Prof. Hull — The Volcanic History of Ireland. 205 



another independently of their isomorphism — which is not thorough 

 — are nearly always found together. 



It will be seen that the majority of the minerals examined are 

 species that are most often found in nature forming component parts 

 of igneous rocks, while two rocks of that class are themselves in- 

 cluded. 



IV. — The Volcanic History of Ireland. 



Being the Anniversary Address delivered to the Eoyal Geological Society of Ireland, 

 February 11th, 1874. 



By Professor Edward Hull, M. A., F.R.S., President; 



Director Geological Survey of Ireland. 



{Concluded from page 150.) 



(PLATE X.) 



Carboniferous Period. — The Lower Carboniferous rocks, both of 

 the North of England, of Scotland, and of Ireland, afford examples 

 of contemporaneous volcanic action of considerable intensity. The 

 so-called " toad-stones " of Derbyshire, and the great sheets of 

 melaphyre, porphyrite, and ashes of the central valley of Scotland, 

 forming the Kilpatrick, Campsie, and Dairy Hills, appear to have 

 been erupted over the bed of the same sea as that in which were 

 poured out similar materials in County Limerick, forming the well- 

 known Carboniferous volcanic rocks of " the Limerick Basin." 

 These rocks have been already so fully described by several 

 observers, that I shall confine myself to a very short description, 

 such as is essential to the brief history of volcanic action which I 

 am here endeavouring to draw up.^ 



The trap-rocks of the Limerick basin, ably described by Professor 

 Apjohn, belong to at least two consecutive periods of eruption ; 

 both, however, included in the Carboniferous Limestone period, 

 and are disposed in concentric sheets round a tract of Upper Lime- 

 stone shale which occupies ' a district around Ballybrood. The 

 " Lower trap-band " is more important and continuous than the 

 Upper, and along with beds of ash and agglomerate is found in 

 several tracts to the north and west of the band itself. In several 

 spots both to the north and south of the " Lower trap-band," are 

 isolated bosses of trap, such as those of Knockdirk, Carnanagh Fort, 

 Cullon, and Maddyboy, which may be regarded as the old " necks " 

 or volcanic throats, from which the sheets of trap and ashes have 

 been erupted or blown out, but from which they are now discon- 

 nected by denudation. 



As regards the composition of these rocks, notwithstanding the 

 variety of names they have received, microscopic examination shows 

 them to belong, with few exceptions, to that series of old augitic 

 lavas usually called " melaphyres," which have undergone consider- 

 able alteration from their original condition. The base is for the most 



1 The composition of the Knockdirk trap is essentially so different from that of the 

 other masses, that I suspect it to belong to an older period of eruption. See author's 

 paper on " The Microscopic Structure of the Limerick Trap-rocks," Geological 

 Magazine, 1873, Vol. X. p. 153, PL VIII. 



