C. I. Gardiner — Paleozoic HocJcs near Balbriggan. 401 



dip to the N.N.W. At the axis of this syncline the beds roll 

 slightly and small dislocations have occurred, and then for about 

 half a mile to the south-east one sees a repetition of the succession 

 one has already passed over while coming from Balbriggan. 

 Unfortunately, just where the graptolitic slates should be exposed, 

 the Palfeozoic rocks are obscured from view by a thick capping- 

 of drift, and when next Primary rocks are seen they are the grey 

 grits occurring below the graptolitic slates, and from this point the 

 shore-line follows the strike of the beds all the way to the village 

 of Skerries. Near this place are three islands showing gritty beds 

 dipping south-east at 30° and higher, while ash beds are seen as well, 

 corresponding no doubt to the ash beds between the igneous series 

 and the grits to the south of Balbriggan. 



Thus the geological evidence of this strip of coast shows that after 

 the Middle Bala period, when the Cardy Rock slates were laid down, 

 there came a gi'eat outpouring of andesitic lavas with occasional 

 scatterings of ashes on the igneous rocks. Subsequently the lavas 

 were covered by ashes, on which grits and slates were deposited. 

 These last rocks are shown by their fossils to be of Silurian age, and 

 the equivalents of the zones of Monograptus gregarius (Lapw.) and 

 M. spinigerus (Nich.) in the Birkhill shales; while higher still 

 come graptolitic bands which, if their fossil evidence be correctly 

 determined, must represent a much higher horizon and are the 

 equivalents of the Wenlock beds. 



Unfortunately, the specimens are very poorly preserved, and Miss 

 Elles states that they " resemble very closely M. Riccartonensis.'* 

 This species, she advises me, is usually found in a definite horizon 

 above the Cyrtograptus Murchisoni zone (which is at the base of 

 the Wenlock Beds), or else in the highest part of that zone. 

 Consequently, if this specific determination be correct, we have 

 between the highest fossiliferous band of the M. spinigerus zone 

 of the Llandoveries 85 feet of rock before we come to the first 

 fossiliferous band of the Wenlocks. 



In County Down somewhat similar graptolitic shales have been 

 described by Swanston.^ There the zone of M. spinigerus is covered 

 by the Portaferry Beds, with M. Biccartonensis. 



The thickness of the beds in the Balbriggan section is not easy to 

 determine, but the following approximate numbers have been 

 arrived at : — 



"Wenlock- Llandovery Beds 2,300 feet shown. 



Igneous series. 



Bala sedimentaries 1,700 feet shown. 



What the date of the diabase and other intrusions was there is no 

 evidence to enable one to find out. 



Further south at Portraiue the igneous series of andesitic lavas 

 was overlain by calcareous rocks of Middle Bala age, so that from 

 the two areas we may conclude that there was a great outflow of 

 andesitic rocks in this part of Ireland in Middle Bala times. There 



1 Report of the Belfast Field Chib, 1876-7. 



DECADE IV. VOL. VI. — NO. IX. 26 



