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O'Reilly — On Jointing in Rocks about DuUin, ^c. 139 



It will be remarked, in the first place, that only five neio directions 

 have been added to the series of 1880, and that in the main there is a 

 fair concordance between the sets of directions observed in the two 

 years. It is presumable, therefore, that were the survey of the various 

 systems of jointing extended over all Ireland, the total number of 

 distinct systems would be limited, and perhaps be not much in excess of 

 the number observed in the district to which the present observations 

 apply. 



Considering the systems of jointing as regards frequency of occur- 

 rence, the Table shows that the most frequent, between Malahide and 

 Skerries, and indeed in the district of observation, is IS". 13° 50' W., 

 which is so well marked and so frequent along the coast of Donabate. 

 This direction represents the coast line at Skerries and the general 

 direction of the coast between. Howth and Dunany Point. It is clear, 

 therefore, that the most frequent direction of jointing in the district 

 observed represents the neighbouring coast line. The prolongation of 

 this direction represents the valley in which runs the Newry canal, 

 the IS". W. shore of Lough JN'eagh, and the course of the Bann river. 

 Its further extension northwards cuts the east coast of Iceland. A 

 parallel to this direction through Wicklow Head northwards represents 

 the coast line between this point and Killiney, and traverses a certain 

 number of localities marked by the occurrence of earthquakes, such as 

 "Wicklow, Bray, Kingstown, Clontarf, and, in the extreme north, the 

 promontory of Innishowen. It would appear that further inland this 

 direction is not only observable, but remarkable by its frequency and 

 regularity. Thus in the Memoirs of the Greological Survey (data 

 and description to accompany quarter sheet 45 S.E., p. 21) mention 

 is made of "numerous well-defined joints striking IS". 10° W." ; also 

 in memoir accompanying quarter sheet 45 S.W., p. 23, describing the 

 district "W. of Ealdowery and Shanballymise, the beds are said to be 

 " jointed in lines running either N. and S., or 15° toW. of N." Also 

 in Memoir accompanying sheet 46 N.W., p. 20 : " To the N.W. of 

 Drangan, in the townland of Ballylusky, numerous very regular 

 and smooth joints, striking N. 10° "W"., cut across the beds. To the 

 south of this locality, and distant about one mile, beds occur, the most 

 remarkable feature of these beds being ' the very regular manner in 

 which they are jointed.' '■ These joint planes have a strike of N. and 

 S., and E. and W., varying occasionally 10° in either direction,' " &c. 



As however all the observations of dips and directions mentioned 

 by this observer are given in round numbers, and with approximations 

 of not more than five degrees, it is evident that this direction N. 10° W. 

 may be in reality any dii'ection between 5° W. and 15° W., and in aU. 

 probability really represents that found in the neighbourhood of 

 Dublin, ]^. 13° 50' W. This view will appear to be sustainable from 

 the consideration of the directions observed in the limestone quarries 

 immediately E. of the Queen's College, Cork. Here may be remarked, 

 jointing, both frequent and well-marked, giving rise in one place to 

 the Dolomitization of the beds, the mean direction found for them was 



R. I. A. PROC, SER. H., VOL. IV. — SCIENCE. U 



