CHAP. XLI 
VENTS OF THE BASALT-PLATEAUX 
27 
nature of the case, impossible in these instances. But examples will be cited 
from the Western Isles and from Faroe, where the vents can be proved to 
belong to the time of the plateau-eruptions, for they are seen to have broken 
through some of the basalt-sheets and to have been buried under others. 
With this clear evidence of relationship in some cases, there need be little 
hesitation in believing that in other instances where no such positive con- 
nexion can be found, but where the vents are obviously such as the general 
structure of the plateaux would have led us to expect, they may be con- 
fidently regarded as part of the phenomena of the plateau-eiuptions. 
Sometimes the vents can be linked with lines ot fissures or dykes. 
This is especially the case where they are small in size. More usually, 
however, no such relation can be demonstrated. It will be remembered that 
among the modern Icelandic eruptions, some eruptive vents, like the later 
cinder-cones of Laki, are ranged in a linear direction along the great fissure, 
while others, of an older series in the same district, almost engulplied 
amidst the more recent lavas, are clustered irregularly in groups. A similai 
diversity of arrangement has been observed among the volcanic cones ol the 
Yelay in Central France. 
Considering as a whole the volcanic necks or eruptive vents which rise 
from the older rocks around the Tertiary basalt-plateaux, and sometimes 
even from the surface of these plateaux themselves, we may conveniently 
follow the same classification as was adopted in dealing with those ol 
Palaeozoic age, and, according to the nature of the material that now fills 
them arrange them in two series: (1) Those occupied by some form ot 
crystalline eruptive rock, and (2) those filled with volcanic agglomerate. 
i. VENTS FILLED WITH DOLERITE, BASALT, ETC. 
These, as the composition of the plateaux would lead us to anticipate, are 
numerous. They perhaps attain their most conspicuous development 111 
Antrim, either on the tableland or among the underlying rocks round its 
edges. The finest example in that district is undoubtedly furnished by the 
lofty eminence called Slemish, which rises above the surrounding basalt- 
terrace, to a height of 1437 feet above the sea (Fig. 294). It is elliptical 111 
ground-plan, measuring some 4000 feet in length by 1000 in breadth. 
Seen from the north, it appears as a nearly perfect cone. The material of 
Which it consists is a coarsely crystalline olivine-dolerite, presenting under 
the microscope a nearly holocrystalline aggregate, in which the lath-shaped 
felspars penetrate the augite, with abundant fresh olivine, and wedge- 
shaped patches of interstitial matter. The rock is massive and amorphous, 
except that it is divided by parallel joints into large quadrangular blocks 
like a granitic rock, and wholly different from the character of the sur- 
rounding basalts. The latter, which possess the ordinary characters of the 
rocks of the plateaux, can be followed to within 80 yards of this neck, which 
rises steeply from them, but their actual junction with it is concealed under 
the depth of talus. At the nearest point to which the two rocks are trace- 
