That the lavas underlie these piles of detritus and extend southwards even up to the very edge of the Silurian Uplands is shown by the rise of a number of successive beds from under the trough into which the conglomerate has been thrown. These lavas, however, are almost immediately cut off by the great boundary fault (f) which flanks the Silurian territory. That they are not met with now to the south-east of the dislocation, where they must once have lain, is an evidence of the great denudation which the district has undergone. [Fig. 94], which gives a section across the broadest part of the area, from the edge of the Muirkirk coal-field to the Silurian uplands, shows the general structure of the ground.
No satisfactory evidence regarding the position of any of the vents of the period has been met with in this district. The rocks to the south of the boundary-fault are older than the Old Red 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 the 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 Red 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 higher parts of the Lower Old Red 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 Red Sandstone area on the mainland of Scotland, from the valley of the Nith to the Firth of Clyde, can only be vaguely indicated.[370] There is a difficulty in ascertaining the south-western termination of the Duneaton 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 Red Sandstone 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 Silurian 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 Nith as part of the Duneaton series. There will still remain a tract of five miles of blank intermediate ground before we enter upon the volcanic rocks of Ayrshire.
[370] The mapping of the Old Red Sandstone volcanic areas of Ayrshire for the Geological Survey was thus distributed:—The district east of Dalmellington was surveyed by Mr. B. N. Peach, that between Dalmellington and Straiton by Prof. James Geikie, and all from the line of the Girvan Valley south of Straiton 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 Explanations.
Owing to complicated faults, extensive unconformable overlaps of the Carboniferous formations, and enormous denudation, the volcanic tracts of Old Red Sandstone age in Ayrshire have been reduced to mere scattered patches, the true relations of which are not always easily discoverable. One of these isolated areas flanks the Silurian Uplands as a belt from a mile to a mile and a half in breadth and about six miles long, but with its limits everywhere defined by faults. A second much more diversified district extends for about ten miles to the south-west of Dalmellington. It too forms a belt, averaging about four miles in breadth, but presenting a singularly complicated geological structure. Owing to faults, curvatures and denudation, the volcanic rocks have there been isolated into a number of detached portions, between some of which the older parts of the Old Red Sandstone, and even the Silurian rocks, have been laid bare, while between others the ground is overspread with Carboniferous strata. A third unbroken area forms the Brown Carrick Hills, south of the town of Ayr, and is of special interest from the fact that its rocks have been exposed along a range of sea-cliffs and of beach-sections for a distance of nearly four miles. Other detached tracts of volcanic rocks are displayed on the shore at Turnberry and Port Garrick, on the hills between Mochrum and the vale of the Girvan, and on the low ground between Dalrymple and Kirkmichael.
The isolation of these various outliers and separated districts is probably not entirely due to the effects of subsequent geological revolutions. More probably some of the areas were always independent of each other, and their igneous rocks were discharged from distinct volcanic centres. We may conjecture that one of these centres lay somewhere in the neighbourhood of New Cumnock, for the lavas between that town and Dalmellington appear to diminish in thickness and number as they are traced south-westward. Another vent, or more probably a group of vents, may have stood on the site of the present hills to the right and left of the Girvan Valley, south of the village of Straiton. A third probably rose somewhere between Dailly and Crosshill, and poured out the lavas of the ridges between Maybole and the Dailly coal-field. The important centre of eruption that produced the thick and extensive lavas of the Brown Carrick Hills may be concealed under these hills, or may have stood somewhere to the west of Maybole. Still another vent, perhaps now under the sea, appears to be indicated by the porphyrites of the coast-section between Turnberry and Culzean Bay.
Owing to the complicated structure of the ground, several important points in the history of the Old Red Sandstone of this region have not been established beyond dispute. In particular, the unconformability which undoubtedly exists in that system in the south-west of Ayrshire has not been traced far enough eastwards to determine whether it affects the volcanic belt east of Dalmellington, or whether the break took place before or after the eruption of that belt. West of Dalmellington it clearly separates a higher group of sandstones, conglomerates and volcanic rocks from everything older than themselves. The structure is similar to that in the Pentland Hills, a marked disturbance having taken place here as well as there after a considerable portion of the Lower Old Red Sandstone had been deposited. These earlier strata were upraised, and on their denuded ends another group of sandstones and conglomerates was laid down, followed by an extensive eruption of volcanic materials.
It is the upper unconformable series that requires to be considered here, as it includes all the volcanic rocks of the Old Red Sandstone lying to the west of the meridian of Dalmellington. The position of these rocks on their underlying conglomerates is admirably exposed among the hills between the valleys of the Doon and the Girvan, as well as on Bennan Hill to the south of Straiton. The andesites rise in a craggy escarpment crowning long green slopes that more or less conceal the conglomerates and sandstones below.
Along the coast-sections the structure of the volcanic rocks may be most advantageously studied. The shore from the Heads of Ayr to Culzean Castle affords a fine series of exposures, where every feature in the succession of the lavas may be observed. Still more instructive, perhaps, is the mile and a half of beach between Turnberry Bay and Douglaston, of which I shall here give a condensed account, for comparison with the coast-sections of Kincardineshire and Forfarshire already described.