TKANSACTIONS OF SECTION C. GOO 



MONDAY, SEPTEMBER 15. 



The following Papers were read : — 



1. On the Structure of Ireland. 

 5y Professor Grenvillk A. J. Cole, F.G.S. 



The more prominent phases of the geological history of Ireland were pointed 

 out, mainly as an explanation of the existing scenic features of the country. 

 Probably very little remains in Ireland of the old Iluronian continent, unless 

 portions of it have appeared again in the cores of Caledonian folds. The stratified but 

 metamorphosed ' Ualradians ' of the west may be Cambrian, or older ; and gneiss 

 is included in the granite of Eastern Tyrone, the latter being probably of Cale- 

 donian age. The gneiss of the ancient moorland between Omagh and Cookstown 

 is, moreover, very probablyprae-Cambrian. The Silurian sea must have covered all 

 the Irish area ; and the subsequent Caledonian folding, with its axes running north- 

 east and south-west, marked out the first distinct lines of the existing country. The 

 arches became filled with molten rock as they rose, and denudation has again and 

 again exposed in them a core of granite. To this folding we owe the guiding lines of 

 Donegal, Sligo, and Mayo; the axis of Newry, which reaches from the sea — and, 

 indeed, from Scotland — down into the midland counties ; and, above all, the long 

 mass of the Leinster Chain, the most important feature of South-eastern Ireland. 

 The granites weather into round-backed moorlands ; the schistose foothills give rise 

 to picturesque ridges and ravines upon their flanks. In the Dublin area, between 

 the foothills and the sea, quartzites and slates, usually regarded as Cambrian, have 

 added the prominent features of Howth, Bray Head, and the two Sugarloaves to 

 an already diversified landscape. 



The Old Red sandstone lakes spread across the hollows of the Caledonian 

 continent, to be succeeded by the inflow of the Carbonifei'ous sea. The Lower 

 Carboniferous beach-deposits are now found on the summits of west Irish 

 mountains, and very little of the country can have escaped submergence. The 

 rlercynian folding produced the second series of structural lines, assisted by the 

 varying resistance of the Old Red sandstone and the Carboniferous limestone to 

 denudation. The east-and-west anticlinal ridges, from the Atlantic to the Irish 

 Channel, with intervening valleys, where the limestone is protected in the 

 synclinals, repeat on a bold scale the structure of South "Wales and Belgium. The 

 folds swung round in the neighbourhood of the pre-existing Leinster Chain ; and 

 the axis of the Kilkenny coalfield, where the coal measures remain on a high 

 synclinal, runs north-east and south-west, like its Caledonian neighbour on the east. 

 The great limestone plain itself is probably to be looked on as a vast shallow 

 synclinal of the same epoch, into which, in later periods, the Caledonian and 

 Hercynian ridges poured down their detritus. 



Marine Permian beds occur near Stewartstown, south-west of Lough Neagh, 

 and also in the north of Co. Down. The terrestrial conditions of the British 

 Trias were continued into the Irish area, but the remaining beds of this period 

 all lie north of Dublin, and mostly owe their preservation to the capping of 

 Cainozoic basalt. The Rhretic sea penetrated as far west as the Caledonian hills 

 of Londonderry, and marine conditions continued during early Liassic times. An 

 uplift then probably occurred, and the sea did not return till the middle of the 

 Cretaceous period. The ' White Limestone,' which forms so distinctive a feature 

 of the Antrim coast, represents the English Chalk. 



The great feature of the north is, however, due to volcanic eruptions of Eocene 

 age. Owing to the immense outpouring of basalt across the uplifted Cretaceous 

 and earlier strata, the counties of Antrim and Londonderry include high igneous 

 plateaux, cut by deep valleys in which the underlying rocks are seen. The land- 

 scapes close around I3elfast reveal the structure of the country in perfection. In 

 the west of Ireland dykes of basalt, running characteristically north-west and 

 south-east, are so frequent as to show that the plateaux once prevailed from Down 

 to the Atlantic coast. Sporadic eruptions occurred even in the Galway area, and 



1902. R R 



