[170] Trans. Roy. Soc. Edin. xxii. (1860), p. 636.
It had been long known that the Scottish Silurian formations, besides having undergone extensive plication, have also been injected by protrusions of igneous material of various kinds. The intrusive character of many of these is so obvious that a similar origin was attributed even to those bosses which could not be proved to be intrusive. Recent work of the Geological Survey, however, and more especially the numerous and careful traverses of my friend and colleague Mr. Peach, have revealed the unlooked-for and important fact that a large number of these supposed intrusions are really portions of a volcanic group brought up on the crests of anticlinal folds, and laid bare by denudation. This group can be traced for at least 100 miles from north-east to south-west over a belt of country sometimes 30 miles broad. Its original limits cannot be ascertained, but they obviously exceeded those within which the rocks can now be seen. Nevertheless the present boundaries embrace an area of nearly 2000 square miles. This Palæozoic volcanic region is thus one of the most extensive in the British Isles. Owing, however, to the constant plication of the strata, and the wide space which the overlying sedimentary deposits are thus made to cover, the volcanic group only comes occasionally into view, and thus occupies but a mere fraction of the superficial extent of the region over which its scattered outcrops appear. These exposures, sometimes only a few square yards in extent, may always be looked for where the anticlinal folds bring up a sufficiently low portion of the Silurian system; they prove that a vast volcanic floor underlies the visible Lower Silurian grits and shales over the length and breadth of the Southern Uplands of Scotland.
Without anticipating details which will properly appear in the official Memoirs of the Geological Survey, I may briefly indicate the visible boundaries of the volcanic group, and refer to some of the localities where it may best be seen. The most easterly points where it has been recognized by Mr. Peach stand on the crests of some sharp anticlinal folds near St. Mary's Loch and near Leadburn and Winkstone in Peeblesshire. Farther westwards it appears at many places along the northern border of the Silurian territory, as at Romanno Bridge, Wrae, Kilbucho, Culter Water and Abington, the length and breadth of each exposure depending partly on the breadth of the anticline and partly on the depth to which it has been cut down by denudation. Near Sanquhar the volcanic series opens out for a breadth of more than a mile, and is seen at intervals across the wild moorlands of Carrick, until from the Stinchar valley it widens out seaward and occupies much of the coast-line of Ayrshire between Girvan and the mouth of Loch Ryan. It probably rises again along a fold near Portpatrick, and it is seen at various points along the southern borders of the Silurian uplands, as near Castle-Douglas, at Glenkiln, Bell Craig near Moffat, and the head of Ettrickdale.
The best sections are those exposed along the coast to the north and south of Ballantrae. When that ground was first examined by the Geological Survey, the hypothetical views in regard to metamorphism already referred to were in full ascendant, and the rocks were mapped on the same general principles as those which had been followed in Wales. Professor Bonney, however, a few years later recognized the true igneous nature of many of the rocks. He found among them porphyrite lavas and agglomerates which he regarded as of Old Red Sandstone age, likewise intrusive serpentines and gabbros.[171]
[171] Quart. Journ. Geol. Soc. vol. xxxiv. (1878), p. 769.
The volcanic rocks of this wide district include both lavas and their pyroclastic accompaniments, as well as intrusive sills and bosses of various materials. They have recently been studied by Mr. J. J. H. Teall, and full descriptions of them by him will appear in a forthcoming volume of the Memoirs of the Geological Survey. He has ascertained that though generally more or less decomposed, the lavas would be classed by German petrographers as diabases and diabase-porphyrites. The former are compact dark-green non-porphyritic rocks, often containing numerous small spherical amygdales; while the latter are markedly porphyritic, enclosing large phenocrysts of more or less altered plagioclase, often measuring half an inch across. These two groups of rock are connected by transitional varieties. They were probably, in the first instance, composed of plagioclase, augite, iron-ores, and a variable quantity of imperfectly crystallized interstitial matter.
Some of these rocks closely resemble in outward appearance the andesites ("porphyrites") of the Old Red Sandstone of the district not many miles to the north, that is, fine purplish-red rocks with a compact base through which porphyritic felspars are abundantly scattered. Occasionally they are markedly slaggy, and show even a ropy surface, while the breccias associated with them contain blocks of similar slag.
Fig. 51.—Structure in finely-amygdaloidal diabase lava, south of mouth of Stinchar River, Ayrshire. The fine dots and circles mark the lines of amygdales.
But the most characteristic external feature of these lavas is their tendency to assume irregularly-elliptical, sack-like or pillow-shaped forms. On a weathered face they sometimes look like a pile of partially-filled sacks heaped on each other, the prominences of one projecting into corresponding hollows in the next. The general aspect of this structure is shown in [Fig. 12], which represents a face of rock about eight feet high and six feet broad. The rocks exhibiting this peculiarity are usually finely amygdaloidal, and it may be observed that the vesicles are grouped in lines parallel to the outer surface of the pillow-like block in which they occur. The diagram in [Fig. 51] represents in ground-plan a surface about twelve feet square on the shore immediately to the south of the mouth of the River Stinchar. In the heart of the spheroids enclosed fragments of other lavas are sometimes observable.