[366] This area is included in Sheets 23 and 24 of the Geological Survey of Scotland. It was mapped and described by myself. (Explanations of Sheets 23 and 24.) Various parts of it have been referred to by earlier writers, particularly Maclaren, Geology of Fife, etc., p. 176.
Another distinct group of volcanoes had its centre about 25 miles south-westward from the Braid vent, and on the same line as those of the Pentland Hills. In no part of the basin can the isolation of the different volcanic clusters be so impressively observed as in the area to the south-west of these hills. On the one hand, the lavas and tuffs from the Braid vent die out, and on the other, as we follow the conglomerates south-westwards, a new volcanic series immediately makes its appearance.
The space between the last extremity of the Pentland lavas and the beginning of the Biggar series does not exceed some 500 yards. It will be remembered that the lower half of the Pentland volcanic series dies out long before it reaches the southern end of the hills, and that it is by lavas on the horizon of some of the dark andesites of Allermuir Hill that the volcanic band is finally prolonged to its extreme southern limit. The most northerly extension of the Biggar lavas lies somewhere on the same general platform. But whereas, at the north end of the Pentland chain, the volcanic sheets rest on the edges of the Upper Silurian shales, at the south end, several hundred feet of coarse conglomerate and sandstone intervene between the Silurian shales and the porphyrites. So rapidly does the bulk of these sedimentary formations increase that in the course of two miles they must be 3000 feet in thickness below the most northerly of the Biggar lavas just referred to. But after that point, when they cross the Lyne Water, they begin to be more and more interstratified with thin sheets of andesite. These lavas, the beginning of the Biggar series, soon number nine or ten distinct bands, and so quickly do they usurp the place of the sedimentary materials that in a distance of not more than twelve miles they form, where traversed by the river Clyde, the whole breadth of the visible tract of Old Red Sandstone, to the exclusion of the conglomerates.
Unfortunately, soon after the lavas make their appearance at the north end they are in great measure overlapped unconformably by the red sandstones at the base of the Carboniferous system, but where the Medwin Water has cut through this covering, they can be seen here and there underneath on their southerly course.
A section through the northern end of the Biggar series, where the successive lavas are dying out northwards among the conglomerates, shows the structure given in [Fig. 91]. The sedimentary strata consist largely of debris of andesite, and the lavas include dark red or purple andesites and also pale felsites, both having the same characters as those of the Pentland Hills.
Fig. 91.—Section across the northern end of the Biggar volcanic group, from Fadden Hill to beyond Mendick Hill.
1. Conglomerates and sandstones; 2. Lavas, the lowest being an olivine-diabase or basalt, the main mass being andesites; 3. Felsites and tuffs; 4. Upper Old Red Sandstone. f, Fault.
In one important respect the volcanic series in the northern part of the Biggar area differs from that of the Pentland Hills, for whereas the uppermost parts of the latter are concealed by faults which bring down the Carboniferous strata against the base of the hills, the lavas at the north end of the Biggar district pass conformably under a thick group of Lower Old Red conglomerates and sandstones. We thus learn that here the volcanic eruptions ceased long before the close of the deposition of the Lower Old Red Sandstone. The overlying sedimentary series is disposed in a long synclinal trough, corresponding in direction with the general north-easterly strike of the volcanic rocks which reappear from under the sandstones and conglomerates along its south-eastern border, where they are abruptly truncated by the fault (f, [Fig. 92]), which brings them against the flanks of the Silurian Uplands. It is interesting to note that by this dislocation the lavas of the Lower Old Red Sandstone are placed almost in immediate contact with those of the Lower Silurian series, which appear here on the crests of numerous anticlinal folds that are obliquely cut off by the fault.
There is yet another feature of interest in the northern part of the Biggar volcanic centre. While the lowest visible lava is an olivine-diabase not unlike parts of the Warklaw group of the Pentland Hills, those which occur above it are partly andesites and partly orthoclase-felsites. The latter form, among the hills near Dolphinton, an important group which reaches its greatest development in the Black Mount (1689 feet). These rocks cover a breadth of more than a mile of ground, and probably attain a thickness of not less than 2000 feet. They so closely resemble in their general characters the corresponding rocks of the Pentland Hills that a brief description of them may suffice. As in that chain of hills, they are so prone to decomposition that they are in large part concealed under a covering of their own debris and of herbage, though their fragments form abundant screes, and numerous projecting knobs of rock suffice to show the main features of the lavas and their accompaniments.
The felsites weather into pale yellow and greyish "claystones," but where fresher sections can be procured they often show darker tints of lilac and purple. They are close-grained, sometimes flinty, generally porphyritic with scattered highly-kaolinized white felspars, but without quartz, often presenting beautiful flow-structure, and not infrequently showing a brecciated appearance, which in the usual weathered blocks is hardly to be distinguished from the breccia of interstratified tuffs.