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] [Footnote 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.