430 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
nearly 500 feet. Here and there the action of atmospheric waste lias 
hollowed out huge crater-like chasms in the crumbling tuff. Standing in 
one of these, the geologist can realize what must have been the aspect of 
the interior of these ancient Carboniferous volcanic cones. The scene at 
once reminds him of the crater-walls of a modern or not long extinct 
volcano. The dull-green rudely stratified tuff rises around in verdureless 
crumbling sheets of naked rock, roughened by the innumerable blocks of lava, 
which form so conspicuous an element in the composition of the mass. The 
ribs or veins of columnar basalt run up the declivities as black shattered 
walls. The frosts and rains of many centuries have restored to the tuff its 
original loose gravelly character. It disintegrates rapidly, and rolls down 
the slopes in long grey lines of volcanic sand, precisely as it no doubt did 
at the time of its ejection, when it fell on the outer and inner declivities 
of the original cone. Some of these features may be partly realized from 
Fig. 145, which represents a portion of the south front of the hill. Sections 
of this neck are given in Figs. 149 and 159. 
(3) Mcks of Tvff or Agglomerate with a Central Plug of Basalt or other 
Lara. It has often happened that, after the explosions in a A^ent have 
begun to decrease in vigour, or have at last ceased, lava has risen in the 
chimney and finally sealed it up. In such cases the main mass of the 
rock may consist of tuff or agglomerate, which the enfeebled volcanic 
activity has been unable to expel from the orifice, while a plug of basalt, 
dolerite, or even more basic material, of much smaller dimensions, may ha\'e 
risen up the pipe in the centre or towards one side. Binns Hill, West 
Lothian, the Beath and Saline Hills of Fife, and Tinnis Hill in Liddesdale 
are good examples of this structure. (See Figs. 26, 148, 149 and 174). 
(4) Nechs of Basalt, Dolerite, etc. — In other cases no fragmental material 
is present in the vent, or possibly traces of it may be seen here and there 
adhering to the walls of the funnel, the prevailing rock being some form of 
lava. Necks of this kind are much less freeprent in tire puy- than in the 
plateau-type. But examples may be found in several districts. The most 
striking with wliich I am accpiainted are those which form so picturesque a 
group of isolated cones around the. volcanic basin of Limerick, to Ire after- 
wards described (ligs. 19o, 196). Ihe veirts there have been filled by the 
uprise of much more acid rocks than the lavas of the basirr, for, as I have 
already stated, they include even quartziferous trachytes. In the basin of 
the Firth of Forth some prornirrent bosses of basalt probably mark the sites 
of former vents, such as Hunearu Hill in Fife, the Castle Bock of Edinburgh, 
arrd Galabraes Hill near Bathgate. Some striking veirts which occur in 
the Jedburgh district, in the debateable laud between the plateau series oir 
the east and the prry-series on the west, show the rrearly complete usurpatioir 
of the funnel by basalt, but with portions of the tuff still renrainirrg rdsible. 
Belatio'ii of the Necks to the Pocks through which they rise. — A. remarkable 
feature among the Carborriferous arrd I’ermian verrts of central Scotlaird is 
presented by the effect which has beerr produced on the strata inmrediately 
surrounding them. In the irrterior of the country this effect is ofteir 
