| Feet | |||
| 1. | 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 | 1448 | |
| 3. | Ditto near the top of Aghla mor, Boylagh, Donegal | 1726 | |
| 4. | Ditto near the top of Glendoan, Kilmacrenan, Donegal | 1738 | |
| 5. | 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 | 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.[1]
- ↑ The direction of the two dykes at the colliery of Tividale in Staffordshire, is also from S.E. to N.W.
