CHAP. XXV 
PLATEAU-VENTS IN SCOTLAND 
403 
in the landscape, presents a precipitous face towards the south, and allows 
the disposition of its component materials to be there seen. The agglomerate 
consists of a succession of rudely stratified beds of coarser and finer detritus, 
whicli on both sides are 
inclined towards the centre, 
where a plug of fine-grained 
olivine- basalt has risen and 
spread oiit into a columnar 
sheet above (Fig. 131). In 
general form this basalt Fia. 131. — Section of south end of Dumbuck Hill. East of 
11 ^ • j • Dumbarton. 
resembles such intrusions 
as that of Largo Law, to be afterwards described (Fig. 226), where 
what may have been the hollow or bottom of the crater is filled with basalt. 
(5) Necks of Andesite, Trachyte, Doler ite, Diabase, or other massive Rock . — 
When the vents have been filled by the uprise of some molten rock, it is 
generally, as we have seen, of a more acid character than the ordinary lavas 
of the plateaux. Frequently it consists of some variety of trachyte or 
andesite, commonly of a dull yellow or grey tint and waxy lustre. Good 
examples may be seen among the remarkable group of necks on either side 
of the valley nortli of the village of Strathblane a, ml in those above Bowling. 
The tliree great necks in East Lothian, already alluded to, — -Traprain Law 
Traprain Law 
Fig. 132. — Section across tlie East Lotiiiaii plateau to show the relative position of one of the necks. 
1. Lower Carboniferous sandstones and shales ; ‘2. Red and green tuffs with a seam of limestone {/) ; 3. Band of 
basic sheets at the base of the lavas ; 4. Trachytes; 0. Phouolite neck. 
(Figs. 132, 133), North Berwick Law (Fig. 109), and the Bass Bock (Fig. 
1 10) — are masses of phouolite and trachyte, obviously related to the trachytes 
of the adjacent plateau. A smaller but very perfect instance of a vent 
similarly filled is to be seen in the same neighbourhood on the shore to the 
east of North Berwick Law.^ 
Examples occur where the funnels of eruption have been finally sealed 
up by the rise of more basic material, and this has happened even in a 
district where most of the lava-form necks consist of trachyte or some other 
intermediate lava. Thus, in the Oampsie Fells, several such bosses appear, 
of which the most conspicuous forms the hill of Dnngoil (1396 feet. Fig. 
128). Further west, among the Kilpatrick Hills, bosses of this kind are 
still more numerous. The group of bosses near Ancrum and Jedburgh is 
mainly made up of olivine-dolerites and olivine-basalts (Fig. 130). This more 
basic composition of itself suggests that these bosses may be connected rather 
with the puy- than with the plateau-eruptiojis. 
^ See “Geology of East Lothian,” Geological Survey Memoir, p. 40. 
