470 PROFESSOR GEIKIE ON THE 
The outpouring of so much tuff and lava as escaped from many of the volcanoes 
would doubtless often give rise to cavities underneath them, and on the decay 
of volcanic energy there might be a tendency in the solid or cavernous column 
filling up the funnel, to settle down by mere gravitation. So firmly, however, 
did much of it cohere to the sides of the pipe, that if it sank at all, it could 
hardly fail to drag down a portion of these sides. So general is this evidence 
of downward movement in all the volcanic districts of Scotland where the 
necks have been adequately exposed, that the structure may be suspected to be 
normal to all old volcanic vents. It has been observed among the shore- 
sections of the volcanoes, of the Auckland district, New Zealand. Mr C. 
HEAPHY, in an interesting paper upon that district, gives a drawing of a crater 
and lava-stream abutting on the edge of a cliff where the strata bend down 
towards the point of eruption, as in the numerous cases in Scotland.* 
Evidence suppled by the Tuff-necks regarding Suberial Volcanic Action.— 
From the stratigraphical data furnished by the Basin of the Firth of Forth, itis 
certain that this region, during a great part of the Carboniferous period, existed 
as a wide shallow lagoon, sometimes overspread with sea-water deep enough to 
allow of the growth of corals, crinoids, and brachiopods ; at other times shoaled 
to such an extent with sand and mud as to be covered with wide jungles of a 
lepidodendroid and calamitoid vegetation. As volcanic action went on inter- 
ruptedly during a vast section of that period, the vents must sometimes have 
been submarine, but may at other intervals have been subeerial. Indeed, we 
may suppose that the same vent might begin as a subaqueous orifice and 
continue to eject volcanic materials, until as these rose above the level of the 
water, the vent became subeerial. I have not been able to determine which — | 
were submarine vents ; but some interesting evidence may be collected to show 
that many actually rose up as insular cones of tuff above the surrounding — 
lagoon. 
The structure of the tuff in many necks suggests subzerial rather than sub- 
aqueous stratification. The way in which the stones, large and small, are 
_ grouped together in lenticular seams may be paralleled in the slopes of 
many a modern volcano. Another indication of this mode of origin is supplied 
by the traces of wood to be met with in the larger tuff-necks. The vents of — 
Fife and Linlithgowshire contain these traces sometimes in great abundance. 
The specimens are always angular fragments, the largest I have observed being 
a portion of a stem about 2 feet long and 6 inches broad, in the neck below St 
Monans church. They are frequently encrusted with calcite. In a neck to the 
west of Largo Law I found many pieces with the glossy fracture and clear 
ligneous structure shown by sticks of well-made wood charcoal. In another 
neck at St Magdalens, near Linlithgow, the wood fragments occur as numerous 
* Quarterly Journ. Geol. Soc, 1860, p. 245. 
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