690 t ME. P. E. COWPEE EEED OX THE [NOV. I9OO, 



IV. The Age op the Rocks. 



In the Geological Survey Memoirs all these igneous rocks, with a 

 few trifling exceptions, are attributed to the Lower Silurian, as are 

 the sedimentary and fossiliferous beds which they pierce. It seems 

 to have been assumed that the great majority were contemporaneous 

 lavas and tuffs, but the evidence now brought forward shows that 

 this generalization is incorrect. Sir Archibald Geikie 1 recently 

 suggested that they belonged to various periods, and were mostly 

 intrusive. The diabase-sheets and elvans have always been recog- 

 nized as intrusive, and therefore of later date than the rest. 



It is possible now to assign a more definite date to many of the 

 intrusive rocks. It is obvious that most are post- 

 Ordovician, though of some this cannot be positively asserted 

 (see p. 660). Prom the following considerations it is indicated 

 that they are also all pre-Carbonif erous. Firstly, the Old 

 Red Sandstone, wherever its relations to them are exposed to view, 

 is seen to rest unconformably upon them, and its basal breccias 

 contain fragments of them. 



Secondly, the Old Red Sandstone of Waterford does not contain 

 any contemporaneous interbedded igneous rocks, and shows no sign 

 of having been deposited during a period of vulcanicity. It 

 should, however, be remembered that the Old Red Sandstone of 

 Waterford is attributed to the upper part of that formation, while 

 the Lower Old Red Sandstone is entirely absent and was probably 

 never deposited over this area. On the west, however, where the 

 Lower beds are present, felsites and ashes are interbedded among 

 the latter, 2 and it might be contended that some, at any rate, of the 

 Waterford felsites are of this age. Waterford was at this time a 

 land-area, judging from the coarse littoral deposits at the base of 

 the Upper Old Red Sandstone and the evidence for the subsequent 

 gradual eastward spread of the waters due to submergence. There 

 is nevertheless no direct proof that any volcanic outbursts took 

 place on land or over the Waterford district in Lower Old Red 

 times. 



Thirdly, since no felsitic or other igneous rocks pierce the rem- 

 nants of the Old Red Sandstone in the county, except two or three 

 so-called ' greenstone '-intrusions on the Reeks of Glenpatrick 

 (marked on the Geological Survey Map, Sheet 167, but which I 

 have not had the opportunity of examining), the natural inference is 

 that all the igneous rocks were of earlier date. Of course, this 

 cannot be asserted positively, because so little of the original 

 covering of Old Red Sandstone is preserved, but the main mass as 

 well as the detached outliers along the coast furnish no contra- 

 dictory evidence. 



Partner to the west and north-west of County Waterford there are 

 well-known instances of Upper Old-Red-Sandstone and Carboni- 

 ferous vulcanicity around Limerick and Bantry Bay 3 ; but there is 



1 ' Ancient Volcanoes of Great Britain ' vol. i (1897) p. 247, etc. 



2 Ibid. p. 346, and references there. 



3 Ibid. vol. i, p. 348, & vol. ii, pp. 41, 49. 



