# Transactions of the Geological Society, 1st series, vol. 3/On the Dykes of the North of Ireland

IV. On the Dykes of the North of Ireland
My object in the following paper is to describe some of the more general characters of Dykes, such as I have lately observed them in the North of Ireland.
I do not know exactly within what geographical limits these curious geological phenomena are to be met with: they are common on the Western coast and in the Isles of Scotland, and I have observed them also in the Isle of Man. I understand that none have yet been remarked in the South of Ireland, and I did not observe any in the Midland counties through which I passed between Dublin and the Northern coast. In England they have been found in the centre of the island, as at the colliery of Tividale in Staffordshire; but in the North of Ireland it is only on the verge of the coast that they abound, and it is there that I have principally examined them. Of more than sixty that I noticed, nearly half were situated on the shore; those which occur in the mountains of Donegal are I believe the remotest from the sea, and those lie within fifteen miles of it.
I have not found their occurrence to depend upon the absolute elevation of the country in which they appear. I have observed them at almost every altitude between that of the shore and those which I have inserted in the following Table, as being the greatest and somewhat uncommon. ​
| | | | Feet |
| 1. | Dyke on Cave-hill, West of Belfast Lough on the Antrim side, above the sea | Dyke on Cave-hill, West of Belfast Lough on the Antrim side, above the sea | 1064 |
| 2. | Ditto on the top of Ballyghnia Cupola, Kilmacrenan, Donegal | Ditto on the top of Ballyghnia Cupola, Kilmacrenan, Donegal | 1448 |
| 3. | Ditto near the top of Aghla mor, Boylagh, Donegal | Ditto near the top of Aghla mor, Boylagh, Donegal | 1726 |
| 4. | Ditto near the top of Glendoan, Kilmacrenan, Donegal | Ditto near the top of Glendoan, Kilmacrenan, Donegal | 1738 |
| 5. | Ditto on the top of Glencarn mountain, Boylagh, Donegal | Ditto on the top of Glencarn mountain, Boylagh, Donegal | 1745 |
| 6. | Ditto near the top of Arragh, the highest mountain in Donegal N.W. side | Ditto near the top of Arragh, the highest mountain in Donegal N.W. side | 1848 |
| 7. | Ditto | South side | 2220 |
It seldom happens in the North of Ireland that dykes occur singly; but they are generally found in groups, several within a short distance of one mother. Thus, at the Giant's Causeway there are 6 within 2 English miles; at the collieries of Ballycastle 5 within the same distance; at Alt-a-dora (a glen in the valley of Dunlughy) 4 within 1 mile; on the N. W. side of Arragh, 6 within 1042 feet; and at Church home in the basin of Dunlughy, 4 within 835 feet.
The uniform direction or parallelism of nearly all the dykes in the North of Ireland is a curious circumstance resulting from my observations: I have subjoined a list of all the dykes that I surveyed with accuracy, in order to put the Society in possession of the data from which I have drawn this conclusion. Many of those in the table were surveyed with a theodolite, others with a very good pocket compass mounted on agate; in all the latter cases I have allowed for a variation of 29° West, upon the authority of Mr. Hanton of Loch-beg in the Rosses, who made two observations at my request in order to determine the point. I have rejected from this table the bearings of all the dykes, which I observed with only a common pocket compass, as not having any pretensions to accuracy; and among these the bearings of the famous dykes of the Giant's Causeway; had all however been admitted, they would all have tended to establish the above-mentioned conclusion. ​
Angle at which the Dyke cuts the stratum :
| 1 | Far-Dunlughy Done | 21 | Primitive blue limestone | 51 | 72 |
| 2 | Church-home Dunlughy, Donegal | 17 | Gneiss and primitive granular limestone | 55$\scriptstyle {\frac {1}{2}}$ | 72$\scriptstyle {\frac {1}{2}}$ |
| 3 | Ditto | 41 | | | 96$\scriptstyle {\frac {1}{2}}$ |
| 4 | Ditto | 27 | | | 82$\scriptstyle {\frac {1}{2}}$ |
| 5 | Ditto | 34$\scriptstyle {\frac {1}{2}}$ | | | 90 |
| 6 | Mein-a-bole, N.W. side of Arnagh, Donegal | 71 | Bluish white limestone | 12$\scriptstyle {\frac {1}{2}}$ | 83$\scriptstyle {\frac {1}{2}}$ |
| 7 | Ditto | | | | |
| 8 | Ditto | 38 | | | 50$\scriptstyle {\frac {1}{2}}$ |
| 9 | Ditto | 37 | | | 49$\scriptstyle {\frac {1}{2}}$ |
| 10 | Ditto | 63 | | | 75$\scriptstyle {\frac {1}{2}}$ |
| 11 | Ditto | 52$\scriptstyle {\frac {1}{2}}$ | | | 65 |
| 12 | Ditto | 44 | | | 56$\scriptstyle {\frac {1}{2}}$ |
| 13 | Ditto | 46 | | | 58$\scriptstyle {\frac {1}{2}}$ |
| 14 | Ditto | 45 | | | 57$\scriptstyle {\frac {1}{2}}$ |
| 15 | Ditto | 35 | | | 47$\scriptstyle {\frac {1}{2}}$ |
| 16 | Alt-na-Calge Dunlughy | 31 | Talcose limestone | 54 | 85 |
| 17 | Ditto | 31$\scriptstyle {\frac {1}{2}}$ | | | 85$\scriptstyle {\frac {1}{2}}$ |
| 18 | Ditto | 22 | | | 75 |
| 19 | Rannagh Point, Isle of Arran-mor, Donegal | 20 | Sienite | | |
| 20 | Pool-a-Phuca, Church-hill, Fermanagh; lesser dyke South limb | 53 | Limestone with madrepores | 89 | 142 |
| 21 | ditto, North limb | 53 | | 31 | 120 |
| 22 | Ditto, great dyke | 35 | | | 124 |
| 23 | Lough-na-Croey, Stranagh-logh moutain, Donegal | 40 | Primitive limestone | 41 | 90 |
| 24 | Dubeng, Giddore, Donegal | 19 | sienite | | |
| 25 | Aghia-mor, Boylagh | 39 | Slaty quartz | 71 | 110 |
| 26 | Trayenagh Bay, between White and Red Castle | 19 | sienite | | |
| 27 | Ditto | 24 | Ditto | | |
| 28 | Lough Foyle, between White and Red Castle | 49 | Sandstone | 16 | 65 |
| 29 | Tory | 41 | Porphyritic sienite | | |
| 30 | Ditto | 18 | | | |
| 31 | Ditto | 35 | | | |
The conclusion to be drawn from this table is that the bearing of the dykes is from S.E. to N.W., and that the dykes all cut the planes of the strata through which they pass, at very considerable angles.
A shift in the direction of a dyke is an accident of rare occurrence; it would probably however be more frequently observed, did not the soil, which covers the surface, prevent us from tracing the dyke to any distance.
The most considerable shift that I ever observed was at Rannagh Point in the Isle of Arran-mor, where it was not less than 47° in 126 feet. At Muir-a-Bole on the N.W. side of Arragh, I observed two other considerable shifts; the one of 22°, the other of 27°.
Dykes differ greatly from one another in their widths, which measure from a few inches to several hundred feet. The latter dimensions are of rare occurrence, and I have only met with three cases of the kind, all in secondary strata; and in two of these the enlargement took place at the bottom of the dyke. One is found in the red bay of Cushendall; the other at Pool-a-Phuca in the county of Fermanagh. The third case occurs between Portrush and Dunbar Castle on the coast of Antrim, and has been noticed by Dr. Richardson. I think I have ascertained that there is a remarkable difference in the average widths of the dykes, according as they are found in primitive or secondary rocks. Out of sixty-two dykes that I have measured, the average width of thirty-eight in the primitive districts is 9 feet; that of twenty-four in the secondary is 24 feet.
I have already noticed some of the more remarkable elevations at which dykes have been found. The height to which a dyke rises above the surface of the stratum, which it intersects, is sometimes ​very considerable. That on the N.W. side of Arragh rises perpendicularly 40 feet, like a partition wall; that on the cupola of Bally-ghuia 8 or 10 feet; at Scrabo Hill near Newtown Ards in the county of Down a dyke appears like a standing pillar at the entrance of one of the freestone quarries. The dyke of Port-na-brock near the Giant's Causeway, juts out into the sea quite isolated to the visible extent of 372 feet. On the contrary, those on the summits of Glendoun, Glencarn, and Aghla-mor, appear like strewed masses, scattered about upon the surface. These might be adduced as instances to prove the wearing away of mountains, if that point stood in need of any additional confirmation.
The depth to which the dykes descend is unknown; and after having observed the sections of a great many along the coast in cliffs from 50 to 400 feet in height, I have not been able to ascertain (except in one or two cases) that their sides converge or have a wedgeform tendency; so that no estimate can be formed of the depth at which they terminate. In this respect therefore they do not seem to agree with the metallic veins.
Moreover, I have not observed that they branch off into slender strings, or (except in some very rare instances) that they swell into (what the miners term) bellies, after the manner of the metallic veins.
The dykes, whether they occur in primitive or secondary countries are nearly vertical. The mean angle of deviation from the perpendicular deduced from nine cases occurring in primitive rocks was 13°, the extremes being 9° and 20°. The same angle deduced from ten other cases was 7° to the N.E. But I am not warranted in drawing any general conclusion as to what point of the compass and in what degree they deviate. The angle of deviation in the two remarkable dykes on Arragh mountain is somewhat considerable, ​as is also that of the dyke in the quarries of Scrabo hill; but I am not able to state it with precision.
There is not that variety in the substance of the dykes, that their numbers, their distance from one another, and the varied nature of the rocks which they intersect would lead one to expect. I have found them composed of the following rocks, which are introduced in the order of their most frequent occurrence. Trap and greenstone, with their associates Lydian stone, flinty slate, greystone and wacke.
I have seen but one dyke of clay porphyry, viz. at Farland point in Donegal, and I conceive it to be altogether of a different class from those dykes to which my principal attention has been given in the present paper.
A dyke is formed either of a number of diminutive pillars aggregated together, or of square rhomboidal pieces piled one upon another like blocks of masonry, the long axes of these figures in either case lying transverse, and perpendicular to the walls of the dyke. These regular figures are often much disintegrated and rounded, and sometimes assume the coated form; the two appearances being often united in the same portion of rock.
Dykes are not, like metallic veins, divided into regular layers of different stony substances; nor do we find in them those drusy cavities which sometimes occupy the middle of metallic veins.
The more compact the trap, the more apt is it to assume the polyhedral form, to be homogeneous, and to be free from the porphyritic texture. In the hard variety I never found imbedded any detached mineral concretions, except a few small specks of soft green steatite. When less compact, it is often set more or less thickly with heterogeneous nodules, but seldom so abundantly as to ​assume a texture decidedly porphyritic. Among these nodules I have found the following minerals.
| | Minerals found imbedded in Dykes. | Locality. |
| 1 | Augite in angular fragments | Isle of Islandowey |
| 2 | Olivine in disseminated grains | Glen of Alta-a-dara in the valley of Dunlughy |
| 3 | Crystallized glassy felspar | Glen of Alta-a-dara |
| 4 | Compact felspar in distinct rounded concretions. | |
| 5 | Radiated zeolite | N.W. ride of Arragh, highest dyke but one |
| 6 | Green soft steatite, in distinct concretions | |
| 7 | Iron pyrites | |
| 8 | Calcareous spar | |
| 9 | Carbonate of lime, mixed with the trap | On the shore on the Antrim side of Belfast Lough |
| 10 | Glassy quartz in distinct concretions | |
| 11 | Sulphate of barytes | |
| 12 | Plates of mica | |
The mean specific gravity of the trap rocks forming dykes may be rated at about 2.86. That of the specimens from Alt-a-dara, containing olivine, being 3.14; that of homogeneous trap from the dykes of the Giant's Causeway being 2.99; and that of the wacke from the shore at Carrick-fergus being 2.45.
The dykes are found traversing both the primitive and the secondary rocks, nor have I ascertained in which they are the most frequent. There appears to be no regular connexion between the substance of the dyke and the rock through which it passes. I have however sometimes found lime in considerable quantity in the dykes that traverse limestone. Glassy felspar I have only found in the dykes of primitive rocks.
In the table of the observations of the dykes some of the rocks cut by them are enumeratsed; I have added however another table ​of the rocks that I have seen intersected, with some additional localities.
| Rocks cut by Dykes. | Localities. |
| Primitive limestone | Vide former list. |
| Sienite | Newry, Bloody Farland, &c. |
| Slaty quartz | Farland point, Donegal |
| Mica slate | Kildrim lead mine in Donegal |
| Gneiss | |
| Transition limestone | Blockhouse Isle, entrance of Carlingford bay. |
| Old red sandstone | Near Newton Glens. |
| Flœtz limestone alternating with sandstone and underlying the coal | East of Bally-castle |
| Coal measures | East of Bally-castle |
| Chalk | East of Glenarm, where the limestone is rendered hard and crystalline in contact with the dykes. One of the dykes branches and encloses portions of the limestone, and many other places, see page 172. |
| Flœtz trap, basalt | Giant's Causeway, and many other places. |
The induration which the secondary rocks undergo when traversed by dykes has often been noticed; it is not my intention now to discuss this subject; I shall only mention that the induration does not extend far from the dyke, and that the phenomena though very frequent are not universal. I have only noticed one instance of remarkable change in a primitive rock contiguous to a dyke. In the case I allude to, viz. in the lead mine of Kildrim in the county of Donegal, mica slate adjacent to the dyke had its texture quite loosened, and was in a dusty state. ​
In general there is no foreign matter between the substance of the dyke and the rock it divides, excepting a slight rusty appearance on the surface of the latter. The contact between the two is pretty close, but they may always be disjoined by the blow of a hammer. When the dyke is prismatic, a hollow interval between the two may sometimes be observed.
Since the average direction of the dykes is from south-east to north—west, and since the average dip of the strata in the north of Ireland is to the south-east, it will follow (independently of the several observations on the bearings of the strata contained in the table) that the direction of the dykes is nearly perpendicular to that of the strata.
Moreover since the longitudinal vallies and the metallic veins of a district are generally parallel to one another, and to the direction of the strata which they intersect, it will follow that the dykes will cross the longitudinal vallies, and that where metalliferous veins and dykes occur together, that one of the two will cut the other. This is the case at the lead mine of Kildrim in Donegal, where the dyke divides the vein. Similar facts are recounted in the Philosophical Transactions, 1790, page 93, by Mr. Mills, as occurring at the lead mines of Persabus and Glasgow-beg in the isle of Ilay; where although the directions of the dykes are not uniform, as in the north of Ireland, yet they cut the veins nearly at right angles. ​
It is very evident from these observations that some of the dykes at least were formed at a later period than the metallic veins; an inference that might be extended to all, could we show them to be all contemporaneous. That they are so is rendered probable by their parallelism, and by the nearly uniform texture of the trap, in whatever rock the dyke is found. The simple minerals too contained in the trap favour this opinion. Calcareous and heavy spar, and lime intimately mixed with the trap abound most in the dykes of secondary rocks; iron pyrites is common to those both of primitive and secondary districts; and radiated zeolite, olivine and augite are common to the dykes of primitive rocks and to the beds of trap of secondary formation.
From this inference, however, I must except a class of dykes which run parallel to the metallic veins, and are probably intersected by that class of dykes which I have been describing in the present paper. An example of this new class of dykes is found at Farland point in the county of Donegal, where alternating strata of slaty quartz and sienite are traversed by a dyke of clay porphyry hearing east of north 21°: a dyke of trap is found at the same place bearing west of north 49°, the angle between the two being 100°. There can be no doubt that they meet one another, but the spot being covered by the sea, I could not discover the point of intersection.
Whatever date and whatever agents we are disposed to assign to the origin of dykes, their uniformly vertical and nearly parallel positions evince that both they and the mountains which they intersect have not undergone any modern disturbance beyond superficial abrasion, but that they remain in the same situation as at the remote period at which they were formed. ​
APPENDIX.
On the Dykes of Monte Somma in Italy,
Extracted from a Series of Letters addressed to the late LORD CHARLEMONT, P.R.I.A.
By the late Rev. GEORGE GRAYDON, Fellow of Trinity College Dublin. Dated Naples.
“ The appearance of the face of the cliff of Monte Somma in its, whole length perfectly coincides with the idea given by its semicircular shape, surrounding Vesuvius, as well as by its sloping back as seen from Naples, and strongly confirms the opinion of its having once formed a much higher conical hill.”
“ The face of the cliff viewed on a horizontal line is by no means smooth, but considerably indented in some places into semicircular or more than semicircular hollows or recesses, with sides nearly perpendicular; in others into hollows with steep sloping sides down which the sand and stones are continually sliding.”
“ It is formed of a great number of successive strata of lavas, in some places perhaps upwards of thirty; these strata in general are thin, that is, not exceeding from three to six feet in depth of solid stone, the intervals between them, which are generally much thicker than the strata themselves, consist of porous red or calcined and usually
respective strata of lava, and belonging to each; but I did not observe any stratum of vegetable mould, properly so called, though I believe there may be some puzzolana and rapilli.”
“ The strata are intersected in many places by walls generally perpendicular to the face of the bank as well as to the direction of the strata, in others inclined somewhat to the latter, and in some to both, and often small ones branching off from the greater perpendicular ones, and inclined to them low down in the substance of the hill itself. These seem evidently to have been fissures or cracks of the whole crust of the hill from top to bottom, into which the lava had flowed and filled them. The lava of which these are composed differs also in the same manner as that of the strata: in some it is compact and almost homogeneous, in which case the joints into which it is divided generally lie across its direction, that is, are nearly horizontal; and it is divided into irregular polygonal parts, in some places assuming a very rude sketch of basaltic columnization; in others it is porous and heterogeneous, particularly in such as are formed of the granitical lava, have a red scorified appearance and an irregularly globular structure.
“ The remarkable analogy between the several circumstances of the face of Somma and of the cliffs of the county of Antrim in Ireland from Bengore head to the river Bush, must strike any one that has seen both. The principal differences between the two are, the constant uniformity of the Antrim strata, the homogeneity of their matter, their basaltic form, and the much greater depth of the strata, which are seldom less than twenty-five or thirty feet, and the less rapid inclination inwards of the strata, which there seems not to exceed an angle of fifteen or twenty degrees, whereas here in Somma the angle is not less than from forty to forty-five degrees, ​which in both amounts nearly to the angle of the dip of the outward surface of the hill which they compose.”
“ These differences seem to have resulted from the greater vicinity of the cliffs at Somma to the seat of the crater as well as to the more rapid slope of the hill, which from the greater degree of fluidity of the lavas, and their more rapid tendency to descend, did not allow them to acquire there any considerable thickness; whereas in Antrim the distance being probably greater from the crater, and the slope much less, the lavas could settle there in greater depth. The basaltic walls also found along the coast of Antrim, and particularly of Ballycastle and Belfast, seem to have a perfect analogy with those of Somma, but are of much greater breadth in general, and the intervening strata of porous and irregular basaltic matter between them correspond exactly with those of scoriæ in all lavas, and so visible in the ancient ones of Somma.”
“ In the side of one of these walls of Somma, I found a crust of completely vitrified matter, covering a schistose cracked and very fragile homogeneous lava, disposed, contrary to the general rule of that kind, in perpendicular joints, and much resembling a kind of schistose hornstone, as well as the upper and superficial covering of Pleaskin.”
“ The whole of the valley between Somma and Vesuvius is covered with repeated irruptions of lava, particularly those of 1767, 1779, and 1787, which have run to the foot of the rock and to a considerable depth.”
This description will be found to agree pretty well with that of M. Breislack (Voyages dans la Campanie. tom. 1. p. 133. Paris 1801.); from both of which it is sufficiently clear, that the walls of Monte Somma are of the same nature with the dykes of the north of Ireland.
* ↑ The direction of the two dykes at the colliery of Tividale in Staffordshire, is also from S.E. to N.W.
* ↑ A dyke also traverses transition limestone, containing magnesia limestone, at Scarlet point in the Isle of Man.
* ↑ Inserted by the Editor
* ↑ The white limestone when thus indurated becomes, as is well known, phosphorescent. I have found limestone, accompanying the undoubted lavas of Andernach in the Palatinate, and containing garnets and augite, not to possess this property.
* ↑ M. Brongniart as well as Dr. Richardson from whom he quotes, (Irish Trans. Vol. 9, page 22,) appears to have misunderstood this passage, and adduces it to prove that dykes are traversed by lead veins, the reverse being the case: Traité de Mineralogie, tom. 1. p. 462. He also quotes some observations said to be made by M. Humboldt on the basalt of Unkeln, from Journal des Mines, No, 19, p. 378, to prove that dykes are sometimes metalliferous; but it does not appear on referring to that number that dykes are there spoken of, nor indeed does the reporter appear to have been satisfied with the expression “ raies metalliques,” or to have relied much on the accuracy of the observations.