CHAP. XXXI 
THE PERMIAN SILLS 
93 
lai'ge sill, which appears nearly two miles further east on the north side 
of that dyke, lies on a lower stratigrapliical horizon, for it cuts the 
Carboniferous Limestone series, and does not reach the top of the Millstone 
Grit. This sill is cut through by two of the later dykes. 
That these great intrusions took place later than the deposition of the 
Coal-measures is obvious. There is no satisfactory evidence to enable us to 
decide to which of the two post-Carboniferous volcanic periods they may 
with most probability be assigned. As one of them is distinctly cut by 
dykes that have been referred to the Tertiary series, it might be plausibly 
argued that it at least is of pre- Tertiary date, and therefore probably 
Permian. On the other hand, as will be shown in a later chapter, some 
portion of the sills appears to be connected with the younger or lertiary 
dykes. This problem must for the present remain unsolved. 
In Ayrshire where, as already described, basic sills traverse the Permian 
volcanic series, other large intrusive sheets are found around the Permian 
basin. On the north side an important group of them, passing through the 
Coal-measures into the Carboniferous Limestone series, runs from Troon 
eastward for more than eight miles to beyond Craigie. On the south side a 
much more extensive series may be traced from the Paver Ayr southwards 
into the Dalmellington coal-field, and thence north-eastwards in a wide 
semicircular sweep into the coal-field of New Cumnock and Airds Moss. 
That some of these sills proceed from the Permian necks has been definitely 
ascertained, and this fact has been already alluded to in connection with the 
vents. I have little doubt that the great majority, if not the whole, oi 
these intrusive sheets are to be referred to the Permian period. 
Some of the sills must be later than a part of the I ennian v olcanic 
eruptions, for they are found in at least three places intercalated in the zone 
of lavas and tuffs. But no instance has been observed of their traversing 
the basin of Permian sandstone which overlies that zone, though a few dykes, 
possibly of Tertiary age, do cut this sandstone. 
