DEVONSHIRE 
35 
region. Many of the “ greenstones ” have been so cleaved as to 1 >ecome 
slaty or almost schistose. I)e la Beclie recognized this change and wrote 
of the “ schistose trappean ash.” A result of this metamorpliism has been 
to impart to rocks originally massive the same fissile structure as the 
adjacent slates possess and in this condition it is often hardly possible to 
distinguish between “ greenstone ” and fine-grained “ ash. 1 here can indeed 
be little doubt that among these Carboniferous volcanic rocks, as we have 
seen to be the case with those of the Devonian system in the same region, 
many lavas or sills have been mapped as tuffs. 
The chief additions to our knowledge of the Carboniferous volcanic 
group of Devonshire since the time of De la Beclie have been made by Mr. 
T. Butley, Mr. W. A. Ussher and General M'Mahon. Mr. Rutley 1 has 
, e mleavoured to trace the respective areas occupied by the different varieties 
°f volcanic rocks in the district around Brent Tor, near Tavistock, and to 
sl >ow the probable connection of the successive bands of lavas and tuffs 
wi th a central, vent of discharge situated at that hill. He believes that 
these bands occur on four different horizons in the sedimentary series. He 
ha » studied the microscopic structure of the rocks, which in his view include 
“amphibolites, gabbros, basalts, pitchstones and schistose ashes or clastic 
r °cks of a doubtful nature.” 2 , 
Mr. Ussher has remapped the tract of Culm-measures on the east side of 
the Dartmoor granite, besides visiting some of the other areas outside of t le 
” V; -mite 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 
f °and by him to be mostly dolerites, some of which, from their parallelism 
wi th the bands of tuff, may be in his opinion contemporaneous lavas, though 
iim majority of them are evidently intrusive. The tuffs are regularly inter - 
stratified among the Culm-measures, their most important band in this dis- 
trict having an" average breadth of about 100 yards, and being traceable for 
at least two miles, possibly considerably further. 8 In going over this tract 
With Mr. Ussher I was led to regard many of the sheets of diabase (dolente) 
0r gabbro as true sills and bosses. Most of them occur as short lenticular 
° r oy al patches tolerably numerous, but not traceable for more than a short 
< istauce, though a connection may often exist which cannot be detected by 
tlle scanty evidence on the surface. One sheet which has been followed by 
M>'. 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, 
f is Overlain with dark Culm-shales and bands of fine shaly tuff, passing 
3 The Eruptive Rocks of Brent Tor,” p. 45. 
‘The British Culm-measures,” Proc. Somerset 
Jr- J-61. 
the west of Dartmoor, with some Notes on the Structure 
vm. Geol. Soc. xxxvi. (1880), p. 286. 
'roc. Somerset Ardtaeol. and Nat. His. Soc. xxxviii. (1892), 
