This singular structure has already ([p. 184]) been referred to as strikingly displayed in a rock at the top of Cader Idris. It is found in dark basic lavas probably of Arenig age, which will be afterwards referred to as occurring along the southern flanks of the Scottish Highlands and also in the north of Ireland. It has been observed by Mr. Teall among the rocks of the Lizard, and has been described as occurring in Saxony and California.[172] In these different localities it is associated with jaspers and cherts, some of which contain abundant Radiolaria. The same structure has been found among the variolitic diabases of Mont Genèvre,[173] and likewise in some modern lavas, as in that of Acicastello already referred to (ante, [p. 26]).
[172] Mr. J. J. H. Teall, Roy. Geol. Soc. Cornwall, 1894, p. 3. Mr. L. Ransome, Bull. Depart. Geol. University of California, vol. i. p. 106.
[173] Messrs. Cole and Gregory, Quart. Journ. Geol. Soc. vol. xlvi. (1890), p. 311.
Fig. 52.—View of Knockdolian Hill from the east.
The volcanic agglomerates and breccias, in the south-west of Ayrshire, attain a great development in several centres probably at or near the original volcanic vents. They present several distinct petrographical types. The remarkable neck-like hill of Knockdolian in the Stinchar Valley is made of a coarse breccia composed mainly of angular pieces of dull greyish-green fine-grained diabase. The breccias and agglomerates of Bennane Head in some parts consist largely of broken-up shales, flinty mudstone, black radiolarian flint or chert, and abundant fragments of andesites and felsites. In other parts the volcanic material predominates, including angular and subangular fragments of various somewhat basic lavas, lumps of vesicular slag and pieces of pumice. Here and there much calcite is diffused through the matrix in strings, veins and patches, which enclose the lapilli. The agglomerate north of Lendalfoot possesses a greenish, somewhat serpentinous matrix, through which immense numbers of tabular felspar crystals are scattered. Similar crystals also occur abundantly in embedded blocks of one of the purplish diabase-porphyrites, which occurs in mass on the shore and inland, and closely resembles the rock of Carnethy in the Old Red Sandstone volcanic series of the Pentland Hills.
Yet another and very distinct type of agglomerate is to be seen on the Mains Hill south-east of Ballantrae. It is a coarse rock, enclosing blocks up to a yard or more in diameter, of a fine compact purplish porphyrite, with large crystals of plagioclase and smaller ones of augite. In some places immense numbers of the small lapilli in the matrix consist of an extremely fine vesicular pumice. Small perfect and larger broken crystals of augite are likewise abundant in some of the greenish, more basic parts of the mass. These greenish serpentinous parts and the numerous augite crystals point to the explosion of some tolerably basic pyroxenic lava. A similar dark green, almost black, rock, with augite crystals, which sometimes measure a quarter of an inch in diameter, occurs near Sanquhar in Nithsdale. It presents a close resemblance to the agglomerate of Rhobell Fawr, already alluded to. So far as these Scottish agglomerates have yet been microscopically examined, they have been found to be composed of crystals, crystal-fragments, and lapilli derived partly from lavas similar to those above described, and partly from felsitic and other rocks which have not yet been observed here in the form of lavas.
The finer tuffs show likewise a considerable range of composition. According to Mr. Peach's observations along the south-eastern parts of the volcanic area, the ejected materials have consisted largely of fine dust (probably in great measure felsitic), which towards the north-east is gradually interleaved with ordinary sediment till the ashy character disappears. As I have already remarked, there is reason to believe that the overlying greywackes and shales derived part of their material either directly from volcanic explosions or from the attrition of banks of lavas and tuffs exposed to denudation.
But besides the interstratified lavas and fragmental rocks there occur numerous intrusive masses which are so intimately associated with the volcanic series that they may with little hesitation be regarded as forming part of it. They consist of various gabbros and serpentines, which are especially developed where the volcanic series comes out in greatest force in the south-west of Ayrshire. They also include more acid intrusions which, as in the case of the rock of Byne Hill, near Girvan, even assume the characters of granite.
The dying out of the volcanic material towards the north-east probably indicates that the vents of the period lay rather in the central or south-western parts of the district. Unfortunately, the limited extent of the exposures of the rocks makes it a hopeless task to search for traces of these vents over by far the largest part of the area. There are two localities, however, where the search may be made with better prospect of success. One of these is a tract to the north of Sanquhar in Nithsdale, which still requires to be studied in detail with reference to the sequence and structure of its volcanic rocks. The other area is that south-western part of Ayrshire which has been already cited as displaying so large a development of the volcanic series. Here the coast-sections reveal the intercalation of fossiliferous bands which show the true stratigraphical horizons of the lavas and tuffs. Under Bennane Head, Professor Lapworth some years ago found, in certain hardened black shales, a group of graptolites which mark an undoubted Arenig platform.[174] Recently the ground has been carefully re-examined by Messrs Peach and Horne, who have detected a number of other fossiliferous zones which confirm and extend previous observations. They have also been able to unravel the complicated structure of the volcanic series, and to represent it on the 6-inch maps of the Geological Survey, of which a reduction on the scale of 1 inch to a mile is now in course of preparation. The following tabular summary, taken partly from notes made by myself during a series of traverses of the ground with Mr. Peach when the revision was begun, and partly from memoranda supplied by that geologist himself, may suffice as a general outline of the volcanic history of this exceedingly interesting and important region.