138 



STKATIGKAPHICAL GEOLOGY 



a conformable passage from the Cambrian into the Ordovician, 

 though the beds along the line of junction are everywhere so 

 mingled together by subsequent plication and crushing that no 

 definite boundary could be drawn between them. The lowest 

 Ordovician Beds are a set of banded slates and fine grits which are 

 generally known as the Eibband Series. They have yielded 

 fossils near Arklow, Courtown, and Kilrea, which included species 

 of Tetragraptus, Phyllograptus, and Didymograptus, and a Bryo- 

 graptus like Kjerulfi. Hence they appear to represent a Tremadoc 

 horizon as well as the Arenig and Llanvirn Series. 



Above the Ribband Series are black slates which contain a 

 graptolite fauna of Llandilo age, i.e. Dicranograptus Nicholsoni, 

 D. ramosus, Ccenograptus gracilis, Diplograptus mucronatus, and 



Lane. 



Chair of Kildare. 



Chair Fa7'm. 



Fig. 43. SECTION ACROSS THE CHAIR OF KILDARE (Reynolds and Gardiner). 

 Horizontal scale, 6 inches to a mile. 



Leptograptus flaccidus. These beds are succeeded by shales and 

 limestones of Bala age which have yielded many fossils. 



At Portraine, on the coast north of Dublin Bay, there is a 

 small inlier of Ordovician, and the cliffs show an interesting, 

 section of Bala deposits. The succession has been described by 

 Messrs. Gardiner and Roberts, 18 and consists of the following ; 

 (1) andesitic lavas, (2) coarse ashy conglomerate, (3) calcareous ash 

 beds with layers of limestone and shale, (4) limestones full of 

 corals, (5) compact grey limestone with many trilobites and 

 brachiopods, including Illcenus Bowmanni, Staurocephalus sp., 

 Trinucleus seticornis, and Sphcerexochus mirus. Thus it appears 

 that after a series of volcanic eruptions in the near vicinity, the 

 water became clear and suitable for the growth of corals, which 

 produced layers and beds of coralliferous limestone, but nothing at 

 all resembling a coral reef. Some change then occurred, probably 

 subsidence, causing a deepening of the water, in consequence of 

 which the corals died out, and their place was taken by trilobites 

 and brachiopods, whose remains are the chief components of the 

 upper limestone. 



A more extensive exposure of the series is found in another inlier 



