222 PROCEEDINGS OF SOCIETIES. 



compact felspar, but to which Professor Sedgwick's appropriate name 

 of felstone has lately been applied by Mr. Jukes. 



In the present state of the geological examination of the country, I 

 think it desirable, however crude, to bring these notes, as made on the 

 spot, before the Society, with a view to future discussion. 



In the last edition of my Geological Map of Ireland I have not made 

 any change in the divisions which, in 1847, I thought it best to adopt 

 between the rocks which I believed to be undoubtedly of igneous 

 origin, and those which presented a kind of mean between igneous 

 and schistose metamorphic rocks, the greater portion of which are 

 now classed by the geological surveyors as volcanic ash. And, although 

 I do not mean on the present occasion to dispute this view, still, on look- 

 ing over my notes and specimens, I am not yet quite a convert to it, at 

 least to the full extent of the views entertained by Mr. Jukes, and his 

 excellent corps of field observers. 



The following are the notes written at Arklow, dated June 26, 

 1847:— 



The most satisfactory and characteristic example of the igneous pro- 

 trusions of the counties of Wicklow and Wexford is exhibited at the 

 hills called generally Arklow Hock, situate about two miles to the south 

 of the town of Arklow. At this place there are two abrupt hills 

 which rise precipitously, one out of the sea on the east side, and the 

 other from the schistose plain on the west. 



If we commence our examination from the south-western point of 

 the western hill called Eock Little, we find a remarkable though irregular 

 vein of white or grayish- white porphyritic felspathic rock, traversing 

 the slate rock in a north-east and south-west direction, where it crosses 

 the road at the south end of Eock Little, and where it has been 

 quarried ; its breadth is about 80 feet, having metamorphic schist on the 

 west side, and a protrusion of greenstone on the east : proceeding north- 

 wards, the vein expands, and is there quarried extensively, and when 

 broken is used for repairing the roads. In some places the rock in- 

 cludes thick elongated masses of schist, which are highly metamorphic, 

 and sometimes present the character of semi-porphyry. 



About 300 feet northward from the first opening of the vein at the 

 road, it presents the appearance of three distinct veins, each having 

 vertical sides, separated from each other by interposed masses of meta- 

 morphic schist, which are stratified horizontally, and have a vertical 

 cleavage. If in this place we make a section across the vein from west 

 to east, we find first a vertical vein consisting of white felspathic rock, 

 having a porphyritic structure, from its containing occasionally imper- 

 fect crystals of felspar, a few crystals of hornblende, and grains of gray 

 quartz ; this vein or branch is 1 6 feet in breadth ; beyond it to the east 

 is a rib of metamorphic schist, presenting the usual cleavage 14 feet 

 broad ; then a second vein of felspathic rock, similar to the first, 50 feet 

 in breadth, again a second mass of metamorphic schist, 70 feet broad, 

 beyond which is a vein of coarse-grained felspathic rock, 70 feet broad 

 at the least, but to the north-eastward it may be considerably more ; 



