On the Physical Geology of the Neighbourhood of Dublin. 141 



that we find the width of the granite exposure to be greatest. 

 All these facts point to the conclusion that the granite protrusion 

 may not have broken through the thick mass of Silurian strata, 

 and that it was brought to the surface by the subsequent 

 denudation, which has wrought parts of it into low ground as 

 about Kingstown, Carlow, St. Mullins, &c. 



The granite, as already observed, his metamorphosed the 

 Lower Silurian slates, all along the line of its contact with them, 

 into regular mica schist ; the alteration extends from the surface 

 of the granite through a thickness of several hundred feet and 

 dies away gradually. The grit bands in the slates are, as might 

 have been expected, but little changed. The width of the 

 metamorphosed rock, as measured on the surface of the ground, 

 is greater on the east, than on the west side of the granite ; which 

 seems to indicate that the bounding surface of the granite 

 descends less steeply beneath the slates on its east, than on its 

 west, side. There seems to be some connection between this 

 fact and that already alluded to, viz., that the small outbursts of 

 granite are on the east and that there are none in the Silurian 

 on the west side of the main granite exposure. 



The contact of the granite with the Silurian rocks is strikingly 

 exhibited on the shore of Killiney Bay, at the base of Killiney 

 Hill. It is there seen that the granite has irregularly penetrated 

 the Silurian slates and sent off veins into them ; it has also 

 caught up what are clearly separated masses of the slate rock, 

 converting all into mica schist and developing therein stellate 

 crystals of chiastolite. Not far off, on the south side of Roches- 

 town Hill, N. and N.W. of Killiney Park, the granite has 

 forced several narrow tongues into the slates, nearly along 

 the direction of the bedding (see the plan of this in the 

 Geological Survey " Explanations," 112, p. 35). 



The Rathmichael relief tank of the Dublin waterworks, on 

 the northern hip of Shankill, was excavated directly on the 

 boundary line of the .two formations. The boundary was dis- 

 tinct ; but not so mitch sO as at Killiney ; and there was a 

 peculiar lumpy, lenticular-nodular structure common to the rocks 

 on each side of the boundary; the greatest extension of the flake 

 lumps being parallel to the surface of separation. At the south- 

 ward end of the Scalp— a remark able physical feature to be men- 



