116 Proceedings of the Royal Irish Academy. 



XIII. — On the Dihections op Maiw Liistes of Jointing obseevable 



IN THE EOCKS about DuBLIN, AND THEIK HeLATIONS WITH ADJACENT 



Coast Lines, and with Lines of Eaitlting and Contact of Geolo- 

 gical FoEMATiONs. (Part II.) By J. P. O'Eeillt, C.E., Professor 

 of Mining and Mineralogy, Eoyal College of Science, Dublin. 

 (Plate I.) 



[Eead, January 28, 1884.] 



In a Paper, read before the Academy in Pebniary, 1880,^ I submitted 

 a series of determinations of directions of jointing observable in the 

 rocks about the Bay of Dublin ; and by grouping them as regards 

 direction and relative predominance I was able to show that a pre- 

 dominating direction of jointing, very frequent about Dublin Bay, 

 represents very correctly the direction of the coast line between Carnsore 

 Point and Wicklow Head, the mean value of these observed joints 

 being IN". 16° 23' E. I further added, " as regards the eastern coast 

 of Ireland, it may be fairly advanced that it is represented by direc- 

 tions which correspond to lines of jointing observable, in greater or 

 lesser number, about Dublin Bay, and which are as follows : — 



" From Carnsore Point to Wicklow Head, . N. 16° 23' E. 



„ Wicklow Head to Clogher Head, . „ 9° 27' W. 



,, Bellagan Point, Carlingford Lough, | , ^o on/ t? 

 to John's Point, Dundrum Bay, j " 



,, Donaghaclee to Bruce's Castle, | 2Q0ii'"W 

 Eathlin Is., ....)" 



'' It should be noticed that the direction 48° 30' E. is almost ex- 

 actly that of the line of porphyritic rocks so markedly characterising 

 the geology of the county Wexford." 



I now proceed to detail a further series of observations of directions 

 of jointing made in the environs of Dublin, and from their summary 

 to draw conclusions as to their relations with the outlines of the 

 country and with the adjacent coast lines : — 



\_The directions are all taken from N. magnetic.'] 



Sandyford : — 



258 Granite rocks, with, well-marked jointing, vertical, 



259 „ ,, „ „ ,, „ ,, ,, 



260 Cross jointing, 



8°-9° 



W, 



10°-13° 



W, 



77° 



E. 



' These Froceedings, ser. ii., vol. iii., p. 295. 



