420 
THE TERTIARY VOLCANOES 
BOOK VIII 
is found by Mr. Gunn to be newer than the porphyry, into which it sends 
sills and dykes. 
A feature observed by the same geologist in Arran offers a further point 
of resemblance to the acid sills and dykes of Skye. He has noticed that 
accompanying the quartz-porphyry of Drumadoon and Bennan, a mass of 
basic rock forms a kind of fringe or selvage round it, sometimes with what 
appears to be a rock of intermediate character between them. Basic sills 
are abundant south of Glen Aslidale, though to the west of Whiting Bay 
most of the intrusive sheets are of acid material. 
Some of the quartz-porphyry sheets are markedly columnar. One of 
them, near Corriegills, displays a divergent grouping of the prisms, not un- 
like parts of the pitchstone sheets of Eigg and Hysgeir, and suggestive of the 
rock having flowed along a hollow like that of a valley. No certain trace, 
however, has been found of any Tertiary lava-stream in Arran, nor has 
evidence of tuffs been detected in any part of the younger igneous series. All 
the rocks appear to be intrusive, though so abundant and varied are they 
as to indicate that they belong to a vigorous eruptive centre, which may 
have poured out at the surface lavas and ashes, since entirely removed by 
denudation. 
The numerous basic dykes for which the south end of Arran has long 
been celebrated have a general northerly trend, and appear to be all of the 
same or nearly the same age. They undoubtedly cut through the quartz- 
porphyries and the coarse-grained basic sills, but are less numerously visible 
in the finer-grained basic sills, while in the felsitic sheets they are seldom to 
be seen. In several places dykes running in an E.X.E. direction cut the 
others, and are therefore of later date. 1 The compound dykes, of Tormore 
on the west side of the island have been already noticed (p. 161). 
VI. THE NORTH-EAST OF IRELAND 
In the north-eastern counties of Ireland there are two regions which 
afford ample material for discussion in connection with the protrusion of acid 
rocks during the Tertiary volcanic period. One of these, which for distinc- 
tion may be called the Carlingford region, embraces the tract of country 
which includes the Mourne Mountains on the north-east side of Carlingford 
Lough and the ranges of Slieve Foye and Slieve Gullion on the south-west 
side. The other lies mainly within the basaltic plateau, the largest of its 
scattered portions forming parts of the hills of Carnearny and Tardree in the 
county of Antrim (Map VII.). 
1. The Carlingford Region 
a. The Mourne Mountains . — This compact and picturesque group of 
hills, about twelve miles long and six miles broad, and reaching a height of 
2798 feet in Slieve Dona.rd, presents a comparatively simple geological struc- 
ture, since it consists almost entirely of granitic rocks which pierce, overlie and 
1 Ann. Rep. of Geol. Surv.for 1894, p. 286. 
