The chief additions to our knowledge of the Carboniferous volcanic group of Devonshire since the time of De la Beche have been made by Mr. F. Rutley, Mr. W. A. Ussher and General M'Mahon. Mr. Rutley[65] has endeavoured to trace the respective areas occupied by the different varieties of volcanic rocks in the district around Brent Tor, near Tavistock, and to show the probable connection of the successive bands of lavas and tuffs with a central vent of discharge situated at that hill. He believes that these bands occur on four different horizons in the sedimentary series. He has studied the microscopic structure of the rocks, which in his view include "amphibolites, gabbros, basalts, pitchstones and schistose ashes, or clastic rocks of a doubtful nature."[66] [Footnote 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.] [Footnote 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.