O'Reilly — On Jointing in Rocks about Dublin, 8fc. 143 



part of the Co. Eoscommon, the continuation of which northwards 

 represents the S. E. boundary of the same formation in the Co. Fer- 

 managh. It corresponds likewise with the western boundary of the 

 same formation in the Co. Clare, and represents the coast line of Clare 

 from Kilkee Bay to Black Head. It thus limits to the east and to 

 the west the remarkable lozenge or parallelogram formed by the N. 

 coast of Galway Bay, the course of the Shannon from Loop Head to 

 Foynes in the south, the west coast of Clare, and the boundary line of 

 the Loughrea and Leitrim mountains, composed mainly of old red 

 sandstone. In the vicinity of Dublin it is well represented by a very 

 remarkable fault, which occurs on the western side of Ireland's Eye, 

 and which is distinctly visible from the main land. This line was 

 deduced theoretically from the great circle, " Southern boundary of the 

 tertiary formation. United States," by the angle 80°, which it makes 

 therewith. The western boundary of the Roscommon old red sand- 

 stone patch, produced southwards, cuts this great cii'cle and that of 

 the W. coast of Africa at their intersection, and so far has a theoreti- 

 cal interest. 



XIII.— N. 79° 11' E., 78° 4' W., and 43° 51' W. If the direction 

 iN". 79° 1 1' E., which occurs at Malahide, be extended westward, it 

 will pass south of the boss of basalt occurring near Philipstown, 

 King's Co., will correspond with the fault marked on the Survey map 

 as occurring at the JN". E. limit of Slieve Aughty, and will cut the 

 "W. coast of Ireland at Doolin point, where it makes an angle of 40° 

 with the southern coast line of Innismore Island, the largest and most 

 westerly of the Arran Islands. This direction forms portions of the out- 

 line of the northern coast of Galway Bay. That part which passes at 

 Cashla Bay, being extended eastward, cuts the eastern coast of Ireland 

 at Laytown, running parallel to the line of junction of the lower Silu- 

 rian with the carboniferous formation. The middle fault of the Slieve 

 Aughty series is also parallel to this direction, and corresponds to the 

 lines of jointing observed at Donnybrook (478, 479). It is also pa- 

 rallel to the southern contact of the Slieve Aughty old red sandstone 

 with the carboniferous formation. The remarkable fault which runs 

 from O'Brien's Bridge to Silvermines, Co. Tipperary, and which is 

 mineralized all along its extent, corresponds also with this direction, 

 and would seem to be the continuation of the system of joints having 

 given rise to the Shannon estuary. The direction of this great Silver- 

 mines fault, being extended eastward, traverses the northern part of 

 the Queen's Co. and Kilkenny coal-field, and corresponds with the break 

 in, or contraction of, the granite range of the Wicklow and Carlow 

 mountains, occurring in the vicinity of Lugnaquilla, as shown on the 

 Geological map. Further southwards this direction shows itself in the 

 lines of contact of the old red sandstone with the carboniferous forma- 

 tion (K. coast of Dingle Bay), and of the old red and lower carboni- 

 ferous slates which run from Sheep Head to Passage, as also the contact 

 of the same rocks at Skibbercen. Finally it corresponds with the part 



