133 
Geological Relations of the East of Ireland . 
to the bay; and it forms an extensive flat on the western side of the 
chain, surrounded by that bold amphitheatre of hills and mountains, 
which extend from the vicinity of Castle Dermot on the west, past 
Baltinglass, toward the foot of Lugnaquilla on the east, thence 
spreading to the south along the western base and brow of the 
Shillelagh hills. This forms the broadest part of the granite soil, 
being about fifteen miles across from west to east, while still more 
south it gradually narrows to a point, constituting however, in this 
quarter, not only the entire mass of Mount Leinster, and Blackstairs 
mountain, with its southern extension called White mountain, but 
forming the bed over which the Barrow flows. To the north of 
Lugnaquilla, the granite tract preserves a more equal and nearly 
uniform breadth of about seven miles, and at its northern extremity, 
between Rathfarnham on the west, and Dalkey Head on the east, it 
is nearly of the same width. 
Such is the extent of that continuous tract of fundamental granite, 
upon which all the succeeding rocks repose. Granite, however, 
re-appears in other quarters; not only protruding and breaking forth 
in isolated, denuded portions of the great base, but occurring, as a 
later production, interstratified with rocks, which unequivocally rest 
upon the fundamental granite. 
§ 17. It might seem almost an idle attempt to pursue the granite 
through the various modifications of its structure and aspect ; but it 
may be observed of this extensive tract, which is full fifty-nine 
miles in length, that it is in general remarkably pure and free from 
minerals not essential to its composition. The felspar is commonly 
of a clear beautiful white, or slightly tinged with yellow or grey, 
very rarely flesh coloured. The quartz is mostly grey, and the 
mica in general greyish white, but it also inclines to brown or black. 
The granite varies much in the size of the grain. Some of the 
