398 C. I. Gardiner — Palceozoic Rocks near Balhriggan. 



IV. — The Silurian and Ordovician Eocks exposed on the 

 Shore near Balbriggan, County Dublin. 



By Charles Irving Gardiner, M.A., F.G.S., Cheltenham College. 



BALBEIGGAN is a village in County Dublin, some nineteen 

 miles north of Dublin, and along the coast in its neighbourhood 

 Ordovician and Silurian^ rocks are exposed for a distance of about 

 five miles. These are described in the Irish Survey Memoirs,^ but 

 from a recent investigation of the locality it seems desirable to give 

 a fresh account of its geology. 



The rocks exposed seven miles to the south, at Portraine, have 

 been recently examined and found to be, in the main, of Middle or 

 Upper Bala age,^ while some unfossiliferous grits were doubtfully 

 assigned to the Silurian period, owing to their similarity to the grits 



ERRATUM. 



P. 399, ith paragraph, 3rd liue : for " we have not got," read " we have noiv got." 



„^,,=„ ^^^v, iciDTOL xuon. ouuvv weii-iuimeu Dieacnea micas witn mucti 

 decomposed felspars. Quartz, calcite, and chlorite have been met 

 with as secondary minerals, while a good deal of apatite occurs. 



Opposite the Cardy Eocks, olive - brown slates occur, much 

 veined with diabase intrusions, and in these slates Zeptmna sericea 

 was found, while the Survey Memoir gives the following fossils as 

 coming from these beds : — * 



Favosites fibrosa. Orthis calligramma. 



Calymene brevicapitata. Orthis porcata. 



Gybele verrucosa. Stropliomena alternata. 



Leptcena sericea. Stropliomena deltoidea. 

 Orthis Actonice. 



These fossils mark the beds as being of Bala age. 

 . The beds now curve round so as to dip S.E., and after a small 

 fault a few bands of brown slates are seen and a garnetiferous ash, 

 and then for nearly two miles the shore is occupied by a series of 

 igneous rocks. These are, in the main, andesites, sections of the 

 rocks showing much decomposed plagioclase felspars in a ground- 

 mass chiefly made up of secondary chlorite and calcite, with 

 numerous small felspar microlites. Amygdales are fairly common, 

 and are generally composed of calcite and chlorite. 



^ The terra Silurian is used in this paper to include rocks from the Llandovery 

 Eeds to the Ludlow Beds. 



"^ Memoir to accompany Sheet 92 (1871), Gaol. Surv. Ireland. 



3 Q.J.G.S., vol. iii (1897).' " 



* Memoir to accompany Sheet 92, p. 21, Geol. Surv. Ireland (1871). 



