DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



101 



material that now fills them. They are either occupied by (a) some form of crystalline 

 eruptive rock, or by (b) volcanic agglomerate. 



(a) Vents fitted with Dolerite, Basalt, &c. — These are by far the most numerous, and 

 as this is what the composition of the plateaux would lead us to anticipate, the fact may 

 be held to confirm the justice of the assumption that these vents were really sources for 

 the plateau-lavas. They are perhaps most conspicuously visible in Antrim, both on the 

 tableland and on 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-terraces to a height of 1437 feet above the sea (fig. 24). It is 

 elliptical in 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 

 noncrystalline aggregate, in which the lath-shaped felspars penetrate the augite, with 

 abundant fresh olivine, and wedge-shaped patches of interstitial matter. The rock is 



Fig. 24. — Slemish, a Volcanic Neck or Vent on the Antrim Plateau, seen from the north. 



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 surrounding 

 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 can nowhere be seen, owing to the depth of talus. At the nearest 

 point to which the two rocks are traceable, the basalts appear somewhat indurated, break 

 with a peculiar splintery fracture, and weather with a white crust. These characters are 

 still better shown on abundant fragments which may be picked up among the debris 

 further up the slope. There can be no doubt, I think, that a ring of flinty basalt, 

 differing considerably in texture from the usual aspect of that rock in the district, 

 surrounds the neck. The meaning of this ring will be more clearly seen from the 

 description of another example in Mull. About four miles to the north-east of Slemish, 

 a smaller and less conspicuous neck rises out of the plateau-basalts. The rock of which 

 it consists is less coarsely crystalline than that of Slemish, but its relations to the 



VOL. XXXV. PART 2. O 



