One of the most obvious features in this volcanic tract is the way in which the erupted materials lie along the lines of hollow or valley in which the red rocks were deposited. This is most distinctly exhibited in the central district. Here a belt of breccias and sandstones, varying from one to three and a half miles in breadth, runs for about five and twenty miles westward in a depression of the Culm-measures. At intervals, the lavas which lie near the base of the red rocks crop out along the margin of the belt throughout most of its extent. But they do not spread out over the older rocks, and they have evidently been erupted from orifices situated along the line of the valley. It is another example of the relation between the trend of hollows and the outbreak of volcanic vents, which I have referred to as so strikingly displayed in the distribution of the Permian volcanic rocks of south-western Scotland. The volcanic materials of the Devonshire Permian district consist mainly of lavas, but include also red sandy and gravelly tuffs. The whole volcanic group is remarkably thin, never attaining even the limited development of the Ayrshire series. No adequate petrographical investigation of these rocks has yet been made. Externally, as seen in the quarries and lanes, the lavas present the closest resemblance to those of the Permian basins of Ayrshire and Nithsdale. They show considerable differences of texture even within the same mass, some portions being dull, fine-grained purplish-red rocks, with scattered pseudomorphs of hæmatite and a few porphyritic felspars, other parts passing into an exceedingly coarse amygdaloid or slaggy pumice. Dr. Hatch, after a microscopical examination of a small collection of specimens, found that while most are olivine-basalts, containing ferruginous pseudomorphs after olivine (Raddon Court, Pocombe, and near Budlake), others are true andesites (Ide, Kellerton Park) and even mica-trachytes (Copplestone, near Knowle Hill).[109] As already remarked, some of the older writers mention the existence of quartz-porphyries.[110] [Footnote 109: _Geol. Mag._ 1892, p. 250. The rocks have been more recently described by Mr. B. Hobson, _Quart. Jour. Geol. Soc._ vol. xlviii. (1892), p. 502. The rock of Kellerton Park is called by Mr. Hobson "mica-augite-andesite," and he gives a chemical analysis of it by Mr. E. Haworth, _op. cit._ p. 507. Mr. Watts has lately found one of the orthoclase rocks to be rich in olivine.] [Footnote 110: See De la Beche, _Report_, pp. 203, 204. My colleague, Mr. Ussher, found close to the Thurlestone outlier of conglomerate near Kingsbridge, Devonshire, a small boss of quartz-porphyry which rises through and alters the Devonian rocks. The actual junction of this mass with the conglomerate is not seen, nor have any fragments of the porphyry been noticed among the pebbles.