224 Dr. Berger on the Dykes 



Feet 



1. Dyke on Cave-hill, West of Belfast Lough on the Antrim side, above the sea 1064 



2. Ditto on the top of Ballyghuia 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 another. 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 sur- 

 veyed 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.* 



* The direction of the two dykes at the colliery of TiYidale ia Staffordshire, is also from 

 S.E. to N.W. 



