148 Prof. SuU — The Volcanic History of Ireland. 



and geologists are now becoming familiar with such expressions as 

 volcanic rocks of Silurian, Carboniferous or Permian periods, and 

 which, from the necessity of the case, must chiefly be the represen- 

 tatives of the sub-marine or sub-lacustrine classes of volcanos. 



Guided, then, by such principles of determination, let us endeavour 

 to review in the order of succession the periods which in this 

 country present us with unquestionable evidences of volcanic 

 activity, and we shall consider them under the three great divisions 

 of geological time, the Palaeozoic, the Mesozoic, and the Cainozoic 

 or Tertiary. 



PalcBozoic Epoch. — The oldest rocks represented in Ireland — the 

 Cambrian beds of the east coast— present us with no evidences of 

 contemporaneous volcanic activity ; but when we ascend into those 

 of the Lower Silurian period, we have numerous examples of 

 volcanic rocks in several parts of our island where these formations 

 occur. Throughout the range of these beds in the counties of 

 Wicklow, Wexford, and Waterford, there are numerous sheets of 

 felstones and porphyries accompanied by beds of ash and volcanic 

 breccia, interstratified with the slates and grits which for the most 

 part make tip the Lower Silurian rocks of those districts. These 

 contemporaneous traps are referable to the acidic felspathic, or highly 

 silicated felstone group ; and amongst them are outbursts of horn- 

 blendic rocks, or diorites, as well as a few melaphyres, which are of 

 intrusive origin and later date than the felspathic rocks themselves. 

 The districts of Stradbally and Kill, near Waterford, seem to have 

 been foci of volcanic action. Similar beds of porphyries, felstones, 

 ashes, and agglomerates are of frequent occurrence in the Silurian 

 district north of the valley of the Boyne, but become less frequent 

 in the districts of Down, Armagh, and Cavan. As all the fossils 

 yielded by these Lower Silurian rocks are of marine origin, we must 

 suppose the whole series to be marine ; and we may thirefore infer, 

 that the volcanic rooks of the period were erupted from vents 

 sporadically breaking out over the sea-bed of the period, and either 

 pouring forth sheets of liquid lava, or vomiting out masses of frag- 

 mental materials which were strewn over the bottom, and were in 

 turn covered over by fresh sediment. 



It is interesting to observe that these rocks are the representa- 

 tives of those great sheets of felstone and porphyry which rise into 

 the lofty escarpments of North Wales, and which have been so ably 

 described b}'^ Sedgwick, Murchison, and Ramsay ; and I am informed 

 that the late Professor Jukes, when engaged with Mr. Du Noyer in 

 mapping out these beds over the Silurian district of the south-east 

 of Ireland, was constant!)' struck by the marked resemblance of the 

 old volcanic rocks of this district to those with whicli he had become 

 familiar amongst the mountains of Snowdon, Moel Wynn, and Cader 

 Idris. 



■ Contemporaneous trap-rocks being so abundant amongst the Lower 

 Silurian rocks of the east of Ireland, where they are generally un- 

 altered, it can scarcely be doubted that they are also represented 



