WICKLOW GEOLOGY
Wicklow is not less remarkable for the variety and importance of its minerals than for the wild and picturesque beauties of its scenery; it comprises the greater portion of the south-eastern mountain chain of Ireland, composed of formations of granite, mica slate, quartz rock, clay-slate, grauwacke, trap, and porphyry. Nearly the whole of the most elevated and wildest part of the mountain range, in a line from north-east to south-west, is composed of granite, which supports, in geological position, all the other beds, and occupies a tract which, to the north of Lugnaquilla, is about seven miles in breadth; but to the south-west of it, where it descends towards the plains of Carlow, it is greatly expanded.
The granite is in general remarkably pure. The size of the grain varies much; some of the largest and most beautifully grained is found at the Scalp and in Glencree; the finest-grained, at the northern foot of Cadeen, in the glen of Imale. It is sometimes porphyritic, as in Glenismaule, Glencree, and the head of the waterfall is Glenmacanass. Numerous other minerals are found imbedded in the granite, and in the veins of quartz that sometimes traverse it, but so small in quantity as to be considered merely adventitious.
The mica slate occurs in direct contact with the granite range on each side, and is found in an uninterrupted range along its eastern border from Shillelagh, by Glenmalur, Glendalough, and Luggelaw, to the Scalp, where it is seen distinctly resting on the granite, as in many other places. It is usually fantastically contorted, on a small scale, and of a dark grey hue; and consists of alternate layers of quartz and mica of various thickness: in some places strata of quartz and of granite, and irregular masses of the latter are imbedded in it. In the lower part of Glenmacanass it contains a bed of talc slate, easily worked with the chisel, and hardening in the fire; which qualities fit it for chimney-pieces, hearth-stones, gravestones, and troughs. Lugnaquilla, though composed chiefly of granite, is capped with mica slate, with some alternating strata of granite.
On the western side of the granite range is a similarly incumbent series of mica slate strata, extending no farther south than Baltinglass; nor is it so regular and continuous in its range from the point where it enters from the county of Dublin, north-east of Blessington. Although the glen of Imale is entirely based on granite, this slate is seen forming the summits of many of the high surrounding mountains on the north, east, and south. Brisselstown hill, and its lateral extension to the west, called Spynan's hill, consist of mica slate, fine and minute granular greenstone, and greenstone porphyry: the mica slate in the western part is porphyritic, containing numerous crystals of felspar; and similar translations, as also into greenstone porphyry by an intimate intermixture of hornblende, are observed in various surrounding localities.
Garnet, in general so constant a companion of mica slate, is seldom seen in the strata of this county, but hollow spar occurs in some places. The low range of hills west of Blessington, and the rest of the northwestern border of the county, are based on clay-slate.
On the eastern side of the county, between the mica slate range and the sea, the prevailing rock is clay-slate, but in detached situations are found granite rising from beneath it, and quartz and trap rocks associated with it. The granite of this tract is very remarkable, as seldom comprising quartz; the chief ingredients being simply felspar and mica, forming in one part pure felspar porphyry. The central and southeastern parts of Dunganstown hill are composed of greenstone; but the prevailing rocks to the south are clay-slate and quartz, extending down the Avonmore and Ovoca, and the varieties which they display are Very numerous.
The varieties of clay-slate, which are here all quartzose, abound in contemporaneous veins of pure quartz, which are more or less metalliferous: the western extremity and the brow of Croghan Kinshela mountain consist of granite, with broad veins of quartz towards the east, succeeded by alternations of granite and clay-slate, terminating in interstratifications of clay-slate and greenstone, beyond which is found only the clay-slate, traversed by veins of quartz, sometimes metalliferous. Beds of granular felspar in the prevailing clay-slate are worked for building on the right bank of the Avonmore, north-west of Rathdrum.
Bordering on the Derry or Aughrim river, and likewise near the Ovoca, in its course from Newbridge, are numerous beds of greenstone. Arklow rocks, on the coast, south of the mouth of the Ovoca, present ill-defined columns of greenstone, with four, five, or six sides: the northern part of the hill consists in general of greenstone: on the north-western side is a variety of the character of basalt. Quartz rock forms a prominent naked ridge on Coollattin hill, in Shillelagh, and constitutes also a very extensive mountain range from the banks of the Avonmore above Rathdrum to those of the Vartrey, comprising the high naked ridge of Carrickmacreilly and the picturesque rock of Cronroe.
In the northern extremity of the county it forms the Great and Little Sugar-loaf, Bray Head, and a great part of the neighbouring hills. In no part of the county have organic remains been found in its rocks. It is also remarkable that there is a total absence of metallic ores on the western side of the great granitic mass, while on the eastern they are found in abundance. A vein of lead has been worked and apparently exhausted in the granite brow of Carrigeenduff, on the banks of Lough Dan; another, called the Luganure vein, wholly in granite, intersects the mountain of Comaderry, and is now very productive.
Another great vein which has been worked crosses the upper part of Glendalough; and in the alternating beds of granite and mica slate on the northern side of Glenmalur is the great vein on which are the lead mines of Ballinafinchogue, and which comprises, besides, galena, white lead ore, blende, and copper pyrites. The above minerals are found at all these places, in true veins; but in the only other metalliferous tract, situated in the clay-slate district, they are found only in beds, in contemporaneous veins, or in alluvial deposits. This tract is about ten miles in length, from Croghan Kinshela, across the northern end of the vale of Ovoca, towards Rathdrum. Its most celebrated produce has been the alluvial gold, found in the gravelly deposits of the streams descending from the eastern side of Croghan Kinshela, and discovered in 1796: of this a further notice will be found under the head of Arklow, in the union of which place it is included.
As no trace of auriferous veins could be found in the mountain by the most persevering efforts, the works necessarily ceased when the stream ore was exhausted. Trials were also made in Croghan Moira mountain, but without effect. Metallic substances, however, are diffused through the whole district in disseminated particles, in slight layers, in contemporaneous veins and strings, and in massy beds, which latter are principally composed of copper pyrites and iron pyrites. The rocks have been perforated in various directions by the works of the associated Irish Mine Company, the line of which, extending into Connery and Tigrony hills, occupies more than one thousand fathoms. These are on the north side of the Ovoca, and there are other productive works on the opposite side, especially in Ballymurtagh.
In Kilcashel some trials have been made, and copper-ore has been met with; and indications of copper in Avondale, and of lead in Knockanode, have also been found in the form of slight strings. The abundance of building stone in every part of the county appears from the previous detail: the granite used in the building of the Bank of Ireland, the library of Trinity College, Nelson's Pillar, and several other of the public buildings of Dublin, was raised from the Golden quarry near Blessington, but the clay-slate is seldom found in layers sufficiently thin for roofing; there are, however, good slate quarries in the parishes of Carnew and Dunganstown.
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