On the Physical Geology of the Neighbourhood of Dublin. 137 



trusive dyke, already mentioned, of coarse crystalline diorite, a 

 rare feature in that neighbourhood. 



Disturbance and Denudation. — Owing to the confused 

 condition of the strata the relation between the Cambrians and 

 the immediately superincumbent rocks is very obscure; so 

 much so that in this neighbourhood it could not be determined ; 

 but in the country south of the Devil's Glen, Co. Wicklow, in the 

 hills near Ashford, it is seen that the Cambrians had suffered 

 considerable disturbance and denudation before the overlying 

 strata were laid down unconformably upon them. The actual 

 junction of the two formations can be seen in Pollshone Harbour, 

 north of Cahore Point, and in Bannow Bay, both on the coast 

 of Wexford. The overlying rocks are Lower Silurian, of 

 Llandeilo and of Caradoc age. It seems most probable that the 

 disturbance and denudation were contemporaneous with the 

 similar actions which produced the unconformability which is 

 now known to exist between the Tremadoc and the Arenig rocks. 



Lower Silurian (below of Llandeilo, above of Bala or Caradoc, 

 age, " Upper Cambrian " of Sedgwick). — The rocks of this 

 formation here consist of thin-bedded, black, and grey, some- 

 times greenish, rarely purple, clay slates and fine greenish, and 

 dark grey, grits, with, very rarely, beds of limestone (while 

 purple slates, rare in this formation, are common in the Cambrian, 

 black slates, common in this formation, have not been found in 

 the other). The thickness of the strata in this district is unknown, 

 but it must be many thousand feet ; in S. Wexford it must be 

 10,000 or 12,000 feet. 



Around Balbriggan, beyond the northern boundary of the 

 map, there is an exposure of this formation, about 40 square 

 miles in area, the southern extremity of which just comes within 

 the limits of the map. Shenick's Island, the largest of the 

 Skerries, is composed of beds of this formation dipping S.S.E., at 

 40° to 50°, on which lies, at one place, a small thin Hake of nearly 

 horizontal beds of conglomerate and sandstone, belonging, if not 

 to the Old Red Sandstone, yet to the base of the Carboniferous 

 formation. The Lower Silurian grits and slates are interstratified 

 with beds of contemporaneous trap, often porphyritic, with 

 layers of trappean ash. Some of the trappean rocks are several 

 hundred feet in thickness. This interesting spot should be 

 visited at low tide. 



