Cole — The Problem of the Bray Series. 9 



Wicklow and Dublin district, and traversed the Bray Series in the region of 

 the Irisli Sea. The Ordovician and Gotlandian covering of the granite core 

 moved north-westward, and the upper part of the Bray Series, sliced off from 

 its foundations, followed across the flanking slates, and even across the 

 metamorphic aureole.^" The north-western front of the overthrust complex, 

 and the scarp that probably was formed by it, disappeared through denuda- 

 tion in early Devonian times. It is interesting to reflect that some of the 

 quartzite pebbles in the Old Red Sandstone cougiomerates at Portraine and 

 west of Rathcoole may have been derived from a block of the Bray Series 

 towering high above the surface revealed to us at the present day. 



Denudation, however, continued. Even in early Carboniferous times 

 granite and schist were exposed, and their fragments are seen in the lime- 

 stone at Eathfarnhani and Blackrock. The Bray Series, in its new location, 

 became truncated and dissected. Its edge was worked back eastwards, and at 

 some epoch, pre-Oarboniferous or post-Carboniferous, the boss of Carrick- 

 gollogau was, by subaerial action, severed from the block of Bray. 



One consideration leads on to another. What becomes of the granite of 

 Dunleary and Killiney when it enters the estuary of the Liffey ? The great 

 bar of crystalline rock, continuous down to the valley of the Nore, rises here 

 500 feet (150 m.) above the sea, under which it disappears. Its north-easterly 

 strike aims across the bay, and, if we continue the margins of its outcrop, we 

 embrace the crumpled block of Howth. Is Howth also a klip, and do the 

 granite and the flanking Ordovician strata pass beneath it, reappearing on the 

 north in Portraine, at Balbriggan, and in Eockabill ? 



The low-lying country between Dublin and Galway has always suggested 

 a band of weakness ; and the north coast of Galway Bay has all the appear- 

 ance of a fault-line. Hence the Leinster granite may well be cut off by a 

 Caledonian fault in Dublin Bay. But the searchlight of geological imagina- 

 tion may play even on tectonics. It must be confessed that a boring of 

 2,000 feet, starting on the Sutton strand, might prove more convincing and 

 effective. 



In this paper I have not touched the question of the southern boundaiy 

 of the Bray Series near Wicklow town, or the long ridge of Carrick Mountain, 

 whicli resembles Carrickgollogan in its strike and form. It is clear that the 



^"This suggestion receives very welcome support from Prof. Olaf Holtedahl's recent 

 paper on "The Scandinavian 'Mountain Problem'" (Quart. Journal Geol. Soc. 

 London, vol. Ixxvi, p. 387, 1921). Holtedahl reasons that, in the long time over which 

 the Caledonian defoi-mation extended, an igneous mass, the intrusion of which occurred 

 in the earlier part of the period of crust-movement, may have become " dead," and even 

 subject to weathering, before the final thrusting took place. 



R.I. A. PROC, VOL. XXXVI, SECT. B. [C'\ 



