Oil the Physical Geology of the Neighbourhood of Dvhlin. 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 thai of 

 Lambay, already mentioned, there are some sheets of felstone 

 porphyry near Boliernabreena, 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, &c. 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 

 Koss,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 S.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 



