CHAP. XXV 
PLATEAU-VENTS IN SCOTLAND 
399 
111 the interior of the country, it is seldom possible to examine the actual 
Junction of necks with the rocks through which they rise, the houndary-line 
being usually obscured by debris or lierbage. On the coast, the vents of the 
plateaux have not been bared l)y tlie sea so fully as in the case of the much 
younger series of the the east of Tife to he described in later pages. But 
where the East Lothian plateau touches the shore, the waves have laid bare 
a number of its minor vents, which have thus been dissected in ground-jdan 
on the beach. As an illustration of these vents an example is given in 
Fig. 126, from the shore east of Dunbai'. Here the sandstones, which are 
inclined in an easterly direction at 20° to 25°, are pierced by an irregular 
mass of tuff. It is observable that in this instance long tongue-like pro- 
j'ectious of the sandstones protrude into the neck ; more frequently the 
material of a neck sends veins or dykes into the surrounding walls. A 
volcanic chimney would seem to have been often much shattered and fissured 
in the course of the volcanic explosions, and the fragmentary material lias 
fallen or been injected into the rents thus caused. As a rule, the rocks 
immediately around the Carboniferous necks are more or less indurated, as 
in this instance from the Dunbar shore. 
The materials which have filled up the vents connected with the plateau- 
eruptions generally consist of (a) agglomerates or tuffs, but occasionally of 
(h) some kind of lava, and frequently (c) of both these kinds of rock 
combined. 
(a) Necks of Afjylonicrate or Tuff . — These materials vary greatly in the 
nature and relative proportions of their constituents. Usually the hicluded 
blocks and lapilli are pieces of andesite, diabase, basalt or other lava, like 
the rocks of the plateanx. But with these occur also fragments probably 
detached from the sides of the funnels through wdrich the explosions took 
place, such as pieces of greywacke, sandstone, limestone and shale. Con- 
siderable induration may be observed among these iion-volcauie ingredients. 
In some cases, as in that of the occurrence of pieces of granite referred 
to on p. 382, the stones have probably been brought up irom some con- 
siderable depth. In others it is easy to see that the blocks have slipped 
down from some higher group of strata now removed from the surrounding 
surface by denudation. Some striking illustrations of this feature will be 
cited from necks of the puy-series in the south of Itoxburglishire (p. 476). 
The lava blocks in the tuffs and agglomerates are usually rounded or 
sub-angular. Pear-shaped blocks, or llatteued discs, or hollow spherical balls 
are hardly ever to be observed, thougli I have noticed a few examples in 
the tuffs of Duuljar. A frequent character of the blocks is that of rougldy 
rounded, highly amygdaloidal pieces of lava, the cellular structure being 
specially developed in the interior, and the cells on the outside being often 
much drawn out round the circumlerence of the mass. Such blocks were 
probably torn from the cavernous, partially consolidated, or at least rather 
viscous, top of a lava column. Most of the stones, however, suggest that 
they were produced by the explosion of already solidified lava, and were 
somewhat rounded by attrition in their ascent and descent The vents filled 
