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. [Footnote 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.] [Footnote 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.] [Footnote 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.