CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 477 
in one place below, in another above, the invading mass, but in the intervening 
ground has been engulphed in it. Similar evidence of the widely separate 
horizons occupied by different parts of the same intrusive sheet is supplied at 
Kilsyth, where the intrusive sheet lies about 70 or 80 fathoms below the Index 
Limestone, while at Croy, in the same neighbourhood, it actually passes above 
that seam.* 
The thickness of the intrusive sheets varies within tolerably wide limits. 
They here and there dwindle down to an inch or less in thickness, running 
away as threads from some thicker mass. But they more usually form masses 
of considerable depth. The rock of Salisbury Crags, for example, is fully 150 
feet thick at its maximum. That of Corstorphine Hill is probably about 350 
feet. The great sheet which runs among the lower limestones from Kilsyth by 
Denny to Stirling has been bored through to a depth of 276 feet, but as the 
bore started on the rock, and not on overlying strata, some addition may need 
to be made to that thickness. 
Area and Horizons —Taking first the area of surface occupied by intrusive 
sheets apart from their geological horizons, we observe that the Falkirk and 
Stirlingshire coal-field is girdled with a great ring of these sheets. Beginning 
at the Abbey Craig, near Stirling, we may trace this ring as a continuous belt 
of high ground from Stirling to the River Carron. Thence it splits up into 
minor masses in different portions of the. Carboniferous system, and doubtless 
belonging to different periods of volcanic disturbances, but yet sweeping as a 
whole across the north-eastern part of the Clyde coal-field, and then circling 
round into Stirlingshire and Linlithgowshire. There are no visible masses to 
fill up the portion of the ring back to Abbey Craig. But through the high 
grounds of Linlithgowshire a number of minor intrusive sheets form an east- 
ward prolongation of the ring, taking in the masses near Edinburgh, and then 
bending northwards into Fife. In the latter county the intrusive masses 
acquire their greatest development. A nearly continuous belt of them runs 
from the Cult Hill near Saline on the west, to near St Andrews on the east, a 
distance of about thirty-five miles. This remarkable band is connected with 
a less extensive one, which extends from Torryburn on the west, to near Kirk- 
caldy on the east. It is remarkable that to the east of the axis of the Pentland 
Hills hardly any trace is to be seen of intrusive sheets. 
If now we examine the geological position of the strata invaded by these 
intrusive masses, we find that by far the larger proportion forms part of the 
Carboniferous Limestone series. The belt between Stirling and Kilsyth keeps 
among the lower parts of that series. On the same general horizon are the great 
sheets of dolerite which stretch through Fife in the chain of the Cult, Cleish, 
and Lomond Hills on the one side, and in the eminences from Torryburn to 
* Explanation of Sheet 31, “ Geological Survey of Scotland,” §§ 43 and 83. 
VOL. XXIX, PART I. 6G 
