ANKIVRRSIRY ADDRESS OF THE TKESIDENT. I5I 



of the two types is due to rapid and constant plication, whereby the 

 two groups of rock, neither of them of great thickness, have been 

 folded with each other in such a way that without the evidence of 

 an established sequence of fossils, or the aid of continuous sections, it 

 becomes extremely difficult to make out the stratigraphical order in 

 any district. When the ground is attacked anew in detail with the 

 assistance of such paltcontological and lithological horizons as have 

 permitted the complicated structure of the southern uplands of 

 Scotland to be unravelled, we may be enabled to tabulate the suc- 

 cessive phases of the volcanic history of the region in a way which 

 is for the present impossible. We have no palasontological evidence 

 of any Arenig rocks in the east of Ireland, nor of the top of the 

 Bala series. The volcanic history of the region is thus unmistak- 

 ably comprised within the later half of Lower-Silurian time, between 

 the beginning of the Llandeilo and the close of the Bala period. I 

 must therefore, in the meantime, content myself with this general 

 limit of geological chronology, and make no attempt to trace the 

 relative antiquity of the igneous rocks in the several districts in 

 which they are distributed. 



Viewing the volcanic region of eastern Ireland as a whole, we 

 are first struck by the feebleness of the manifestations of eruptivity 

 in the north and their increasing development as we trace them 

 southwards. At the northern end of the Silurian area in County 

 Down, thin bands of "felstone" and "ash" have been recognized by the 

 Survey, interstratified with the highly-inclined and plicated Silurian 

 rocks. As the latter are plainly a continuation of the strata which 

 have been mapped out zone by zone in the south of Scotland, the 

 volcanic intercalations may possibly lie in the same general pla-. 

 form which is there so well marked *. Far in the interior, some 

 70 or 80 miles to the south-west, a strongly-marked horizon of tuffs, 

 breccias, and coarse agglomerates runs through the counties of 

 Monaghan and Cavan f. Farther south a more important volcanic 

 centre lies on the borders of Louth and Meath, where a group of 

 lavas and tuffs may be followed for a distance of about twelve miles J. 

 Unfortunately, large tracts of Carboniferous strata now conceal the 

 Lower-Silurian rocks ; but where the latter rise to the surface be- 

 tween Drogheda and Balbriggan, several groups of felstoue-sheets 

 and tuffs are observable, indicative probably of some minor vents 



* See Sheet 49, Geol. Survey Ireland, and Explanation thereto, 

 t Ibid. Sheet 69, and Explanation of Sheets 68 & 69, pp. 9, 13, 15. 

 \ Ibid. Sheets 81 k 91, and Explanation. 

 VOL. XLVII. I 



