468 PROFESSOR GEIKIE ON THE 
instructive examples of this character. As the veins thin away from the main 
mass of basalt, they become more close-grained and lighter in colour ; and ; 
when they enter dark shales or other carbonaceous rocks, they pass into that 
peculiar white earthy clay-like variety known as “ white-rock ” or “ white-trap.” 
Junction of the Necks with the surrounding Rocks.—In a modern volcano no 
opportunity is afforded of examining the effects which have been produced upon 
the rocks through which the volcanic vent has been opened, except now and 
then among the detached fragments ejected. But in the Basin of the Firth of 
Forth a numerous series of coast sections lays bare this relation in the most 
satisfactory manner. The superincumbent cones have been swept away, and 
we can exainine, as it were, the very roots of the old volcanoes. The margin 
of a neck or volcanic vent is thus found to be almost always sharply defined. 
The rocks through which the vent has been drilled have often been ‘cut across, 
as if a huge auger had been sunk through them. This is well displayed in the 
beautifully perfect neck already cited at Newark Castle, near St Monans 
(fig. 11). The strata through which this neck 
rises consist of shales, sandstones, thin coal, 
and encrinal limestones, dipping in a westerly 
direction at angles ranging from 25° to 60°. 
At the south end of the neck they are as 
sharply truncated as if by a fault. Elsewhere 
they are much jumbled, slender vein-like por 
tions of the tuff being insmuated among the 
projecting portions. A large vertical bed of 
sandstone, 24 yards long by 7 yards broad, 
stands up as a sinuous reef on the east side of 
the vent/(s, fig.11). It is a portion of some of the 
surrounding strata, but is entirely surrounde i 
gtig, Ul Piano! volcan nee ae with agglomerate, so far as can be seen at the 
agglomerate on beach near St Monans. ‘4 
me Hioticcdt-Arath tidiistiig’eimouee off sane surface. Here and there the shales have been 
stone (s), and piercing sandstones and shales excessively crumpled, and at the north end 
with beds of limestone (/,/,), and a thin seam ‘ v ae 
of coal (c); B, basalt “white trap” dyke. have been invaded by a vein of basalt which, | 
Tho seatowe eHonigie iaiot ieee where it runs through them, assumes the — 
usual clay-like character. As shown in the drawing (fig. 11), the strata have 
been blown out, and their place has been occupied by a corresponding mass — 
of volcanic agglomerate. They have undergone comparatively little alteration. — 
In some places they have been hardened, but their usual texture and structut 
remain unaffected. . 
In other instances the boundary-line between the neck and the surroundin 
rocks is less sharply marked. Not infrequently the latter, as laid bare on beach- — 
sections, protrude in tongues and irregular projections into the neck, while the — 
