Prof. Hull — The Volcanic History of Ireland. 207 



volcanic rocks may be taken at 2,200 to 2.300 square miles. Yet 

 this area, large as it is, is but a fragment (though doubtless a large 

 one) of the original surface covered by augitic lavas, which once 

 stretched as far south as the mountain ranges of Slieve Croob or 

 Mourne, and eastward as far as the Sperrin Mountains in the County 

 of Derry. 



The volcanic rocks of the north-east of Ireland are clearly refer- 

 able to three distinct periods of eruption — all, however, more recent 

 than the Upper Chalk — and therefore referable to the Tertiary 

 period. During the earliest of these periods, eruptions of highly 

 silicated lavas, which have solidified into trachytes, rhyolites, and 

 pitchstones, took place from several distinct vents — two of which, 

 at least, have been determined. One of these is situated west of 

 Hillsborough, amongst the Lower Silurian rocks, and the other 

 amongst the hills north-east of the town of Antrim, at a distance of 

 twenty miles from the former. At what precise period this eruption 

 of trachytic lavas took place, we have no means of knowing; but 

 as it preceded that of the overlying augitic lavas which we know 

 to have been of Miocene age, it is probable that these oldest volcanos 

 were in activity as far back as the Upper Eocene period. Thus it 

 would appear that the volcanic fires burst forth contemporaneously 

 over the north-east of Ireland, and the region of Mont Dore in 

 Central France, where the eruptions have been shown by Lyell and 

 Scrope to have taken place at this period. This earliest period of 

 volcanic activity was followed by one of, we may suppose, prolonged 

 repose, upon the close of which a second and more extended out- 

 burst of volcanic materials took place over the whole area, extend- 

 ing from the slopes of the Slieve Croob range northwards. The 

 lavas of this period are essentially augitic, consisting of dark 

 amygdaloidal basalts and dolerites often decomposed, and parted by 

 bands of bole and red ochre, which were formed by decomposition 

 of the upper surfaces of successive flows. The most southerly of 

 the vents of this period seems to have been situated at Scrabo Hill, 

 in Co. Down, on the south side of Belfast Lough, and many others, 

 mostly hidden from view, are scattered over the surface of Co. 

 Antrim. The sheets of lava poured forth from these vents in suc- 

 cessive flows attained a thickness of 500 or 600 feet, and were 

 accompanied by showers of ashes, after which there was a cessation 

 of volcanic activity ; and the second period came to a close. 



It was during this period of repose that those remarkable and 

 valuable beds of lithomarge and pisolitic iron-ore were formed, as 

 I believe under the waters of a lake, or chain of lakes, several 

 times larger than Lough Neagh. The streams which entered these 

 lakes — strongly impregnated with iron — also carried down leaves 

 and stems of plants which grew on the basaltic uplands along their 

 shores. In other places plants actually grew over extensive lagoons, 

 and beds of lignite were formed. The plants from these beds have 

 been described by Professor Harkness, F.R.S., ^ and more recently 

 by Mr. W. H. Baily, from specimens obtained from the railway 

 1 Brit. Assoc. Report, 1856, p. 66. 



