Haigh — Carboniferous Volcanoes of Philipstoion, King's Co. 23 



difference noticeable in the rocks of this higher member of the limestone 

 group [Middle limestone], it seems obvious that corresponding differences of 

 an interesting character prevailed in the conditions of contemporaneous deposi- 

 tion. There is little doubt that the sudden change, and diversity in character 

 and contents, in the higher series, was accompanied, if not caused, by the 

 commencement and continuance for a time of volcanic activity in the 

 region." 



The movements which occurred immediately preceding or during the 

 volcanic eruptions may have caused the local undulations in the sea-floor, 

 and, as suggested for the Limerick district, may have caused the abrupt 

 alteration and diversity of conditions which gave rise to the deposition of the 

 argillaceous type of limestone. 



The ash of the Croghan Hill district bears a close resemblance to much 

 of that which occurs in the Limerick area, and is similar to that of 

 Carboniferous Age in Scotland and central England. 



It is of a pale colour, very uniform in character, and contains much 

 calcareous material. 



Another rather striking feature, which tends to connect the Limerick and 

 Croghan Hill volcanic areas, is that they lie on a line which is parallel to the 

 Caledonian axes of folding, which played so conspicuous a part in the topo- 

 graphy of the country. It is not a little remarkable that such a line marks 

 the boundary of the rocks of Atlantic affinities, occurring in the Lower 

 Carboniferous series in Great Britain and Ireland. 1 This line passes just 

 south of the Scottish carboniferous volcanic areas, and the series of rocks 

 both in Ireland and Scotland belong to a province of a distinctly Atlantic 

 type. In the south of Scotland the Atlantic type is overlapped by a series of 

 dykes and sills of late Carboniferous age, or even later, consisting of sub- 

 alkaline rocks, which belong to a Pacific province ; but no such type of rock 

 has been found in the Carboniferous intrusions in Ireland. 



Description of the Volcanic Bocks. 



The igneous rocks occurring in the district are roughly divisible into two 

 main types, which can be classed as pyroclastic and intrusive. The pyro- 

 clastic rocks are breccias and tuffs. The fragmentary material composing 

 these breccias and tuffs consists mainly of basic lapilli, which were blown out 

 of the vent, being in a liquid form at the time of the explosion, since this 

 glassy material contains oval, much elongated, and tubular bodies produced 

 by the drawing out of steam-cavities, most of which have now become filled 



'■ A Harker, " Natural History of Igneous Rocks," 1H09, p. 107. 



