O'Reilly — On Jointing in Bocks about Dublin Bay, Sfc. 305 



There now remains to be shown the relations which those direc- 

 tions bear to the coast lines on the east side of Ireland ; and, further- 

 more, their relations with the lines of directions corresponding to the 

 principal mountain chains, river valleys, main lines of jointing, and 

 geological lines of boundary of the interior of the country. 



Taking the first direction, 16° 23' E., the mean of twenty-six 

 observed directions (but not necessarily the mean of all the similar 

 directions which might be observed in Ireland), it will be found that 

 this is the direction of the coast line between Carnsore Point and 

 "Wicklow Head, as exactly as possible; also the line of direction 

 between Carmel's Point, IST. W. "Wales, and St. David's Head, S. "W. 

 Wales. It also represents the boundary line limiting the granite of 

 "Wicklow, on the west side, between Castleclermot and Goresbridge, 

 and the general direction of the mountains between Kippure and 

 Achavannagh mountains, as shown on Griffith's general map of Ireland. 

 It represents, therefore, a coast line and a line of mountain direction. 

 It also represents a band, or zone, of parallel joints which must extend 

 over a considerable distance, and over a certain breadth of country. 

 If we examine the extension of this direction on the map of Great 

 Britain, it will be found to pass through the Mull of Galloway, 

 near Cairngarroch Bay, to pass at Ayr, Ben Mac Dun chain, and 

 to come out at Elgin — this direction being that of the north-east 

 coast of Scotland, between Eifeness and Peterhead. The continuation 

 of the line of direction passes through the Shetland Islands, on the 

 western side, parallel to their general or longer axis, and to the 

 geological boundary line of the southern promontory of Mainland 

 Islknd. If it be further examined, with reference to the Great Circle 

 of which it forms a part, it will be found that its extension traverses 

 the IS^orthern Ocean, parallel to a line joining Bremanger Land (the 

 most westerly point of l^orway) with Great Ice Cape (the north-east 

 point of JSTova Zembla), at about a distance of 3° 47' from those two 

 extreme points. It then crosses Siberia from the mouth of the Lena 

 to Okhotsk, where it emerges, making with the coast lines east and 

 west angles of about 70°. It traverses the North Pacific, crosses New 

 Caledonia, passes through New Zealand, from Mount Cook to Dunedin, 

 and finally cuts the west coast of Africa, from Bathtirst to Cape 

 Bojador. If we compare the extent of land traversed to that of ocean 

 surface, it will be found that these extents stand to one another some- 

 what in the ratio of 1 : 1 5 ; that is to say, that this Great Circle is one 

 of considerable contraction of the surface towards the Earth's centre, 

 and it will be easily recognised that there are not many other Great 

 Circles of greater contraction. Its importance, physically and geolo- 

 gically, has, therefore, a certain significance, lying, as the line or Great 

 Circle does, on the boundary between the Old and New World. 



The second direction, 25° 52' "W., is remarkable from more than 

 one point of view. As a coast direction it only shows itself about 

 Killiney, representing the axis of Dalkey Sou.nd, and the depression 

 between the first and second Hills, or between the Telegraph and 



E. I. A. PROC, SER. II., VOL. III. — SCIENCE. 2 A 



