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 1s 
now known to exist between the Tremadoc and the Arenig rocks. 
LOWER SILURIAN (below of Llandeilo, above of Bala or Caradoe, 
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 8. 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 8.8.E., at 
40° to 50°, on which lies, at one place, a small thin flake 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 contemporancous trap, often porphyritic, with 
Jayers of trappean ash. Some of the trappean rocks are several 
hundred feet in thickness, This interesting spot should be 
visited at low tide, 
