The intrusive rocks, probably referable to the same volcanic period, consist chiefly of dolerites and basalts which occur as dykes, sills and bosses, and are more particularly developed in the south-west of Ayrshire.
ii. GEOLOGICAL STRUCTURE OF THE VOLCANIC DISTRICTS
1. Ayrshire, Nithsdale and Annandale
(1) Interstratified Lavas and Tuffs.—It will be convenient to consider first the volcanic chronicle as it has been preserved in the south-west and south of Scotland, where the existence of Permian volcanoes in Britain was first recognized. The volcanic rocks in the middle of the Ayrshire coal-field rise from under a central basin of red sandstone, which they completely enclose. Their outcrop at the surface varies up to about a mile or rather more in breadth, and forms a pear-shaped ring, measuring about nine miles across at its greatest width (Map V.).[91]
[91] Mr. Gunn has recently detected among the newest red sandstones of Arran a small patch of volcanic rocks which may be of this age. Mr. A. Macconochie has also found what may be traces of a similar volcanic band below the Permian sandstones of Loch Ryan, in Wigtonshire.
This volcanic ring runs as a tract of higher ground encircling the hollow in which the Permian red sandstones lie, and forming a marked chain of heights above the Carboniferous country around. It is built up of a succession of sheets of different lavas, with occasional partings of tuff or volcanic breccia, which present their escarpments towards the coal-field outside, and dip gently into the basin under the inner trough of brick-red sandstones. Good sections of the rocks are exposed in the ravines of the River Ayr, particularly at Ballochmyle, in the Dippol Burn near Auchinleck House, and in the railway cutting near Mossgiel.
That these are true lava-flows, and not intrusive sills, is sufficiently obvious from their general outward lithological aspect, some of them being essentially sheets of slag and scoriæ. Their upper surfaces may be found with a fine indurated red sand wrapping round the scoriform lumps and protuberances, and filling in the rents and interspaces, as in the case of the Old Red Sandstone lavas already referred to. As an example of these characteristics, I may cite the section represented in [Fig. 200]. At the bottom lies a red highly ferruginous and coarsely amygdaloidal basalt (a). Over it comes a volcanic conglomerate three feet thick, made up of balls of vesicular lava like that below, wrapped in a brick-red sandy matrix (b). Lenticular bands of sandstone without blocks occur in the conglomerate, and others lie in hollows of its upper surface (c). This intercalation of detrital material is followed by another basic lava (d), about six feet thick, highly amygdaloidal in its lower and upper parts, more compact in the centre. The amygdales and joints are largely filled with calcite. The slaggy bottom has caught up and now encloses some of the red sand of the deposit below. Another lava from three to six feet thick next appears (e), which is remarkable for its slaggy structure, and is so decomposed that it crumbles away. Like the others it is dull-red and ferruginous and full of calcite. It must have been at the time of its outflow a sheet of rough slag that cracked into open fissures. That it was poured out under water is again shown in the same interesting way just referred to, by the red sand which has been washed into the interspaces between the clinkers and has filled up the fissures, in which it is stratified horizontally between the walls. Above this band, and perhaps passing into it as its slaggy base, lies another more compact lava (f) like the lower sheets.
Fig. 199.—General section across the Permian basin of Ayrshire.
1. Highest group of the Coal-measures; 2. Volcanic tuffs and ashy brick-red sandstones; 3. Lavas with interstratified tuffs and brick-red sandstones; 4. Brick-red Permian sandstones; 5, 5. Necks of volcanic agglomerate; 6. Boss of dolerite.
Throughout the series of lavas, as indicated in the foregoing section, traces of the pauses that elapsed between the separate outflows may be seen in the form either of layers of red sandstone or of tuff and volcanic breccia. Here and there, under the platform of bedded lavas, the brick-red sandstone is full of fragments of slag and fine volcanic dust. But the most abundant accumulation of such detritus is to be seen at the top of the volcanic series, where it contains the records of the closing phases of eruption. Thick beds of tuff and volcanic breccia occur there, interleaved with seams of red sandstone, like the chief mass of that rock, into which they gradually pass upward. Yet, even among the sandstones above the main body of tuff, occasional nests of volcanic lapilli, and even large bomb-like lumps of slag, point to intermittent explosions before the volcanoes became finally extinct and were buried under the thick mass of red Permian sandstone.