l6o PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



and grits of Upper-Silurian age. The coarseness and thickness of 

 these conglomerates, and their appearance on so many successive 

 horizons, probably indicate a time of considerable terrestrial dis- 

 turbance. It was during such a time that volcanic action broke 

 out in this region. Lavas and agglomerates were thrown out 

 together with fine tuffs. These rocks extend in a nearly continuous 

 band along the hillsides that form the north-western boundary of 

 Lough Mask to beyond Lough Nafooey. According to the mapping 

 of the Geological Survey, the volcanic rocks lie in the higher part of 

 the Upper-Silurian deposits *. My own observations in this district 

 have been too limited to allow of my offering a satisfactory summary 

 of its volcanic geology. I found in the country about Lough Nafooey 

 that the lavas consist of finely crystalline or compact porphyrites , 

 varying from a dull-green to a chocolate-purple colour, frequently 

 amygdaloidal, and becoming cellular on weathered surfaces by the 

 decay of the amygdules. The vesicular structure is not equally 

 diffused through the mass, but appears in slaggy patches, the cavities 

 being often elongated in the direction of flow. Tlaese rocks present 

 the closest resemblance externally to the porphyrites of the Lower 

 Old E-ed Sandstone. An average specimen prepared as a thin slice 

 shows under the microscope a congeries of minute lath-shaped 

 microliths of felspar with disseminated patches of iron ore. 



Intercalated with the lavas are bands of coarse agglomerate, 

 made up chiefly of large and small blocks of similar porphyrites, 

 often remarkably slaggy, and embedded in a compact matrix of more 

 finely comminuted materials of the same nature. Among the frag- 

 ments may also be observed pieces of horny felsite, red jasper, and 

 baked shale. The whole volcanic group, which includes a number 

 of successive beds of lava and pyroclastic materials, rests upon con- 

 glomerates, grits, and shales, and dips underneath fossiliferous sand- 

 stones, shales, and fine conglomerates. 



In that wild part of County Kerry known as the Dingle pro- 

 montory, traces of contemporaneous volcauic rocks are to be observed 

 at various localities and on several horizons. To the cast, near 

 Anascaul, on the northern shore of Dingle Eay, some tuffs occur in 

 what are believed to be Llandovery strata. But it is on the western 

 coast, among the headlands and coves that lie to the north and 

 south of Clogher Head, that the best sections are to be seen. The 

 succession of the rocks in this locality was well worked out by Du 

 Noyer, and the memoir prepared by him, with the general introduc- 



* See Sheets 84, 85, 94, and 95 of the Geol. Survey of Ireknd, with corre- 

 sponding Explanation. 



