32 



The Irish Natiwalist. 



were themselves upheaved, and formed island-masses, round 

 which new strata were unconformabh' laid down. The fossils 

 of this second system of rocks prove it to be of Ordovician 

 age, the term ''Ordovician" being now generally adopted in 

 place of the much disputed "Lower Silurian" cmplo3^ed by the 

 Geological Sun'C}^ and many text-books. We cannot enter 

 here into the long war of words which made both the terms 

 "Upper Cambrian" of Sedgwick, and "Lower Silurian " of 

 Murchison distasteful to disinterested combatants. Professor 

 Lapworth has fortunatel}' given us a name which both parties 

 can accept with dignit}'. 



The Ordovician system includes, in the home of the ancient 

 Ordovices, (i) the Arenig series, with its huge volcanic masses, 

 Arenig-fawr, Cader Idris, and the Arans ; (2) the Llandeilo 

 series, with the famous Ffestiniog slates, and with some continu- 

 ance of volcanic deposits ; and (3) the Bala series, with richly 

 fossiliferous limestones, and an enormous outpouring of tuffs 

 and lavas, to the hardness of which we owe Snowdon, and 

 many other precipiced mountains in North Wales. 



In County Dublin the Ordovician rocks have been altered 

 on the south b}^ the invasion of the granite ; but at Skerries, 

 in the north, the}' come out clearl}' below the Carboniferous, 

 and crop out again in the interesting promontor}- of Portraine. 



Fig. 4. 



Lanibay Island from Portraine. Irregularly disturbed limestones and 

 ^ shales on the headland in foreground. 



The fossil species show that this Portraine series is of Bala 

 age. The coast is easily struck east of Donabate station ; and 

 the oldest rocks exposed lie directly opposite Lambay Island. 

 Here a mass of gre}' compact limestone, representing an old 



