The Geology of Dublin and its Suburbs

[From the Dublin Penny Journal, Vol. 1, No. 16, October 13, 1832]

The advantages of soil and climate by which this country is so pre-eminently distinguished, are the theme of every statistical writer, and, being pretty generally admitted, do not require any special enforcement. It is not, however, so generally known that Ireland is a country, not only suited to agricultural improvement, but one abounding in every object which can attract or reward the attention of the philosophic student of nature. It would indeed be difficult to point out a single district of our favoured land which does not present, not only curious objects of antiquarian research, but those rare and instructive natural productions, which excite an interest more vivid and enduring than the sublimest of the creations of art. A most fertile and diversified surface, and a variety of climate not only dependant upon the length to which it stretches along the meridian, but upon the altitude of its mountain chains, and the number and extent of its lakes and rivers, confer upon Ireland a Flora second to that of no other country in Europe for number and variety of species; while the pursuer of mineralogical or geological studies is equally ensured, within its limits, ample means of illustration and instruction. To the scientific tourist Ireland also presents another source of attraction almost peculiar to herself. While the harvest of science has been repeatedly gathered elsewhere, and the soil as it were exhausted by repeated cultivation, Ireland is still an, unexplored wilderness, and requires but the hand of industry and skill in order to the disclosure of its fertility, and the rich rewarding of those who may have the judgment to select it as a theatre of philosophical enterprise. To sustain these positions in their fullest extent by a reference to facts, though an easy task, would carry us far beyond the limits within which this communication must of necessity be confined. We purpose, however, singling out one department of science - that of geology, and shewing that it may be studied with the greatest advantage in this city, inasmuch as the structure of the adjacent country suffices to illustrate some of its most imposing theories, and exemplify some of its most striking phenomena.

Dublin is situate at the embouchure of the river Liffey, by which it is divided into nearly equal portions, and is encircled on the side of the land by a chain of lofty and picturesque mountains, composing an amphitheatre interrupted only towards the north-west. To the north of the city the Tolka, a small stream, empties itself at Ballybough bridge; while on the southern side, the Dodder, curving northwards for some distance, terminates with the Liffey in the harbour; both rivers, the Tolka and the Dodder, thus constituting natural boundaries or limits to which, though the city has not yet reached, it is rapidly extending. The latter river, for a few miles towards the base of the Dublin mountains, in which it has its source, serves as a convenient line of division between districts geologically distinct. To the south of the Dodder we encounter none but the primitive rocks of Werner; while in proceeding northwards from it, formations are met with termed secondary, in the language of that distinguished systematist. In the former direction the massive granite and its accompanying schist arrest the attention of the most uninstructed observer. The havoc which has been made of the Killiney hills with a view to the construction of the magnificent Pier at Kingstown, has familiarized every class of our citizens with the appearance and physical characters of granite; while the Scalp, a chasm in one arm of Shankhill mountain, within a couple of miles of Enniskerry, presents the mica-slate at its junction with the subjacent granite, and exhibiting a singularly contorted and dislocated appearance probably due to that convulsion of nature by which it was exposed. These are the rocks which constitute the chief substratum of the southern portion of the environs of Dublin. To the student in geology they are interesting in the highest degree. They are unquestionably the oldest deposits found upon the oxidized crust of our planet, as would appear proved by the absence from them of all organic remains; or, if of more modern origin, they must have been upheaved by the agency of subterranean heat. Whichever hypothesis theorists may adopt respecting their manner of formation, they are from their geolological position, and the simplicity and uniformity of their composition, those which should be first examined by him who enters upon the study of the structure of the earth.

But granite and mica-slate are not the only rocks to be met with to the south of the Dodder, and within a short distance of Dublin. The greater and lesser Sugarloafs are composed exclusively of quartz rock, a circumstance to which with the Paps of Jura, they owe the conical contour of their highest points. The rugged ridge of Brayhead is also in a great measure quartz rock, but here it is inter-stratified with clay-slate, and also with green-stone, a member of the trap family of rocks. The green-stone however here is distinguished from every modification of the same stone which the writer has seen elsewhere, by its fracture being studded with minute glistening scales of mica. What theory will account for the structure of Bray-head? Is it to be traced to the agency of fire? Or was it the result of aqueous depositions? Neither of these rival hypotheses seems to us adequate per se to the explication of all the phenomena. But, were we forced to decide between them, we would not hesitate to avow that existing appearances, in the present as well as in many similar cases, would seem better accounted for by attributing to all the formations denominated primitive, an igneous rather than an aqueous origin.

The primitive rocks to the south of Dublin are not destitute of other sources of scientific attraction. In them are found a number of simple minerals, distinguished some by their rarity, others by their commercial value. And they are also the depositories of metallic ores. At Killiney alone several curious fossils have been met with. Spodumene occurs here in considerable abundance, a mineral very rare in other countries, and in great request with chemists and collectors, in consequence of its including about 8 per cent. of lythia, the new alkali not long since detected by arf-wedson in Ocialite. Here also a new mineral has been discovered by Dr. Taylor, of Cork, closely resembling spodumene, but divided from it by unequivocal distinguishing characters. He has called it Killinite from its locality, and minutely described it in the Transactions of the Royal Irish academy. Garnets also are frequently met with at Killiney of small size, but beautifully transparent, and of the most regular crystalline form. To the preceding may be added beryl, apatite - a mineral resembling the aqua-marine in form, but totally different in its composition and structure, - tourmaline, shorl, &c. &c.

The only metallic ore which has been found in quantity in this district is galena, or the common sulphuret of lead. At Killiney, opposite to Dalkey island, it was once abundantly raised; but the works have long since been abandoned. The remains of a shaft and adit connected with the mine are still visible immediately over the strand, not far from the point at which occurs a remarkable junction of the schist and and granite. To the south-west of Killiney, on the side of Shankhill, a much more productive vein is at present worked by the Mining Company of Ireland. The ore is, as usual, chiefly galena, but the white lead or carbonate has also been found, though sparingly. On Shankhill also there is a tower for the manufacture of shot, and in its immediate vicinity, at Ballycorus, there are blast and wind furnaces, to which the ore is carried for smelting, not only from the adjacent mine, but from others in the possession of the same company at the Seven Churches. The sulphate of barytes or heavy spar, which occurs very generally in the matrix of the ores of lead, may be picked up in abundance on Shankhill in the neighbourhood of the shaft, and is frequently sought for by the Dublin Chemists with a view to the preparation of a very powerful medicine, and certain indispensible chemical tests.

Having disposed in this rapid and very imperfect sketch of those objects to the south of the Dodder, which claim the attention of the geologist, we shall now pass to the more recent formations on which the metropolis is situate, and which are found upon its northern and western sides.

Dublin is built upon a great alluvial deposit of sand, clay, and gravel, whose depth is various in different parts of the city. In some places, as, for example, Castle-street and High-street, after sinking to a certain depth, peat has been encountered, which, when dried, has afforded a very excellent turf. This curious fact was first publicly announced in a letter lately addressed by Philip Molloy, Esq. of Merrion square, to the secretary of the Geological Society of Dublin. When this alluvial bank is pierced through, a stone of a peculiar kind is met with denominated calpe, a term first applied to it by our celebrated countryman, Mr. Kirwan. This calpe has a close fracture, a very dark colour, and is very generally intersected with veins of hornstone and calcarious spar. Besides carbonate of lime it includes considerable quantities of silex and alumen, traces of the oxides of iron and manganese, and an appreciable per centage of free carbon, to which its colour is due. It is in fact a rock intermediate between limestone and clayslate, and has by some geologists been considered as arising from an intimate admixture of both. In consequence of the amount of foreign earths and oxides which it includes, this stone does not admit of being burned into lime. It is also extremely ill adapted to the purposes of building, for, by exposure to air and moisture, it becomes soft and friable, and undergoes a very rapid disintegration, a fact unfortunately too well attested by the dilapidated exteriors of some of our public buildings. To the geologists, however, it is particularly interesting, being a stone of a very characteristic composition, and one almost peculiar to Dublin and its vicinity. In this district it alternates with; beds of limestone of remarkably dark colour, and which fre-quently include fully ten per cent. of silex. It is also frequently studded with cubical crystals of iron pyrites, and associated with thin strata of slate clay, containing imbedded spheroidal masses of a species of clay iron stone, a substance which, when burned and mixed with quick lime, affords a cement capable of setting under water. These different beds, were it not that they are obviously a part of a very extended formation which exhibits elsewhere far different characters, might be fairly classified with the earlier calcarious rocks; for, like these, they are, as far as the observation of the writer goes, almost entirely destitute of animal or vegetable fossils. In this particular how strikingly are they distinguished from the Kildare limestone, brought into the city by the Grand Canal, which appears to be little more than an aggregation petrified shells and corals. This is the stone which is chiefly employed by the lime burners of Dublin.

Within the city the calpe does not any where crop out, or appear at the surface. In the suburbs, however, there are several quarries which yield it, and its accompanying black flinty limestone, the largest and most extensively worked of which are situate near the commons of Crumlin, and at Rathgar upon the property of Mr. Osbrey. In proceeding westward from Dublin along the banks of the Liffey the rock is every where concealed by alluvium, which occurs as a series of undulating hills, seldom reaching 150 feet in height, and composed of layers of sand, and water-worn pebbles of calpy limestone. In examining these mounds the debris of other rocks are also occasionally found, rolled pebbles, for example, of chert, clay slate, greenstone, and granite. The chert is undoubtedly derived from the veins of this substance which we have already noticed as existing in calpe, while the balls of granite, clay slate, and greenstone, are as clearly traceable to the prevailing rocks of the north-western termination of the Dublin mountains, and their subordinate beds of trap. Upon reaching Lucan, and turning off by the road on the north side of the river which leads by the back of Colonel Vesey's demesne to Leixlip, the rock is at length found exposed on the face of a high bank which hangs on the right hand side over the road, and exhibits appearances of a singularly curious and fantastic description. The limestone of this district when undisturbed, as in the quarry at Lucan under Mr. Gandon's house, occurs in parallel beds usually separated from each other by seams of rotten calpe, dipping uniformly at a small angle to the magnetic east north east, and bearing north north west and south south east. In the bank, however, just spoken of this regularity is entirely wanting. The strata of limestone and accompanying calpe are thrown into sinuosities which may be aptly compared to those in virtue of which the serpent, without the aid of extremities, is nevertheless capable of progression. Of these sinuosities, three or four are distinctly visible, and so abrupt are they, that the flexures, if viewed as the areas of incomplete circles, would, in some cases, constitute at least two thirds of their respective perispheries. A very cursory inspection is also sufficient to show that, at the time when these contortions were effected, the strata must have been in a soft or yielding state, for individual slabs of limestone, forming part of the disturbed beds, have themselves an arched outline, so as to conform to the general curvature of the strata. These singular phenomena are not without parallel in other parts of Ireland. The limestone in several districts is similarly waved. But in no place are the undulations more remarkable than in the celebrated rock which constitutes the basis of the ruins of the cathedral of Cashel. We will not stop here to devise an hypothesis in explanation of these extraordinary appearances; but cannot resist observing, in passing, that they would seem to constitute one of many difficulties which beset a theory recently put forward with confidence, and supported by much ingenuity and eloquence - that all known geological facts admit of being referred to existing causes, or such as are now observed to be in actual operation.

The limestone district, of which we have been speaking, is but a portion of the immense field which extends throughout the greater part of Ireland, being found in every county save Derry, Antrim, and Wicklow, constituting the substratum of the great valleys, and resting on the rocks of sandstone, schist, conglomerate and trap, which compose the mountain chains and insulated hills. The depth of this field is no doubt different in different places, but upon the average it may be set down as 200 feet. In some places, however, it must be 700 feet at least, as is inferable from the ascertained heights of the heads of the Grand and Royal Canals above the level of the sea. The dip also of the beds is subject to considerable variation, the only general rule being that the limestone always conforms to the position of the subjacent rocks. Between, however, the Dodder and the Tolka, and for some distance to the north of the latter river, the bearing of the strata is due east and west, and they dip at an angle of 15° to 20° to the south. Within this compass, no metallic veins have been discovered, or at least none such are at present worked.

To complete this sketch of the geology of the vicinity of Dublin, it only remains to glance at the structure of Howth, the bold promontory which constitutes the northern boundary of our spacious and beautiful bay. This hill is in the main composed of quartz rock and clayslate; a stone of an intermediate character being frequently found interposed between them. These rocks form beds of varying thickness, and frequently alternate with each other. There are twelve of these alternations upon the great scale, and smaller ones almost without number. A huge bed of porphyritic greenstone is also visible on the southern side, running from the water edge into the heart of the hill, and separating at some distance into two lesser veins, which gradually diverge from each other. On the same side of Howth, or that which looks towards the metropolis, other objects of geological interest are also to be seen. A bed of dolomite, accompanied by greyish limestone, which first appears at a few points to the south of Skerries, after dipping beneath the sands of Portmarnock and Malahide, re-appears near the harbour of Howth. This same bed, sweeping round the base of the promontory, is next found near Sutton, at its south-western point. Here it is quarried, and hence at no distant period it was exported to England, where the magnesian earth was extracted from it, and converted into a series of valuable preparations. The following is an outline of the process, by the prosecution of which Dr. Henry of Manchester is said to have realized a splendid fortune: The dolomite, which consists of a mixture of carbonate of lime and carbonate of magnesia, is broken into fragments of the size of an egg, and burned as usual in a common kiln. By this process the carbonic acid or fixed air is expelled, and the earths remain. Upon a known weight of these, such a quantity of pyroligneous acid is next digested, as is inferred from previous experiment to be exactly adequate to the saturation of the lime. This latter earth, in virtue of its superior affinity, is exclusively taken up, and washed off in the form of acetate of lime; and the magnesia left behind is converted into sulphate or Epsom salt, which is purified by processes well known to the practical chemist. From the purified Epsom, the carbonate of magnesia is easily thrown down by the addition of a suitable quantity of an alkaline carbonate, and from the carbonate the calcined magnesia is obtained, by the application of the minimum degree of heat necessary for the expulsion of the carbonic acid. The impure acetate of lime, also, formed in the commencement of the process, is a product of considerable value. When subjected to a gentle torrefaction so as to destroy the bitumen which adheres to it, it yields, when properly treated with oil of vitriol, acetic acid in its purest form, and of any degree of strength which the manufacturer may choose. It is in fact possible, by a modification of this process, to procure it so strong as to include but 15 per cent. of water, and to congeal, when exposed to a temperature some degrees above the melting point of ice. Why should not this process be practised in our native city. Or wherefore is it that our most valuable minerals become productive only when worked by English hands. We will not venture upon supplying any response to these interrogatories, but will merely express a hope that the period is not far distant, when Irishmen will merge party in national objects, and when the bounties of Providence shall cease to be marred by the folly or the wickedness of man.

Before concluding this paper which, we are afraid, has already grown to an unreasonable length, we are anxious to state that the black oxide of manganese is also found at Sutton, and in the immediate vicinity of the magnesian limestone. It has been raised and prepared for sale in considerable quantity, and sold to the manufacturers of the bleaching salt of lime, and of the different other more recently fabricated compounds, of which chlorine is the active element.