits profoundly

modified the surface of the country, whether they resulted from the

melting of the ice-sheets of the time of maximum glaciation, or from

the movements of local glaciers. Boulder-clays and sands, and gravels

rearranged by water, occur throughout the lowlands; while the eskers

or "green hills," characteristic grass-covered ridges of gravel, rise

from the great plain, or run athwart valleys and over hill-sides,

marking the courses of sub-glacial streams. When the superficial

deposits are removed, the underlying rocks are found to be scored and

smoothed by ice-action, and whole mountain-sides in the south and west

have been similarly moulded during the Glacial epoch. In numerous

cases, lakelets have gathered under rocky cirques behind the terminal

moraines of the last surviving glaciers.

There is no doubt that at this epoch various movements of elevation

and subsidence affected the north-west of Europe, and modern Ireland

may have had extensions into warmer regions on the west and south,

while the area now left to us was almost buried under ice. In

post-Glacial times, a subsidence admitted the sea into the Lagan

valley and across the eastern shore in several places; but elevation,

in the days of early human occupation, brought these last marine

deposits to light, and raised the beaches and shore-terraces some 10

to 20 ft. along the coast. At Larne, Greenore and in the neck between

Howth and Dublin, these raised beaches remain conspicuous. To sum up,

then, while the main structural features of Ireland were impressed

upon her before the opening of the Mesozoic era, her present outline

and superficial contours date from an epoch of climatic and

geographical change which falls within the human period.

See maps and explanatory memoirs of the _Geological Survey of

Ireland_ (Dublin); G. Wilkinson, _Practical Geology and Ancient

Architecture of Ireland_ (London, 1845); R. Kane, _Industrial

Resources of Ireland_ (2nd ed., Dublin, 1845); G. H. Kinahan, _Manual

of the Geology of Ireland_ (London, 1878); E. Hull, _Physical Geology

and Geography of Ireland_ (2nd ed., London, 1891); G. H. Kinahan,

_Economic Geology of Ireland_ (Dublin, 1889); A. McHenry and W. W.

Watts, _Guide to the Collection of Rocks and Fossils, Geol. Survey of

Ireland_ (2nd ed., Dublin, 1898). (G. A. J. C.)