224 PROCEEDINGS OF SOCIETIES. 



bluish-gray, with crystals of hornblende, in others, dark bluish-gray, 

 compact or splintery felspar without quartz. At the summit the rock 

 is composed of a base of dark-gray compact felspar, with disseminated 

 imperfect crystals of white felspar. Descending the declivity to the 

 eastward, the base of the rock still preserves its dark-gray colour, but 

 it becomes very fine-grained, and presents the character of clinkstone 

 porphyry, being translucent at the edges. 



This rock has been much quarried ; it very much resembles the fels- 

 pathic porphyry of Penmonmawr in Carnarvonshire, used for the pave- 

 ment of London. Continuing to descend to the eastward, the same 

 rock continues to the base of the steep declivity, where the surface rock 

 presents a white colour, and is similar to that of the vein first-men- 

 tioned, but on this mass being quarried beyond the weathered surface, 

 it presents the dark bluish-gray colour of the sub- crystalline felspar 

 porphyry of the summit of the hill. Still continuing to descend to the 

 eastward, the rock, which presents a white felspathic mass at the sur- 

 face, is followed by dark bluish-gray compact felspathic porphyry, 

 which extends nearly to the coast. 



The cliff immediately above the strand is composed of a brown and 

 apparently irregularly stratified mass of rock, dipping S. W., angle 80°, 

 and presents a very doubtful character. At the base close to the 

 shore, near the old mine adit, the rock consists of thin beds of black 

 Lydian stone, passing into flint slate, in which graptolites have been 

 discovered. At the adit a bed of breccia occurs, having a close-grained 

 quartzose or hornstone base, with angular fragments of white quartz and 

 some Lydian stone ; but time did not admit of these rocks being examined 

 with sufficient care. Leaving the section, and proceeding in a "S. E. 

 direction towards Arklow and the sea, we find the greenstone and 

 columnar felspathic rock extends uninterruptedly from the forge in a 

 north-easterly direction to the sea-shore, at the northern extremity of 

 Rock Big, varying frequently from ordinary greenstone to the dark 

 gray compact felspathic rock. On the sea-shore there is a rude facade 

 of basaltic columns similar to, but more perfect than, those already 

 mentioned, which are being quarried for the pier at Arklow ; and the 

 columnar structure is in consequence far less striking than it has for- 

 merly been, but it is still sufficiently visible. The most remarkable cir- 

 cumstance is, that these columns sometimes consist of a porphyritic 

 felspathic rock, composed of dark-gray compact felspar, containing 

 imperfect crystals of hornblende ; while other portions of the mass are 

 composed of splintery, fine-grained, dark-gray compact felspar, having a 

 conchoidal fracture translucent at the edges, and in hand specimens 

 resembles fine-grained quartzite. 



The coast south of this columnar rock is composed of the ordinary 

 amorphous unstratified rock, varying in character and colour, as already 

 mentioned ; but it is remarkable that the dark-gray rock is in one in- 

 stance at least traversed by a vertical vein of white compact felspar 

 rock, about 9 inches in thickness, exactly similar in structure to that 

 first described. 



