Trof. Hull — The Volcanic History of Ireland. 149 



amongst the beds of the same great series in the west and north- 

 west of the island, where they have undergone, for the most part, 

 various degrees of metamorphism. In the Galway district, Mr. 

 Kinahan considers that such rocks may be recognized amongst the 

 hornblende schists of the metamorphosed districts ; ^ but while the 

 general conclusion may be admitted, the great amount of alteration 

 which these rocks have undergone, together with their structural 

 complications, renders actual determination in special cases some- 

 what uncertain. 



The Lower Silurian period may, therefore, be considered as having 

 been one of great volcanic activity in this country, as well as in 

 Wales and Cumberland ; the products being, for the most part, 

 felspathic and highly silicated, and such as were erupted at various 

 points over the bed of the sea. 



Upper Silurian Period. — The Upper Silurian rocks are not very 

 largely represented in our island; nevertheless, they present un-. 

 mistakable evidences of contemporaneous volcanic action. Nothing 

 can be more conclusive on this head than the phenomena observable 

 in the mountainous region of West Mayo and Galway. Here, on 

 ascending the flanks of Muilrea from the north shore of Killary 

 harbour, we cross over the edges of great beds of gi'it, slater, and 

 conglomerate, alternating with others of quartz -porphyry, each bed 

 capped by its covering of volcanic ash; as if showing, that after 

 each successive outpouring of lava, there had been a grand eruption 

 of fragmental materials from the volcanic vents. In all probability 

 these vents, like those of the Lower Silurian period, were sub- 

 marine, nor is there any very marked distinction in the general 

 character of the volcanic products. 



Another district of volcanic action of this period is that lying 

 along the western shores of Lough Mask, where beds of felstone, 

 quartz-porphyry, felspathic ashes, and agglomerates are associated 

 with fragmental strata, supposed to belong to the Upper Llandovery 

 series, but which may also represent the earlier stage of the Muilrea 

 volcanic rocks.^ 



The Promontory of Dingle, where it projects into the Atlantic 

 Ocean, together with some of the Blasket Islands, affords evidence 

 of having been another district of volcanic action during the Upper 

 Silurian period. Great beds of ash, agglomerate, and a few of trap 

 form the coast from Clogher Head southwards, for a considerable 

 distance, and again are found at Beginish, Young's, and Inishvickillane 

 Islands, and have been carefully laid down on the maps of the 

 Geological Survey by the late Mr. Du Noyer.^ The thickness of 

 these volcanic products has been estimated by the late Professor 

 Jukes at 2,500 feet, and they are iaterstratified with sedimentary 

 strata richly stored with marine fossils of the Wenlock stage. As 

 the beds of felstone and ash of Muilrea probably correspond to this 



1 Mem. Geol. Survey, Explanation Sheets, 105 and 114. 

 ^ See Geological Survey Map, Sheet 85. 



3 Sheet 171. See "Explanation" of Sheets 160, 161, 171, etc., by Messrs. Jukes 

 and Du Noyer, pp. 12, 46, etc. (1863). 



