Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/232



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.