CHAP. XX 
ERUPTIONS IN A YR SHIRE 
331 
the dislocation, where they must once have lain, is an evidence of the great 
denudation wliieh the district lias undergone. Fig. 94, wliich gives a section 
across the broadest part of tlie area, from the edge of the Muirkirk coal-tield 
to the Silurian uplands, shows the general structure of the ground. 
Xo satisfactory evidence regarding the position of any of the vents 
of the period has been met with in this district. The rocks to the 
soirth of the boundary-fault are older than the Old Eed Sandstone, and 
as they must have been for some distance overspread by the conglomerates, 
sandstones and volcanic series, we might hope to find somewhere among 
them traces of necks or bosses. The only mass of eruptive rock in 
that part of tlie district is the tract of Spango granite which has been 
already referred to in connection with the subject of the vents and granite 
protrusions of Old Eed Sandstone time. This mass, about four miles long 
and two miles broad, rises through Silurian strata, and by means of the- 
boundary fault is brought against the higher group of conglomerates and 
sandstones. The Silurian shales and sandstones around the granite have 
undergone contact-metamorphism, becoming highly micaceous and schistose. 
The ascent of this granite must have taken place between the upheaval and 
contortion of the Upper Silurian strata, and the deposition of the liigher 
parts of the Lower Old Eed Sandstone of this region. Its date might thus 
come within the limits of the volcanic period. But one must frankly own 
that there is no positive evidence to connect its production with the volcanic 
history. 
8. The Ayrshire Group of Vents 
The original limits of the volcanic districts in the remaining portion of the 
Old Eed Sandstone area on the mainland of Scotland, from the valley of the 
Xith to the Firth of Clyde, can only be vaguely indicated.^ There is a diffi- 
culty in ascertaining the south-western termination of the Duueaton area, 
and in deciding whether the lavas and tuffs of Corsincone in Nithsdale 
should be assigned to that district or be placed with those further to the 
south-west. Between Corsincone and the next visible volcanic rocks of the 
Lower Old Eed Saiidstone there intervenes a space of six miles, along which, 
owing to the effect of the great fault that flanks the north-western margin 
of the Southern Uplands, the Carboniferous Limestone and even the Coal- 
measures are brought against the Siluiian formations, every intermediate 
series of rocks being there cut out. It may therefore be, on the whole, 
better to include all the volcanic rocks on the left side of the Xith as part 
of the Duneaton scries. There will still remain a tract of five miles of 
blank intermediate ground Iiefore we enter upon the volcanic rocks of 
Ayrshire. 
> The mapping of the Old Ked Sandstone volcanic areas of Ayrshire for the Geological Survey 
was thus distributed : — The district east of Dahnellington was suiweyed hy Mr. B. N: Peacli, that 
between Dahnellington and Straitou hy Prof. Janies Geikie, and all from the line of the Girvan 
Valley south of Straiten westward to the sea by myself. The ground is embraced in Sheets 8, 13 
and 14 of the Map of Scotland, and is described in accompanying Exiilanations. 
