S98 C. I. Gardiner — Palmozoic 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 

 occurring at Balbi'iggan amongst which it was announced that 

 several species of Monograptiis had been found. 



The river Delvin runs into the sea about two miles north of 

 Balbriggan, and it is on the south side of its estuary that, coming 

 from the north, one first meets with Paleeozoic rocks, after the long 

 strij) of coast occupied by Quaternary deposits south of Drogheda. 

 The beds exposed near the Delvin are green grit, and slates dipping 

 S.S.W., and are the lowest beds observed along the coast. Near 

 Lowther Lodge black slates are seen, and a good deal of intrusive 

 diabase and two small dykes of kersantite are noticeable. Sections 

 from this latter rock show well-formed bleached micas with much 

 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 LeptcBna sericea 

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

 coming from these beds : — * 



Favosites Jihrosa. Ortliis calligramma. 



Calyinene hrevicapitata. Orthis porcata. 



Gyhele verrucosa. Strophomena alternata. 



Leptmna sericea. Strophomena 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. 



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

 Beds to the Ludlow Beds. 



2 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). 



