[65] "The Eruptive Rocks of Brent Tor and its Neighbourhood," Mem. Geol. Surv. 1878. "On the Schistose Volcanic Rocks occurring on the west of Dartmoor, with some Notes on the Structure of the Brent Tor Volcano," Quart. Journ. Geol. Soc. xxxvi. (1880), p. 286.
[66] "The Eruptive Rocks of Brent Tor," p. 45.
Mr. Ussher has re-mapped the tract of Culm-measures on the east side of the Dartmoor granite, besides visiting some of the other areas outside of the granite mass. While confirming the general accuracy of De la Beche's survey, he has been able to improve the mapping by inserting more detail, separating especially the tuffs from the "greenstones." The latter have been found by him to be mostly dolerites, some of which, from their parallelism the bands of tuff, may be in his opinion contemporaneous lavas, though the majority of them are evidently intrusive. The tuffs are regularly interstratified among the Culm-measures, their most important band in this district having an average breadth of about 100 yards, and being traceable for at least two miles, possibly considerably further.[67] In going over this tract with Mr. Ussher I was led to regard many of the sheets of diabase (dolerite) or gabbro as true sills and bosses. Most of them occur as short lenticular or oval patches tolerably numerous, but not traceable for more than a short distance, though a connection may often exist which cannot be detected by the scanty evidence on the surface. One sheet which has been followed by Mr. Ussher from Combe to beyond Ashton, a distance of nearly two miles, presents in the centre a somewhat coarsely crystalline texture which rapidly gives way to a much closer grain, and the rock then becomes highly vesicular. It is overlain with dark Culm-shales and bands of fine shaly tuff, passing upward into a granular tuff. Some layers of this tuff assume a finely foliated appearance by the development of pale leek-green folia, which show slickensided surfaces parallel with the bedding. The rock then presents one of the usual appearances of schalstein. This structure seems obviously due to mechanical movement along the planes of stratification.
[67] "The British Culm-measures," Proc. Somerset Archæol. and Nat. His. Soc. xxxviii. (1892), p. 161.
Bands of black chert and cherty shale are interpolated among the tuffs, which also contain here and there nodular lumps of similar black impure earthy chert—an interesting association like that alluded to as occurring in the Carboniferous volcanic series of the Isle of Man, and like the occurrence of the radiolarian cherts with the Lower Silurian volcanic series already described.[68]
[68] Cherts containing numerous species of radiolaria have recently been found by Dr. Hinde and Mr. Howard Fox to form an important part of the Lower Culm-measures of Devonshire, Quart. Journ. Geol. Soc. vol. li. (1895), p. 609.
The volcanic belt in the valley of the Teign can be followed for about two miles. It is undoubtedly interstratified among the dark Culm-measures, which are distinctly seen dipping under and overlying it.
General M'Mahon has recently shown what may be done by careful and detailed examination of the ground broadly sketched in by De la Beche. He chose for study a strip of "greenstone" shown on the Geological Survey Map to extend for about three and a half miles along the north-west margin of the Dartmoor granite. He has found that what is represented under one wash of colour on that map includes both tuffs and lavas. The tuffs, in spite of the alteration which they appear to have undergone from the proximity of the great granite mass, are found by microscopic investigation to be made up of fine volcanic dust containing minute lapilli of various lavas. Sometimes as many as six or seven different kinds of lava may be represented in the same microscopic slide. These include felsitic or rhyolitic and trachytic rocks together with fragments of dark glassy lava full of magnetite dust. With the tuffs are intercalated sheets of felsite and trachyte. In the same district coarse volcanic agglomerate occur, made up of blocks of different lavas and pieces of different sedimentary rocks.[69]
[69] Quart. Journ. Geol. Soc. vol. l. (1894), p. 338.
These observations are of special interest, inasmuch as they point to the eruption of a much more acid series of volcanic lavas and tuffs than had previously been known to exist in the Culm-measures. Until the ground has been more accurately mapped, it is impossible to say whether these rocks are older or younger than those that lie around Brent Tor, a few miles to the south-west. General M'Mahon has noted the presence of more basic eruptive rocks in the same district. He specially cites the occurrence of mica-diorite, of basaltic lavas altered into a serpentinous mass, and of a dolerite which may possibly mark the actual vent of the old Brent Tor volcano. His observations on the influence of the Dartmoor granite in inducing new mineral rearrangements in the igneous rocks of the Culm-measure series are full of interest.