[104] Siluria, 4th edit. (1867), p. 333. See also Berger, Trans. Geol. Soc. vol. i. (1811), pp. 98-102; Conybeare and Phillips, Geology of England and Wales, p. 313, footnote; De la Beche, Report on the Geology of Cornwall, Devon and West Somerset (1839), chap. vii. p. 193. Messrs. Hull and Irving (Quart. Journ. Geol. Soc. vol. xlviii. 1892, pp. 60, 68) have more recently discussed the subject, and follow the view of Murchison.
[105] Murchison cogently argued that as no signs of volcanic activity were known in the Trias, but were abundant in the Permian system, the Devonshire rocks might be regarded as appertaining to the older series, op. cit. Triassic volcanic rocks, however, are now well known on the Continent.
No proper account has yet been written of the volcanic group which I now propose to describe.[106] De la Beche was, I think, the first to recognize the true volcanic nature of the rocks and their contemporaneous interstratification in the red sandstone series.[107] As traced by him on the Geological Survey maps, these rocks lie at or near the base of the red sedimentary deposits, resting sometimes directly on the Culm-measures, sometimes on an intervening layer of red strata. He found them in three separate districts in the neighbourhood of Exeter, the most northerly lying near Tiverton, the central extending from Kellerton for a few miles up the Yeo Valley, beyond Crediton, and the third stretching from the City of Exeter to Pen Hill, about five miles to the south-west. He recognized the amygdaloids as slaggy lavas, and saw that the volcanic breccias and tuffs are interleaved with the sandstones. With regard to the probable vents from which these materials were ejected, he thought that the chief centre of activity lay at Kellerton Park, while in other localities he believed the bosses of igneous rock "to descend in mass downwards, as if filling up some crater or fissure through which these rocks had been vomited."[108] He speaks also of "quartziferous porphyries" occurring among them, a statement which, if petrographically accurate, would suggest the uprise of a later more acid lava in some of the vents.
[106] An outline of some of their characters will be found in a paper by Mr. W. Vicary in Trans. Devonshire Assoc. 1865, vol. i. part iv. p. 43.
[107] See his "Report" cited in the note above. De la Beche quotes J. J. Conybeare as pointing out the intimate connection of these igneous and stratified rocks (Annals of Philosophy, 2nd series, vol. ii. (1821) p. 165); but this author wrote at the time of the Plutonist and Neptunist controversy, and does not commit himself to any distinct expression of opinion on the subject.
[108] Report, p. 201.
More recently the ground has been revised by Mr. W. A. E. Ussher of the Geological Survey, who has ascertained that the volcanic rocks appear in many more places than those where they were noted on the older maps, and likewise extend for some miles further to the north and west.
It now appears that in the central and chief district the lavas can be followed westward from Spray Down near Kellerton to Greenslade near North Tawton, a distance of about twenty-one miles. Their most northerly outcrop is at Thorn above Loxbere in the Tiverton district, and their most southerly visible portion passes under the Cretaceous rocks of Pen Hill. The distance between these extreme points is likewise about twenty-one miles. The whole display of volcanic phenomena is comprised within an area of less than 400 square miles.
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]