Kinahan — On Geological Unconformabilities. 285 



section (Counties Tyrone and Armagh) are evidently sandstones of 

 Carboniferous age. 



Carboniferous. 



The Upper Old Red Sandstone, now known to be the lowest 

 group of the Carboniferous, usually has at its base a conglomerate 

 or breccia that rests unconformably across the edges of older 

 strata. These conglomerates nearly always comply with the recog- 

 nised formula for a typical unconformability, a conglomerate lying 

 across the upturned edges of older strata ; yet in a few places this 

 rule is not followed, as under the conglomerate there may be sand- 

 stones, or even shales ; or, as in the Co. Clare, a peculiar lime- 

 stone or quartz rock ; while in the Slieve Aughta, Co. Galway, 

 in places, both the older and later strata lie horizontally. 



Devonian. 



Rocks belonging to this group, that is, the passage or " gap 

 rocks," between the Carboniferous and the Silurian, seem to be 

 represented only in S.W. Ireland. 



In the Co. Kerry they have a well-marked basal conglo- 

 merate lying unconformably on the Silurian {Dingle Beds) ; while 

 in the Co. Cork they are a regular passage group, passing con- 

 formably upwards and downwards into the Carboniferous and the 

 Silurian [Qlengariff grits), respectively. 



Silurian. 



Some of the Silurians which are of the Lower Old Red Sandstone 

 type have a basal conglomerate lying unconformably on upturned 

 older strata, but more usually the basal boundary is more or less 

 complicated. 



At Cushendun, Co. Antrim, the basal conglomerate lies uncon- 

 formably on metamorphic rocks. 



In the Pomeroy district, Co. Tyrone, the boundary between 

 them and the Ordovicians is so obscure that it has not been 

 satisfactorily determined ; while at Lisbellaw, Co. Fermanagh, 

 there is a massive basal conglomerate, as pointed out in a previous 

 communication. 



