KILDARE GEOLOGY
Planting has been carried on for many years extensively and successfully. Many of the demesnes are ornamented with full-grown timber. The timber sallow thrives particularly well in the wet grounds with which the county abounds; beech and larch are also of very quick growth. In the demesne of Moore Abbey is one of the best-planted hills in Ireland; and the woods of Carton and Palmerstown are extensive, and the timber remarkably fine. In draining the bogs remains of ancient forests have been discovered. The great mountain range of granite of which the county of Wicklow is nearly composed, terminates in this county at Castledermot. Thence by Ballitore, Kilcullen, and to the south-east of Naas, nearly as far as Rathcoole, is clay-slate; the rest of the county belongs to the great field of floetz limestone which covers the greater part of the flat country of Ireland, and which is here interrupted only by the chain of central hills.
The low group of hills west of Rathcoole, which includes Windmill Hill, Athgoe, Lyons, and Rusty Hill, is composed of clay-slate, grauwacke, grauwaeke-slate, and granite. The grauwacke consists of small and finely rounded and angular grains of quartz, numerous minute scales of mica, small fragments of clay-slate, and sometimes portions of felspar. The rock at Windmill Hill ranges 10° north of east and south of west, which is the general direction of these hills, exhibiting also at times an undulating curved slaty formation: the dip is towards the south-west, and generally at an angle of about 45°.
The grauwacke-slate of Windmill Hill is remarkable for containing subordinate beds of granite, the uppermost at the depth of four fathoms; they are 50 or 60 yards apart, separated by the grauwacke-slate, and all dip from 45° to 50° to the south-east. Some of these granite beds may be traced westward to the turnpike road opposite to Rusty Hill: they consist of a small and finely grained intermixture of yellowish and greyish white felspar, greyish vitreous transparent quartz, and flakes or scales of mica, white and silvery, with some scattered portions of schorl: the grains are sometimes so minute that the stone appears almost compact. Sometimes also small particles and cubical crystals of iron pyrites are disseminated through the rock, which, when decomposing, communicate an iron-shot spotted appearance to the stone.
The red sandstone conglomerate occurs in situ at the northern foot of the Hill of Lyons, where it is exposed for about 10 fathoms in length, in strata four feet thick, ranging east and west, dipping 30° to the north, and resting on grauwacke-slate; it re-appears in the central range. Red Hill, Dunmurry Hill, and the western foot of Grange Hill, consist of alternating beds of finely grained grauwacke, grauwacke-slate, and clay-slate, ranging 10° north of east and south of west, and dipping 60° towards the south-east, but in many places being nearly vertical. At the northern foot of Red Hill is a small patch of red sandstone conglomerate, which was quarried for mill-stone some years since. Enough of the firm rock is visible to show that the strata range east and west, and dip 17° west.
The Chair of Kildare consists of floetz limestone, extending southwards to the northern foot of Dunmurry Hill, and covering the grauwacke and slaty rocks. To the north it rests on the trap of Grange Hill, which also covers the same kind of rock. Strictly speaking, these two elevations are but parts of the same hill, with a slight hollow between them. The floetz limestone of the latter appears to be disposed in massy strata, from four to five feet thick, dipping 45° towards the south-east: it is generally greyish white, but sometimes mottled reddish brown, being intermixed with shades of blueish white and grey; and it contains bivalves and entrochites. In its outgoing to the north-west the limestone presents a rocky face, or small escarpment, beyond which is a slight hollow forming the southern face of Grange Hill. In the road leading to the hill, above the rock which appears at the surface, is compact greenstone, in some places porphyritic: but near the surface it is easily frangible, and being much decomposed acquires almost the appearance of wacke. From the dip of the limestone and the general form of the Chair of Kildare, it is highly probable that the green-stone is subjacent to the limestone; an opinion confirmed by the fact that the greenstone just described contains marine exuviae, and, where adjacent to the limestone, it appears to be intermingled with calcareous matter.
The organic remains are principally bivalves, ammonites, and terebratulites, with entrochites in smaller number. These organic remains seem to be confined to that portion of the rock which is in the vicinity of the limestone; for none can be discerned in the remaining mass of the hill, which exhibits everywhere rocky protuberances from one continuous body of greenstone and porphyry. The only other rock visible is clay-slate, standing in strata nearly vertical: it appears low down on the western side, and at the base of the hill in that quarter. The Hill of Allen is separated from Grange Hill by an intervening vale, their summits being about two miles apart: it is composed of one great body of granular and compact greenstone and greenstone porphyry, which appears all round the base, on the sides, and on the summit, in numerous protuberant rocky masses, without any mark of stratification. Some of the greenstone is remarkably crystalline, consisting of large masses of hornblende, with crystals of felspar. Whether this hill be a distinct mass or connected with Grange Hill is not easily ascertained, from the depth of the alluvial soil.
About a quarter of a mile from the northern extremity of the Hill of Allen is a slight eminence called the Leap of Allen, composed of red sandstone conglomerate, arranged in beds which vary from 9 to 18 inches and even to 2 ½ feet thick, and are separated by thin layers of reddish sandy slate-clay. It contains the same components as the conglomerate already noticed, with the addition of fragments of grauwacke-slate, which are, however, comparatively rare: it is quarried for mill-stones. The beds range north-north-east and south-south-west, dipping south-south-east at an angle of from 150° to 20°, and therefore they probably underlie and support the trap of the Hill of Allen. Indications of copper having been observed in the Dunmurry hills, miners were employed to explore them in 1786, during whose operations detached masses of sulphuret of copper were found of nearly 40 per cent. purity, accompanied with a strong vitriolic water: the principal bed seemed to lie deep in the hill, and even to dip under the adjoining valley. Near the base of the hill was also found an alkaline argillaceous earth of a light grey colour, possessing many of the qualities of fullers' earth. In the veins of the rocks, and in the matrix of the ore, were quantities of fine yellow ochre proper for painting. The surface of the Hill of Allen also presents indications of copper. The loose stones and the projecting points of rock appear as if vitrified by fire, and in many places impregnated with carbonate of copper.
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