CHAP. XXXI 
THE PERMIAN SILLS 
9i 
by denudation but for the protection afforded to the tuff by the intrusion ot 
the basalt. The upper dotted lines in the figure are inserted merely to 
indicate hypothetically how the cone may originally have stood. O 11 the 
west side the sheets of tuff which were thrown out over the surrounding 
Fig. 226. — Section across Largo Law. 
1 1, Lower Carboniferous strata ; t, tuff of cones j t', tuff of plain -beyonrl the cones ; 11 B, basalt ascending vents 
and sending out veins ", B , basalt wliic.li lias probably flowed out at the surface. The dotted lines aie 
suggestive of the original outline of the hill. 
country have been almost entirely removed, but on the east and south they 
still cover an extensive area. (See Fig. 208). 
(2) Sills . — In the Clyde coal-field and in the basin of the Firth of 
Forth, among the vast number of sills which there traverse the Carboniferous 
formations, it is possible that some belong to the Permian volcanic period (see 
vol. i. p. 474). Where the sheets have been intruded along horizons that lie 
below the upper stratigrapliical limit of the puy eruptions, they may not un- 
naturally he held to belong to these manifestations of volcanic energy, though 
it is obviously quite conceivable that some of them may be of much later date. 
But where they lie above the highest platforms of Carboniferous lavas and 
tuffs, they may be assigned to a younger volcanic period. We know as yet 
of only two such periods after the deposition ot the Carboniferous Limestone 
series in Scotland — Permian and older Tertiary. Unless, therefore, these 
higher sills formed part of some other display of subterranean activity 
which is not known to have culminated in eruptions at the surface, they 
must he looked upon as probably either Permian or Tertiary. 
In the great coal-field of Stirlingshire and Lanarkshire, among the 
large sills that break into the Millstone Grit and the Coal-measures, 
one lies entirely in the Coal-measures, and covers about six square miles 
of ground, stretching from near Caldercruix Station, a little east ot Airdrie, 
to near Kirk of Shotts, a distance of about four miles. A group of 
smaller sheets, possibly connected with the larger mass, runs for four 
miles further west to beyond New Monkland. Another chain of sills, which 
may also be part of the same great intrusion, extends from tire Cant Hills, 
near the Kirk of Shotts, for more than eight miles in a north-easterly 
direction. The largest mass in this chain stretches from Blackridge, west 
of Bathgate, for upwards of three miles, covering an area of about three 
square miles and terminating on the north at the line of dislocation 
which has been followed by one of the east and west dykes. Another 
