152 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



along the borders of the counties of Meath and Dublin *. Th6 

 alternation of lavas and tuifs is here well displayed, some of the 

 volcanic breccias being coarse accumulations in which pieces of 

 shale derived from some older part of the Palaeozoic formations are 

 abundant. 



Crossing over the broad belt of Carboniferous Limestone through 

 which the Liffey flows into Dublin Bay, we come to the great 

 continuous tract of older Palaeozoic rocks which stretches south- 

 ward to the cliffs of Waterford. Through this tract runs the huge 

 ridge of the Wicklow and Garlow granite. On the west side of this 

 intrusive mass bands of " greenstone-ash," as well as " felspathic 

 ashes," have been traced among the Silurian rocks by the Geological 

 Survey. But it is on the south-east side of the granite that the 

 volcanic intercalations are best displayed. Indeed, from Wicklow 

 Head to Dungarvan Harbour there is an almost continuous develop- 

 ment of igneous rocks, rising into rocky eminences, trenched into 

 ravines by the numerous streams, and laid bare by the waves in fine 

 coast- cliffs. It is in this south-eastern part of Ireland, comprising 

 the counties of Wicklow, Wexford, and Waterford, that the Lower- 

 Silurian igneous rocks can best be studied. I shall accordingly ask 

 your attention specially to the rooks there displayed, and the records 

 they furnish regarding the history of volcanic action. 



There are obviously various distinct centres of eruption in this 

 long belt of country. The Eathdrum and Castletimon tract forms 

 one of these. Another of less size culminates in Kilpatrick Hill, a 

 few miles to the southward. Arklow Head marks the position of a 

 third. The lavas and tuffs which set in a few miles to the south 

 of that promontory, and may be said to extend without interruption 

 to the south coast, were probably thrown out by a series of vents 

 which, placed along a north-east and south-west line, united their 

 ejections into one long submarine volcanic bank. There can be no 

 doubt that the most active vents lay at the southern end of the belt, 

 for there the volcanic materials are piled up in thickest mass, and 

 succeed each other with comparatively trifling intercalations of 

 ordinary sedimentary material. Some of these vents, as I shall 

 relate in the sequel, have been cut open by the sea along a range of 

 precipitous cliffs. 



The comparatively feeble character of the volcanic energy during 

 Lower-Silurian time in the south-east of Ireland is shown by the 

 great contrast between the thickness of the volcanic intercalations 

 * Ibid. Sheets 91 & 92, and Explanation. 



