CHAP. XXVII 
VENTS OF THE SCOTTISH PUYS 
427 
reiice of abundant vertical rents, which have been filled partly with the 
same material that forms the nodules, and pai'tly witli sandstone. 
The formation of the neck took place after the deposition of the Index 
Limestone, and probably about the time of the accumulation of the next 
limestone, which lies immediately to the west somewhat higher in the 
series. It would appear that the eruption which produced this funnel gave 
forth only gaseous explosions, and occurred on the sea-fioor ; that the low 
crater-walls were washed down to such an extent that the sea entered and 
carried some of its characteristic organisms into the lagoon or m.aar within ; 
further, that as the silt gathered inside, successive suhsidences occurred, 
whereby the sediment was rent by cracks into which sand and calcareous 
mud were washed from above.^ 
Many necks occur wherein non-volcanic materials, though not forming 
the whole of the agglomerate, make up by far the larger part, with only a 
slight admixture of volcanic tuff between them. Among the Burntisland 
necks of Life, for instance, abundant fragments of the well-marked cyprid 
limestone and shale may be observed, while at Niddry in Linlithgowshire 
blocks several yards in length, and consisting of different layers of shale and 
cement-stone still adhering to each other, may be seen imbedded at all angles 
in the tuff. 
Wliere oidy the debris of non-volcanic rocks occupies a vent, we may 
infer that the volcanic action was limited to the explosion of steam, whereby 
the rocks were dislocated, and an orifice communicating with the surface 
was drilled through them,, and that while no true volcanic rock in such a 
case appeared, the pipe was filled up to perhaps not far from the surface by 
the falling hack of the shattered detritus. A little greater intensity or 
farther prolongation of the volcanic action would bring the column of lava 
up the funnel, and allow its upper part to be blown out as dust and lapilli ; 
while still more vigorous activity would he marked by the rise of the lava 
into rents of the cone or its actual outflow at the surface. Every gradation 
in this scale of progress may he detected among the Carboniferous volcanoes 
of the basin of the Firth of Forth. 
2. Necks of Tuff and Agglomerate . — The majority of the necks con- 
nected with the puys consist of tuff or agglomerate. Externally they 
generally appear as smooth rounded grassy hills that rise disconnected fi’om 
other eminences. In some districts their materials consist of a greenish 
granular often stratified tuff, enclosing rounded balls of various basic lavas and 
pieces of sandstone, shale, limestone or other strata through which they have 
been drilled. This is their usual character in the Forth region. But in some 
cases, the tuff becomes a coarse agglomerate, made up partly of large blocks 
of basalt and other volcanic rocks and partly of the sedimentary strata 
around them, of which large masses, many cubic yards in bulk, may be seen. 
Among the enclosed fragments it is not unusual to find pieces of older 
stratified tuff. These resemble in general petrographical character parts of 
the tuff among which they are imbedded. Sometimes they have been 
^ The vent is sliown in' Sheet 39, Geol. Hurv. Scotland. 
