Kinahan — On Irish Arenaceous Rocks. 519 



SILURIAN and DEVONIAN. 



[Except in south-west Ireland (Cork and Kerry), these rocks seem to be rather 

 mixed up. The Devonian proper are the equivalents of the "Lower Old Red Sand- 

 stone, ' ' or Passage beds between the Silurian and Carboniferous ; but in many places, 

 either stratigraphically or lithologically, it is hard to determine whether the rocks 

 should be called Devonian or Silurian, as the lower beds of the Silurian (Smerwick beds), 

 the upper beds of the Silurian (Dingle beds), and the Devonian, are all, lithologically, 

 more or less identical. Their exact age, therefore, cannot be positively stated, except 

 in such places as Cork and Kerry, where good continuous sections across the strata are 

 exposed (see Kerry, p. 567). The lower rocks in the Silurian are usually reddish, or 

 purplish, and over these are light-coloured fossiliferous rocks (shades of grey, green, 

 and blue) ; but still higher up on these, in all the Irish tracts, there are rocks more or 

 less similar to those below. Hereafter, in these descriptions, the reddish rocks will be 

 called of the "Old Eed Sandstone type," and the lighter- coloured rocks "Typical 

 .'Silurians."] 



In some of the new maps there has been a curious dividing up 

 of the Silurians : this is especially conspicuous at Lisbellaw, Co 

 Fermanagh. This is an interesting locality, as the condition under 

 which the " Lisbellaw Conglomerate " accumulated, must have 

 been very identical with what is now going on at the Chesil Bank. 

 In Lyme Bay the " flow-tide " current runs from the westward ; 

 and this current, accelerated by the wind-waves, carries the Chesil 

 Beach along with it, to be accumulated in the bight behind, or west- 

 ward of, Portland Bill, which acts as a groyne. Chesil Bank, or 

 lbeach, becomes coarser and larger as it is followed east, till it forms 

 a massive heap of shingle to the west of the Bill ; but eastward of 

 the Bill, in Weymouth Bay, there are finer accumulations. In 

 Silurian times similar forces were at work in the neighbourhood of 

 Lisbellaw. Running north-eastward from Lisbellaw was a coast- 

 line, while west of the village there was a spit, or " Bill," of Or- 

 dovician, and west of the latter a bay. Along the north-east and 

 south-west shore the " flow-tide " current ran south-west to Lis- 

 bellaw, the shore accumulations increasing in magnitude and 

 coarseness from the north-east towards the south-west. Thus we 

 find at the north of Lough Eyes their conglomerates lying un- 

 conformably on the Ordovician ; to the south-west is the massive 

 " Lisbellaw Conglomerate " accumulated against the Ordovician 

 spit, that acted as a groyne ; while in the bay, west of the latter, 

 sandstones and shales accumulate. Thus, there is a parallel in 

 both places, as along the shore-lines the beach gets coarser and 



