CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 455 
them ; their absence makes it certain that the numerous volcanic vents were 
confined to the low grounds. 
C. STRUCTURE OF THE VOLCANIC MASSES. 
The volcanic rocks associated with the Carboniferous formations in the 
Basin of the Firth of Forth may be conveniently grouped into four sub-divisions 
according to their mode of occurrence with reference to the surrounding strata. 
Ist, Necks, that is, masses of volcanic material occupying the space of former 
yents or orifices out of which the volcanic eruptions proceeded. 2d, Intrusive 
Sheets, Dykes, and Veins. These are portions of lava which never succeeded in 
forcing their way to the surface, but after penetrating some way upward, were 
arrested in their progress, and consolidated among the rocks. 3d, Interbedded 
or Contemporaneous Lavas, that is, masses of molten rock which were emitted 
at the surface, flowed out there in streams, and consolidated into sheets that lie 
conformably among the strata with which they are geologically contempor- 
aneous. 4th, 7ugs, which occur in large stratified masses, or in small beds, 
either interstratified with ordinary sedimentary deposits, or accompanying sheets 
of lava. 
1. Volcanic Necks. 
General Characters.—A volcanic neck is a pipe or funnel which has been 
blown out of the earth’s crust, and has been filled up with the solid materials 
ejected by the first or subsequent explosions. Viewed geologically, it may be 
regarded as a column of extraneous material usually in the main of volcanic 
origin, which descends from the surface to an unknown depth beneath. 
Unless disturbed by posterior subterranean movements, this column may 
be considered to be vertical, though any tilt subsequently affecting the rocks 
of the locality may have given it an inclination to one side. In the basin of 
the Firth of Forth there has been comparatively little displacement of this 
kind. 
In their external aspect the necks form conspicuous features among the 
volcanic districts in which they occur. In the great majority of cases they rise 
as isolated cones or dome-shaped hills, circular or elliptical in outline, and for 
the most part with smooth grassy slopes. Where a dyke or boss of a hard 
rock, such as basalt, occurs in them, it usually stands out as a crag or knoll. 
Where the whole neck consists of an enduring rock of that kind, it forms 
a bolder, more abrupt eminence. Largo Law (fig. 2) may be taken as a 
singularly perfect example of the cone-shaped neck. Traprain Law and North 
Berwick Law illustrate the contour assumed when the rock is of a more 
