Fig. 200.—Section of lavas east side of Mauchline Hill.
Fig. 201.—Section of the top of the volcanic series near Eastside Cottage, Carron Water, Nithsdale.
There is good reason to believe that both the volcanic sheets and the red sandstones overlying them, instead of being restricted to an area of only about 30 square miles, once stretched over the lowlands of Ayrshire; and not only so, but that they ran down Nithsdale, and extended into several of its tributary valleys, if indeed, they were not continuous across into the valley of the Annan.[92] Traces of the lavas and tuffs are to be found at intervals over the area here indicated. The most important display of them, next to their development in Ayrshire, occurs in the vale of the Nith at Thornhill, whence they extend continuously up the floor of the Carron Valley for six miles. They form here, as in Ayrshire, a band at the base of the brick-red sandstones, and consist mainly of bedded lavas with the basic characters above referred to. These lavas, however, are followed here by a much thicker development of fragmental volcanic materials. Abundant volcanic detritus is diffused through the overlying sandstones, sometimes as a gravelly intermixture, sometimes in large slaggy blocks or bombs, and sometimes in intercalated layers of tuff, while an occasional sheet of one of the dull red lavas may also be detected. The final dying-out of the volcanic energy in a series of intermittent explosions, while the ordinary red sandy sediment was accumulating, is here also admirably chronicled. As an illustration of these features the accompanying section is given ([Fig. 201]). The last of the lavas (a) presents an uneven surface against which the various kinds of detritus have been laid down. First comes a coarse volcanic breccia (b) made up of angular and subangular blocks of different lavas imbedded in a matrix of red ashy sand. This deposit is succeeded by a band of dull red tufaceous sandstone, evidently formed of ordinary red sandy sediment, into which a quantity of volcanic dust and lapilli fell at the time of its accumulation. Some of the ejected blocks which lie inclosed in the finer sediment are upwards of a foot in length. A more vigorous discharge of fragmental material is shown by the next bed (d), which consists of a coarse nodular tuff, mingled with a little red sandstone and crowded with blocks of the usual lavas. Beyond the locality of this section these tuffs are found to pass up insensibly into the ordinary Permian sandstone.
[92] See Memoirs of Geol. Surv. Scotland, Sheet 15 (1871), p. 35; Sheet 9 (1877), p. 31.
Fig. 202.—Section of two outliers of the Permian volcanic series at the foot of Windyhill Burn, Water of Ae, Dumfriesshire.
But we can detect the edges of yet more distant streams of lava emerging from under the red sandstones and breccias to the east of the Nith. On the farther side of the Silurian ridge that forms the eastern boundary of the Nith valley, above which it rises some 700 or 800 feet, there is preserved at the bottom of the valley of the Capel Water, which flows into Annandale, another small outlier of a similar volcanic band. Three miles to the south-east of it two little fragments of the volcanic group lie on the sides of a small tributary of the Water of Ae. Since these may serve as a good illustration of the extent to which denudation has reduced the area of the Permian volcanic series, a section of the locality is here given (Fig. 202). The general foundation rocks of the country are the Silurian greywackes and shales in highly inclined and contorted positions (a). Each outlier has, as its basement material, a volcanic breccia (bb) in which, together with the usual lava-fragments, are mingled pieces of the surrounding Silurian strata. In the smaller outlier lying to the north-east, this detrital layer is only about one foot thick. It is overlain by a slaggy amygdaloid of the usual character (cc), which in the lower outlier is covered with boulder clay (d). There can be little doubt that these detached fragments were once united in a continuous sheet of lava which filled the valley of the Water of Ae and that of its tributary. That the lava stretched down the Ae valley for some distance is proved by the occurrence of another outlier of it two miles below.
But there is still additional evidence for the wide extension of these volcanic sheets. It appears to be certain that they stretch far to the eastward, under the Permian sandstones of the Lochmaben basin of Annandale, for breccias largely made up of pieces of the bedded lavas are found close to the northern edge of the basin on the west side of the River Annan. To this remarkable adherence of the lavas and tuffs to the bottom of the Permian valleys I shall afterwards more specially refer.