CHAP. XXXI 
A YRSH1RE, NITHSDALE AND ANNANDALE 
63 
some fragmentary outliers of the Permian lavas capping the Upper Coal- 
measures ; and if we merely crossed from the Nith into the tributary valley 
of the Carron Water, we should see preserved in that deep hollow a great 
series of Permian lavas, tuffs and agglomerates. It is only by a happy 
accident that here and there these superficial volcanic accumulations have 
not been swept away. There was probably never any great thickness of 
them, but they no doubt covered most, if not all, of the district within 
which the vents are found. 
The Permian necks are, on the whole, smaller than those of the Carbon- 
iferous period. The largest of them in the Ayrshire and Nitlisdale region 
do not exceed 4000 feet in longest diameter; the great majority are much 
less in size, while the smallest measure 20 yards, or even less. Those of Fife, 
to be afterwards described, exhibit a wider range of dimensions, and lia\e 
the special advantage of being exposed in plan along the shore. 
These necks, from their number and shapes, form a marked feature in 
Fig. ‘204.— Patna Hill from the Doon Bridge, Ayrshire ; a tuff-neck of Permian age. 
the scenery. They generally rise as prominent, rounded, dome-shaped, or 
conical hills, which, as the rock comes close to the surface, remain per- 
manently covered with grass (Pigs. 203 and 204). Such smooth green 
puys are conspicuous in the heart of Ayrshire, and likewise further south in 
the Dalmellington coal-field, where some of them are locally known as 
“ Green Hill,” from their verdant slopes in contrast to the browner vegeta- 
tion of the poorer soil around them (Fig. 203). 
As in those of older geological periods, the necks of this series are, 
for the most part, irregularly circular or oval in ground-plan, but some- 
times, like those of the Carboniferous system, they take curious oblong 
shapes, and occasionally look as if two vents had coalesced (Fig. 205). 
Hero and there also the material of the vents has consolidated between the 
walls of a fissure or the planes of the strata, so as to appear rather as a 
dyke than as a neck. Descending, as usual, vertically through the rocks 
which they pierce, the necks have the form of vertical columns 
volcanic material, ending at the surface in grassy rounded hillocks or lulls. 
