161 
Geological Relations of the Hast of Ireland. 
lines of granular protuberances. The purest specimens of this 
mineral that I have obtained in the whole of the mica slate tract, 
were from Cadeen, in reddish or hair brown translucent crystals, 
with a lamellar structure parallel to two sides of the prism, which 
is rifted transversely ; the fracture, small conchoidal or splintery. 
But the characters of this substance are subject to great variation 
from a more or less intimate incorporation with mica. Andalusite 
in this vicinity forms almost an essential ingredient in the mica slate, 
of which scarcely any specimen is wholly free from it : hence this 
rock acquires a varied aspect and different states of tenacity. 
§ 46. Brisselstown hill and its lateral extension to the west 
called Spynan’s hill, consist of mica slate, fine and minute granular 
greenstone, greenstone slate, and greenstone porphyry. Mica slate 
appears on the eastern side of Brisselstown, beneath which is seen 
the granite base, but the southern side and summit are composed 
of the trap rocks just mentioned, principally the two first, which 
form a great bed in the mica slate, while to the west this rock ap- 
pears again ; all these beds ranging 20° east of north and west of 
south, and dipping 65° toward the east. The mica slate in the 
western part is porphyritic, containing numerous crystals of felspar, 
and it passes into greenstone porphyry by an intimate mixture of 
hornblende. Similar relations may be observed in the banks of the 
Slaney, at Davidstown, and farther north toward Donard, and again 
in Blackmoor hill ; white to the south they may be remarked in 
Kilranela hill. A gradation from mica to hornblende is frequently 
observable in these rocks ; and finely disseminated iron pyrites is 
common in the greenstone. 
Brisselstown hill forms a conspicuous feature in the landscape of 
this neighbourhood. Craggy protuberances stud the upper part and 
form the summit of the hill, which is encircled by three concentric 
Vol. V. x 
