4o8 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
other dykes and sills on the mainland, which traverse the Coal-measnres, 
and would thus he not older than late Carboniferous or I’ermian time. 
“ The fourth groujj of dykes intersects all the others, and is probably of 
lertiary age. The prevalent direction of these dykes in the Cumbraes is 
X.N.W.” Tlie Tertiary dykes are more fully described in Chapters xxxiv. 
XXXV. 
The great group of tuffs which underlies the lavas of the East Lothian 
plateaii is traversed by numeious dykes and sills, of which many good 
examples may be seen in the coast-cliffs of North Berwick. Among these 
rocks are beautiful olivine-basalts with singularly fresh olivine, as on the 
sliore at North Berwick. Some of them are still more basic, as in the ease 
of a limburgite intrusion at the Gin Head, Tantallon Castle. 
2. In the necks, dykes are sometimes abundant, and they may be 
Fig. 1-34. — Veins ami (tyke.s traver.'iiiig the agglomerate and tuft' of the great Reufrew.shire vent. 
observed occasionally to travei’se the surrounding lavas. They consist of 
similar materials to those found outside tlie plateaux. Some of the larger 
necks are intersected by a network of dykes and veins. The great vent or 
group of vents among the uplands of Bentrewsliire, already described (Fig. 
129), furnishes some admirable examples of this cliaracteristic volcanic 
feature. An illustration from that locality forms the subject of Fig. 134. 
The agglomei’ate which fills the large liollow among the Campsie Hills may 
be quoted as another illustration (Fig. 128). Further instances will be 
found in some of the sections given in preceding pages (see Figs. 124, 
125, 127). The general aspect of a dyke in the volcanic series is shown 
in Fig. 135. 
Tlie Sills associated witli tlie plateau-type of Carboniferous volcanic 
action form a less prominent feature than they do among the earlier 
