On the Physical Geology of the Neighbourhood of Dublin. 139 
W. by S. from Dublin, has afforded fossils of the same type. The 
great probability is that much of the unfossiliferous portion of the 
formation is of the same age as the fossiliferous, though the lower 
part of it may be, as some of it certainly is, of Llandeilo age. 
The general strike of the beds, throughout the area now in ques- 
tion, is N.N.E. and S.S.W.; this obtains on both sides of 
the granite exposure, the longest axis of which has nearly the 
same direction. All along the sides of the granite, the rocks are 
changed into mica slate. | 
. Besides the contemporaneous felstone of Portrane and that of 
Lambay, already mentioned, there are some sheets of felstone 
porphyry near Bohernabreena, which are most probably contem- 
poraneous ; being interbedded with the Lower Silurian strata. 
There are also masses of basalt and dolerite at Ballynascorney, 
which are probably intrusive; though rudely conforming to the 
strike of the slates, &e. These two places are at the mouth of 
the interesting valley of Glennasmole, three or four miles S. of 
Tallaght. We may here mention that, in Wicklow and Wexford, 
of the long ranges of igneous rocks, whose trend corresponds gene- 
rally with the strike of the Lower Silurian strata, the felstones 
are usually contemporaneous, the others principally intrusive. 
GRANITE.—As we are following chronological order, we must 
now turn our attention to the granite, before proceeding to the 
next sedimentary formation. The granitic exposure of this 
neighbourhood, which is the largest continuous one in the British 
Islands, extends from Kingstown, on the north, to near New 
Ross, in Wexford, on the south, a distance of nearly seventy miles. 
It has a width of from seven to seventeen miles. It must extend 
northward from Kingstown, beneath the sea, into Dublin Bay, 
and probably farther still, as we find the small island Rockabill, 
five miles off Skerries, and just outside the northern boundary of 
the map, to be composed of granite of the same type. But of 
course the Rockabill granite, though evidently belonging to the 
same mass, may not belong to the same surface exposure thereof; 
as is the case with the Carnsore granite at the 8.E. point of the 
Co. Wexford. There are some small isolated granite protrusions 
in the Co. Wicklow, which differ importantly, as to composition, 
from that with which we are now concerned; these, however, 
are outside of our present subject. 
The age of the granite of the main mass is determinable 
