O'Eeilly — On Jointing in Bocks about Dublin Bay, 8(c. 295 



XLIX. — On the Dieections op Main Lines of Jointing obseevable 



IN THE EoCKS ABOUT THE BaT OF DtTBLIN, AND THEIK RELATIONS 



WITH ADJACENT CoAST LiNES. By J. P. O'Reilly, C.E., Professor 

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



[Bead, February 23, 1880.] 



"When submitting to the Academy, in January, 1879, a memoir upon 

 the correlations of lines of direction on the Earth's surface (vide 

 Transactions, vol. xxvi., p. 617), I expressed the hope of being able to 

 bring forward a series of observations in support of the theory therein 

 involved, to be taken, in the first place, from the three kingdoms, the 

 Geology of which has been so thoroughly worked out in most of its 

 details. 



During the last two summers I made a series of determinations of 

 directions of main lines of jointing, on the north and south sides of 

 the Bay of Dublin, which I have the honour of submitting to the 

 Academy, with a summary of the consequences which I think are 

 fairly deducible therefrom, and which go in support of the general 

 theory of correlation already referred to. 



I have in the first place simply detailed the measurements and 

 observations made, and have then grouped the directions of jointing 

 into series, showing their relations one with the other and with the 

 coast lines. 



Commencing at Blackrock, the most northerly outcrop of the 

 Dalkey or Wicklow granites, there occur on the shore, just beside 

 the railway station, patches of granite which extend out in small 

 promontories in a N.E. direction. 



One patch of this rock occurs close under the Blackrock station, 

 and forms there part of the sea wall. It presents the constitution of 

 a well-marked granitic Breccia^ which has not, that I am aware of, 

 any representative in or about the Dublin coast. It evidently re- 

 presents the consequences of a fracture having taken place in the 

 immediate vicinity, and is in all probability related to the cause which 

 determined the northern limit of the Dalkey granites. This is to a 

 certain extent indicated by the direction^ of the jointing observed in 

 this granite : — 



Blackrock : — 



1. General direction in a rough joint, .... jST™* 67° W. 



2. Cross jointing, ,, 13° E. 



3. Jointing in rocks at ladies' bathing-place, under Idrone- 



terrace, ,, 67° W. 



4. Another jointing in the same place, . . . ■ ,, 67° W. 



' See ' ' Notice of some Remarkable Appearances of the Granite to the Soutli of 

 Dublin," by the Eev. H. Lloyd, F.T.C.D., M.E.I. A., Proc. Geol. Soc. : Dublin, 

 Nov. 10, 1833. 



^ The directions in this Table are magnetic. The magnetic variations beuag, for 

 the date of the observations, about 22° "W. 



