338 Proceedings of the Royal Irish Academy. 



LV. — EuKiTEs, OR Basic Felstoiies of Silitrian Age. By G. H. 



EjNAHAj^ AIS-D GEPaiAED A. KlKAHAN. 



[Eead, April 26, 1880.] 



Ix the Irish Sihirians, that is, including under this name both the 

 rocks containing the typical Silurian fossils, and also those commonly 

 called Lower Old Red Sandstone (the " Glengariii grits," and "Dingle 

 heds " of Jukes), there are felstones Tvhich seem to he typical of the 

 formation. They are basic, the "Eurites" of Daubuisson (fromewreo, 

 to flow easily, on account of the facility with which they melt before 

 the blow-pipe), and belong to the "Hybrid rocks" of Durocher. 

 The geological position of the Silurian eurites is fully given in the 

 Report on the Rocks of the Ballaghaderreen and Pintona districts. 



In Munster, the granitic roots of these rocks have not been ob- 

 served, although in places in the intrusive portions they partake more 

 or less of elvan characters, as they contain free quartz ; in Galway, 

 however, they can be traced down into a true granite, and in 

 IS". E. Mayo into elvan ; while in Tyrone there are granites and elvans 

 which are evidently the granitic roots of the interbedded eurites. 

 "WTiat may be an important character in these rocks is the presence of 

 carbonates of iron and lime ; sometimes abundantly. In this Report, 

 it is proposed to describe some of the euritic rocks of Kerry, I^. W. 

 Galway, and S. "W. llayo. 



We are indebted for valuable aid to Professor O'Reilly, M.R.I. A., 

 of the Royal College of Science, who allowed us the use of his well- 

 arranged mineralogical laboratory and his microscopes, besides free 

 access to his excellent type collections of rocks, minerals, and micro- 

 scopic slides. 



During our examination we have consulted several works on the 

 microscopical study of rocks, especially those of E. Rutley, on The 

 Study of Rocks ; F. Zirkel, Microscopic Fetroyra])hy ; and F. Fouque, 

 Santorin et ses e?'iiptions. 



Paut I. — Rocks of Kerry, N.JF. Galway, and 8.W. Mayo. 



In the counties Kerry and Cork, south of a line drawn east and 

 west along Dingle Bay, the Silurian rocks are of the type called by 

 Jukes " Glengariff grits." In these rocks there are masses of eruptive 

 rocks appearing in the vicinity of Valentia harbour, and southward of 

 Killarney, in the neighbourhood of Lough Guitane, between Man- 

 gerton and Glenflesk. 



In the vicinity of Yalentia harbour the majority of the eruptive 

 rocks are so basic that they must be classed as either gabbros or 

 dolerites ; but those near Lough Guitane are eurites of Daubuisson, 

 and belong to the typical basic felstones of the Irish Silurians. 



