704 Scientific Proceedings, Royal Dublin Society. 
their junction with the slates and sandstones above and below. 
Some of the grit stones are so siliceous, and some of the quartz rocks 
become so granular, that it is not always easy to determine 
whether to call any individual mass of rock a siliceous grit or 
quartzite. The principal masses marked on the map are very 
decided in character, and have always been called quartz rock by 
every geologist who has examined them. They vary in thickness 
from twenty to several hundred feet.” 
The facility of approach afforded by the road from 
Shankill Castle, across the hill, already referred to, along 
with the excellent quality of the quartzite as a material for 
road-metal, has led to the opening up of quarries along the 
outcrop which borders this road; and in these quarries, may be 
examined, as well the rock, as the systems of jointing and 
fissuring to which it is subject. As might a priori be expected, 
certain directions of jointing dominate, and have already been 
noticed for this locality in a paper read by me before the 
Royal Irish Academy in 1889. (“On the Directions of the 
main Lines of Jointing observable in the Rocks about the Bay 
of Dublin, and their relations with adjacent Coast Lines,” 
part i1., p. 245. See also Proceedings Royal Irish Academy, 
2nd ser., vol. iv., Science.) 
The more important jointings are further characterised by 
being filled to a greater or lesser extent with a silicate of iron and 
manganese, dark-brown to black in colour, and intimately associated 
with quartz crystals. The jointings thus marked may be referred 
to four principal directions :— 
(No. 1.) N. 6° : 15’ W.—Corresponding to the coast line 
direction between Six-mile-point and Ballygannon (lower New- 
castle district). 
(No. 2.) N.37°: 34 E.—Corresponding to the general direction 
of the 8.E. edge of Carrickgollaghan Hill. 
(No. 3.) N. 62°: 30° W.—A well-marked system of jointing, 
according to which there had evidently been repeated movements, 
having given rise to breccia formation along the joints, with inter- 
position of the iron-manganese silicate already referred to. This 
direction corresponds very exactly with that of Glencullen Valley, 
in the part of it which traverses the granite formation. 
(No. 4.) N.70:47’ W.—Corresponds to the systems of jointing 
